https://theultimatetone.com/ Mon, 11 May 2026 12:47:38 +0000 MyBB https://theultimatetone.com/Thread-Pro-Reverb-Mod-Ideas Tue, 10 Feb 2026 16:13:23 +0000 foreverstrung]]> https://theultimatetone.com/Thread-Pro-Reverb-Mod-Ideas To qualify, the last few years I've built about a dozen diff amps ranging from 18w to 50w.
Currently I've picked up a 1979 70w Pro Reverb that appears to have been dormant for quite awhile. Rusty chassis straps, dank grill cloth and it appears to have all of the original tubes. I don't want the 70w vanilla PA type sound we get from this amp. I'm looking for ideas to modify. I considered gutting the thing and building a 6G16 amp. I believe I can use the same PT and OT, chassis and cabinet. Or I could try to bring this 70w PA sounding amp to the 40w early 70's version with maybe more breakup at a lower volume.
I'm getting ready to disassemble it. Clean up the hardware. Grill cloth and tolex. I could just restore it as it is and resale it. I paid $600 for it and reverb and tremolo work great. Speakers are great. I really would like more of a project then just restoring.
Appreciate the feedback]]> To qualify, the last few years I've built about a dozen diff amps ranging from 18w to 50w.
Currently I've picked up a 1979 70w Pro Reverb that appears to have been dormant for quite awhile. Rusty chassis straps, dank grill cloth and it appears to have all of the original tubes. I don't want the 70w vanilla PA type sound we get from this amp. I'm looking for ideas to modify. I considered gutting the thing and building a 6G16 amp. I believe I can use the same PT and OT, chassis and cabinet. Or I could try to bring this 70w PA sounding amp to the 40w early 70's version with maybe more breakup at a lower volume.
I'm getting ready to disassemble it. Clean up the hardware. Grill cloth and tolex. I could just restore it as it is and resale it. I paid $600 for it and reverb and tremolo work great. Speakers are great. I really would like more of a project then just restoring.
Appreciate the feedback]]> https://theultimatetone.com/Thread-Split-rail-PA-with-Higher-Front-end-Voltages Thu, 05 Jun 2025 16:01:51 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Split-rail-PA-with-Higher-Front-end-Voltages
Most modern solid-state power amps use symmetric supply voltages, commonly referred to as "split rails". This just means there is, say, +/-50Vdc supporting the circuit. In absolute terms each rail is the same voltage, only the polarity is different.

The supply voltage has to be sufficient to accommodate the peak signal amplitude into the speaker load at full load. So, the +/-50V will accommodate a 40Vpk signal into 8-ohms, corresponding to 200Wpk or 100Wrms. There is 10V extra, which accommodates losses over the transistors and current-sense resistors (emitter resistors). If the supply stays solid at 50V at full load, there should a few more volts possible into the load as we do not want to rate the power of the amp right at its limit lest it may not reach that value under low-mains or other conditions. On any case, the supply rails need to be in proportion to the power output desired.

Most PA circuits live within the same supply voltage as the output stage requires. However, there are some designs where the front-end circuitry requires a higher voltage, and these higher rails can be created in various ways. The two broad methods are supply stacking and having independent sources.

Supply stacking is exactly as it sounds: a second supply is stacked on top of the main supply. Where a typical PSU has a center-tapped winding, a full bridge over the whole winding and filter caps and bleeder resistors from each of the bridge outputs to the CT, the stacked supply adds another small supply on top of each rail. This smaller supply is often a small OPT with dual secondaries, where each secondary has a full bridge across it, followed by a cap and bleeder resistor. These small supplies are identical and one has its negative end tied to the positive main rail, while the second small supply has its positive end tied to the negative main rail. We end up with V+HI, V+, CT (0), V-, V-HI. Note that the V+/-HI are relatively low-current and would not be suitable in a class-G or class-H amplifier.

To size the small PT, we look at the current draw of the front-end circuitry. The typical differential input stage draws constant current. The transimpedance stage may have a constant current source for one side, but the other side may draw more current under load. Predrivers or driver stages draw modest currents but can sink and source much higher currents, so these should be assessed in simulation or in a real circuit. How much current the small supply must support depends on the circuit load, and we most often have drivers and predrivers supported by the main supply, alleviating some of the variable loading on the small supply.

We also consider the nominal voltage that the front-end circuit operates from. Because it sits atop the main supply, which will sag under load, we have to account for that sag plus any sag that may occur due to low-mains to both PTs. This generally means that the small supply voltage might be twice the difference we need between the idling main rail and the idling front-end rail. For example, a mosfet amp might have +/-65V main rails with +/-75V for the front-end. We intuitively view this as needing 10V boost supplies. If that is all we add, then as the 65V sags, so too will the 75V, and this may not be good for the stability of the quiescent point for the output stage, or possibly for the overall circuit stability.

If we double the boosted supply to 20V and add a ground-referenced regulator to provide 75V, then we accommodate up to 10V of sag for the front-end. The reality is that we need a bit more sacrificial voltage here.

As a reference,the inherent regulation of typical toroidal PTs is quite good, varying with the VA rating:
80VA   10%
120VA  9%
160VA  7.7%
225VA  7.2%
300VA  7.1%
500VA  5.1%
625VA  4.1%

Some front-end circuits incorporate active current sources while others use resistive ones. For the latter, a regulated supply maintains bias stability, but means the amp might be unstable while powering up. As mentioned, the regulator must be ground-referenced and it is simplest to use a discreet circuit for this. For active CS circuits, we may still wish to regulate the voltage, or at least have an active hum filter for each boosted rail.

The PT for the boosted supplies may be quite small depending on the circuit, maybe 10-15VA, but could be up to 50VA for a large amp or when significant drive currents must be supported by the boosted rail. Each boost supply is derived from one secondary, so whatever the current and voltage product is for one boost supply, we must double it for the PT VA rating.

The small PT VA goes up proportionately if we use it to create fully independent high rails that are ground-referenced. The circuit is identical to the main supply, just a smaller PT with higher voltages.

The boost supply PT can be a small toroid, semi-toroid or EI type. EMI from this PT should be low as the load is balanced over the secondaries and fairly constant.

Have fun]]>
Most modern solid-state power amps use symmetric supply voltages, commonly referred to as "split rails". This just means there is, say, +/-50Vdc supporting the circuit. In absolute terms each rail is the same voltage, only the polarity is different.

The supply voltage has to be sufficient to accommodate the peak signal amplitude into the speaker load at full load. So, the +/-50V will accommodate a 40Vpk signal into 8-ohms, corresponding to 200Wpk or 100Wrms. There is 10V extra, which accommodates losses over the transistors and current-sense resistors (emitter resistors). If the supply stays solid at 50V at full load, there should a few more volts possible into the load as we do not want to rate the power of the amp right at its limit lest it may not reach that value under low-mains or other conditions. On any case, the supply rails need to be in proportion to the power output desired.

Most PA circuits live within the same supply voltage as the output stage requires. However, there are some designs where the front-end circuitry requires a higher voltage, and these higher rails can be created in various ways. The two broad methods are supply stacking and having independent sources.

Supply stacking is exactly as it sounds: a second supply is stacked on top of the main supply. Where a typical PSU has a center-tapped winding, a full bridge over the whole winding and filter caps and bleeder resistors from each of the bridge outputs to the CT, the stacked supply adds another small supply on top of each rail. This smaller supply is often a small OPT with dual secondaries, where each secondary has a full bridge across it, followed by a cap and bleeder resistor. These small supplies are identical and one has its negative end tied to the positive main rail, while the second small supply has its positive end tied to the negative main rail. We end up with V+HI, V+, CT (0), V-, V-HI. Note that the V+/-HI are relatively low-current and would not be suitable in a class-G or class-H amplifier.

To size the small PT, we look at the current draw of the front-end circuitry. The typical differential input stage draws constant current. The transimpedance stage may have a constant current source for one side, but the other side may draw more current under load. Predrivers or driver stages draw modest currents but can sink and source much higher currents, so these should be assessed in simulation or in a real circuit. How much current the small supply must support depends on the circuit load, and we most often have drivers and predrivers supported by the main supply, alleviating some of the variable loading on the small supply.

We also consider the nominal voltage that the front-end circuit operates from. Because it sits atop the main supply, which will sag under load, we have to account for that sag plus any sag that may occur due to low-mains to both PTs. This generally means that the small supply voltage might be twice the difference we need between the idling main rail and the idling front-end rail. For example, a mosfet amp might have +/-65V main rails with +/-75V for the front-end. We intuitively view this as needing 10V boost supplies. If that is all we add, then as the 65V sags, so too will the 75V, and this may not be good for the stability of the quiescent point for the output stage, or possibly for the overall circuit stability.

If we double the boosted supply to 20V and add a ground-referenced regulator to provide 75V, then we accommodate up to 10V of sag for the front-end. The reality is that we need a bit more sacrificial voltage here.

As a reference,the inherent regulation of typical toroidal PTs is quite good, varying with the VA rating:
80VA   10%
120VA  9%
160VA  7.7%
225VA  7.2%
300VA  7.1%
500VA  5.1%
625VA  4.1%

Some front-end circuits incorporate active current sources while others use resistive ones. For the latter, a regulated supply maintains bias stability, but means the amp might be unstable while powering up. As mentioned, the regulator must be ground-referenced and it is simplest to use a discreet circuit for this. For active CS circuits, we may still wish to regulate the voltage, or at least have an active hum filter for each boosted rail.

The PT for the boosted supplies may be quite small depending on the circuit, maybe 10-15VA, but could be up to 50VA for a large amp or when significant drive currents must be supported by the boosted rail. Each boost supply is derived from one secondary, so whatever the current and voltage product is for one boost supply, we must double it for the PT VA rating.

The small PT VA goes up proportionately if we use it to create fully independent high rails that are ground-referenced. The circuit is identical to the main supply, just a smaller PT with higher voltages.

The boost supply PT can be a small toroid, semi-toroid or EI type. EMI from this PT should be low as the load is balanced over the secondaries and fairly constant.

Have fun]]> https://theultimatetone.com/Thread-Toroidal-PT-and-OT-Suppliers Fri, 28 Mar 2025 16:31:48 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Toroidal-PT-and-OT-Suppliers
My own toroidal transformer guru retired a while ago and that has caused me to think about what direction I want to take with my amp line. His rates for custom power and audio transformers were very much on the low side and he did not penalise you for prototypes or low quantities.

Building preamps is not a problem as there are many suppliers with appropriate off-the-shelf products that are suitable. It is really the support of power amps that is tricky. You can find good output transformers but the PT designs I have seen all leave something to be desired.

Note that when using a toroidal OT it MUST be FULL-POWER-BANDWIDTH. You CANNOT use the OT to restrict the frequency range as it will overload in an extremely non-musical manner.

The nearly universal problem with toroidal PT offerings is that there is no proper bias winding.

In a tube power amplifier the bias supply is the most important voltage in the entire circuit.

As readers of TUT (The Ultimate Tone) know, a separate bias WINDING on the PT is preferred in the method for achieving lowest-noise and to properly support a Power Scale solution. A bias TAP on a center-tapped plate winding is workable, but far from ideal.

Toroidal transformers are nearly ideal in their functioning and because of this we have to specify them and use them a bit differently than all other transformer types. This mainly manifests itself in how we configure rectification inasmuch as half-wave rectification should be avoided at all cost. This means that a bias TAP is not a good idea AND that the basic CTed plate winding is also not appropriate unless you are in fact using a tube rectifier - there are other ways to incorporate a tube rectifier which use a non-CTed winding. As TUTs point out, the CTed plate winding is really just two half-wave circuits over-lapped and each winding has to be able to support the full load - a real waste of wire and winding space. The discontinuous conduction of half-wave rectifiers injects noise into the PT, which couples into all the other windings and back to the mains. The efficiency of toroids makes this a much worse situation than with traditional transformer designs.

So, toroids need to be specified with windings that require a full-wave full-bridge rectifier, usually referred to simply as a "bridge rectifier" and often attained as an integrated 4-pin package.  The winding is used to support the load continuously over the full AC cycle of the mains to minimise EMI. The plate winding should be like every other secondary inasmuch as it is a single winding with no CT.

The plate winding may have a tap to allow different maximum output voltage using either a switch or hard-wiring. As TUT4 illustrated, this tap will not provide an alternate but lower voltage at the same time as the full winding is being used, nor the reverse, where the full winding cannot provide a higher voltage while the tap is in use.

The capacitive-coupled bias supply some manufacturers use with the proper plate winding is clever in all the wrong ways.  It has a high-ish impedance and imposes a discontinuous load on the winding, injecting nose. It has all of the wrong characteristics to supply what is the most important voltage in the entire chassis and will not support a bias regulator. It is truly a false economy.

In my survey of toroidal PT offerings for tube amps, I see the CTed plate winding BUT no 5V heater winding for the presumed tube rectifier?  The use of such windings continued well past the reign of tube rectifiers, and this can only be due to lethargy on the part of both the amp builders and the transformer suppliers - an unspoken agreement. In any case, many of the toroidal manufacturers today are newcomers to the industry and it must be assumed they are simply unwittingly copying the mistakes of legacy manufacturers without realising it. Or, they have some gaps in their understanding about tube power amps?

Heater windings tend to be plentiful to over-abundant in modern toroidal PTs for tube amps. Most have at least two 6v3 windings with many providing a CT to at least one of those. The CT is fairly useless since a faux-CT works as well, and in audio the use of a DC-stand-off for the heater winding is inexpensive and provides as much hum rejection as  DC heaters but without all the trouble and wasted heat.

You have to remember that with a toroid EVERY winding must fully cover the core to help shield it, which means that it must be made with enough wire to wrap around the full circle of the toroid and end right beside where it began. A CTed winding is in fact two separate windings that each must wrap all the way around the core, and then have the appropriate leads tied together and brought out as a center-tap. Doing this for low-voltage windings imposes some difficulty upon the manufacturer. The wire lead-outs impose unavoidable gaps in the shielding, so minimising the number of these is helpful in reducing EMI.

TUT4 presented London Power's 6/12 heater system. This uses a 12V CTed winding to supply a mix of 6V and 12V heaters. For lowest-noise from wiring, using 12V for preamp tubes that accommodate it allows the triode leads to never have to cross the heater wiring. We can do this in both hand-wired and PCB layouts. In a power amp, the 6V (octal) tube heaters can be distributed on either side of the CT (and with separate wire runs can use #22 wire). These loads do not have to be equal about the CT for the reasons cited above. With many of the tube amp toroidal PTs on offer, the two 6V windings can be wired this way and tied to a DC-stand-off. If one of the 6V windings has a CT we simply do not use the CT.

So, most of the available toroidal PTs for tube amps have no bias winding and have no 5V winding to support a tube rectifier. The latter is not really a problem for most builders. To have a proper bias supply, we end up adding an auxiliary supply of one form or another to create a bias supply of sufficient voltage and impedance.

In the US, Antek has a range of PTs and OTs
https://www.antekinc.com/transformers/

In Poland, Toroidy has PTs and OTs, serving all of Europe
https://sklep.toroidy.pl/en_US/index

There are certainly many other companies providing off-the-shelf devices for hobbyists and builders.

Note that the information for each device is often scanty and there is no "match-up" suggestions of PT + OT.

There are often a range of finishes from standard tape-wrapped, to available cans, and then potted centers or potted in a can.]]>
My own toroidal transformer guru retired a while ago and that has caused me to think about what direction I want to take with my amp line. His rates for custom power and audio transformers were very much on the low side and he did not penalise you for prototypes or low quantities.

Building preamps is not a problem as there are many suppliers with appropriate off-the-shelf products that are suitable. It is really the support of power amps that is tricky. You can find good output transformers but the PT designs I have seen all leave something to be desired.

Note that when using a toroidal OT it MUST be FULL-POWER-BANDWIDTH. You CANNOT use the OT to restrict the frequency range as it will overload in an extremely non-musical manner.

The nearly universal problem with toroidal PT offerings is that there is no proper bias winding.

In a tube power amplifier the bias supply is the most important voltage in the entire circuit.

As readers of TUT (The Ultimate Tone) know, a separate bias WINDING on the PT is preferred in the method for achieving lowest-noise and to properly support a Power Scale solution. A bias TAP on a center-tapped plate winding is workable, but far from ideal.

Toroidal transformers are nearly ideal in their functioning and because of this we have to specify them and use them a bit differently than all other transformer types. This mainly manifests itself in how we configure rectification inasmuch as half-wave rectification should be avoided at all cost. This means that a bias TAP is not a good idea AND that the basic CTed plate winding is also not appropriate unless you are in fact using a tube rectifier - there are other ways to incorporate a tube rectifier which use a non-CTed winding. As TUTs point out, the CTed plate winding is really just two half-wave circuits over-lapped and each winding has to be able to support the full load - a real waste of wire and winding space. The discontinuous conduction of half-wave rectifiers injects noise into the PT, which couples into all the other windings and back to the mains. The efficiency of toroids makes this a much worse situation than with traditional transformer designs.

So, toroids need to be specified with windings that require a full-wave full-bridge rectifier, usually referred to simply as a "bridge rectifier" and often attained as an integrated 4-pin package.  The winding is used to support the load continuously over the full AC cycle of the mains to minimise EMI. The plate winding should be like every other secondary inasmuch as it is a single winding with no CT.

The plate winding may have a tap to allow different maximum output voltage using either a switch or hard-wiring. As TUT4 illustrated, this tap will not provide an alternate but lower voltage at the same time as the full winding is being used, nor the reverse, where the full winding cannot provide a higher voltage while the tap is in use.

The capacitive-coupled bias supply some manufacturers use with the proper plate winding is clever in all the wrong ways.  It has a high-ish impedance and imposes a discontinuous load on the winding, injecting nose. It has all of the wrong characteristics to supply what is the most important voltage in the entire chassis and will not support a bias regulator. It is truly a false economy.

In my survey of toroidal PT offerings for tube amps, I see the CTed plate winding BUT no 5V heater winding for the presumed tube rectifier?  The use of such windings continued well past the reign of tube rectifiers, and this can only be due to lethargy on the part of both the amp builders and the transformer suppliers - an unspoken agreement. In any case, many of the toroidal manufacturers today are newcomers to the industry and it must be assumed they are simply unwittingly copying the mistakes of legacy manufacturers without realising it. Or, they have some gaps in their understanding about tube power amps?

Heater windings tend to be plentiful to over-abundant in modern toroidal PTs for tube amps. Most have at least two 6v3 windings with many providing a CT to at least one of those. The CT is fairly useless since a faux-CT works as well, and in audio the use of a DC-stand-off for the heater winding is inexpensive and provides as much hum rejection as  DC heaters but without all the trouble and wasted heat.

You have to remember that with a toroid EVERY winding must fully cover the core to help shield it, which means that it must be made with enough wire to wrap around the full circle of the toroid and end right beside where it began. A CTed winding is in fact two separate windings that each must wrap all the way around the core, and then have the appropriate leads tied together and brought out as a center-tap. Doing this for low-voltage windings imposes some difficulty upon the manufacturer. The wire lead-outs impose unavoidable gaps in the shielding, so minimising the number of these is helpful in reducing EMI.

TUT4 presented London Power's 6/12 heater system. This uses a 12V CTed winding to supply a mix of 6V and 12V heaters. For lowest-noise from wiring, using 12V for preamp tubes that accommodate it allows the triode leads to never have to cross the heater wiring. We can do this in both hand-wired and PCB layouts. In a power amp, the 6V (octal) tube heaters can be distributed on either side of the CT (and with separate wire runs can use #22 wire). These loads do not have to be equal about the CT for the reasons cited above. With many of the tube amp toroidal PTs on offer, the two 6V windings can be wired this way and tied to a DC-stand-off. If one of the 6V windings has a CT we simply do not use the CT.

So, most of the available toroidal PTs for tube amps have no bias winding and have no 5V winding to support a tube rectifier. The latter is not really a problem for most builders. To have a proper bias supply, we end up adding an auxiliary supply of one form or another to create a bias supply of sufficient voltage and impedance.

In the US, Antek has a range of PTs and OTs
https://www.antekinc.com/transformers/

In Poland, Toroidy has PTs and OTs, serving all of Europe
https://sklep.toroidy.pl/en_US/index

There are certainly many other companies providing off-the-shelf devices for hobbyists and builders.

Note that the information for each device is often scanty and there is no "match-up" suggestions of PT + OT.

There are often a range of finishes from standard tape-wrapped, to available cans, and then potted centers or potted in a can.]]> https://theultimatetone.com/Thread-Power-Tube-Tone Sat, 08 Mar 2025 18:02:58 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Power-Tube-Tone
Vacuum tubes aka valve, allowed the electronics age to begin and grow into something indispensable in our modern lives. For most things, newer technologies have taken over from tubes, but there are still places where tubes are either the only possible gain element to use, or they are simply preferred for their unique characteristics. For example, microwave ovens use a magnetron tube to generate the microwave energy used to stimulate food to cook it. Add a wave guide and a capacitor and this is the heart of every microwave oven regardless of what the front panel looks like. Very high-power radio transmitters are still tube-based although modular solid-state system are working their way into this realm.

For audio amplification, tubes are still used by many manufacturers, hobbyists and music enthusiasts even though dramatically lower distortion can be attained using solid-state circuits. Nominally, a playback system is not supposed to change the sound, but every listener assembles a system to his own liking, and this is because our individual hearing is unique as is our aesthetic sense of how things should sound. Tubes are considered to be almost "organic" in how they handle music, which can be explained in scientific terms of their structure and resulting nonlinearities. The varying transfer curve of what the output looks like for any given input level gives the tube a "pleasant" character, and makes it doubly useful for musical instrument applications where the amplifier is "the other half of the instrument".  MI is about tone creation, so we want the character of the tubes to shine through, and to be able to combine these characters to achieve our sonic goal.

Fortunately for both hifi and MI, there are a lot of power tube types that share a common base type and pin-out, making them more or less plug-and-play compatible - with the necessary check of the idle condition to assure safe operation. The sound of each tube is unique, but we can group them all based on their sonic texture, which strongly follows their internal structure.

The first broad strike of distinction of power tube tone is based on the use of beam-forming plates. This was an RCA development in the 1950s, and it resulted in a reduction of distortion contributed by the tube. Tubes using beam-forming plates are still called "pentodes" or "tetrodes" as there are still five or four elements within the tube, respectively, but the screen grid is replaced by plates. There is no need for a critical alignment of the various grids, and the electron stream is focused towards the plate.

Beam-forming plate tubes:
6CA7
6L6GB/GC
5881
6550
7027
7481
7491
8417
These tubes have a neutral tone with no specific emphasis of frequencies. Compared to the types below, this group may sound "clean", or "less bright", or "less harsh".

"Kinkless tetrodes" is Mullard's term for their power tetrodes with critically-aligned  grid wires. This group includes the famous KT-series:
KT-66
KT-77
KT-88
KT-100*
KT-120*
*Devised well after Mullard went out of business and the trade name was bought by Mike Matthews of Electroharmonix. So, the structure may not be true to the legacy?

Kinkless tetrodes tend to have a higher distortion than the beam-forming plate tubes, with this distortion being perceived as "thick", "rich", "brittle", or "harsh", "muddy". Personally I find this group to have a muddy tone, but it is a huge component of the distinctive Hiwatt sound.

Structural pentodes have grid wires for the control grid, screen and suppressor. As a power tube there is some critical alignment necessary. This group includes:
6BQ5
6V6
6973
EL-34
EL84
These tubes tend to have a "brittle", "bright" or "harsh" tone, except for 6V6 which is either  "creamy" or "muddy". Being a 9-pin miniature base type, the 6BQ5, EL-84 and 6973 have no substitutes per se, as all have the identical tone.

Each group has variants of many of the tube types within it and there are no doubt other tubes not mentioned here.

Note also that the tones described are for the wiring connection using the tube elements as generally perceived to be "standard", where signal is applied to the control grid, the screen is tied to a fixed voltage, and signal is taken from the plate through an output transformer. The character of each tube will change when alternate operating modes are used, such as triode wiring or the use of ultralinear taps on the OT.

Triode tone is generally perceived to be "clean", but can be deemed "dark" when compared to a reference tone that contains more distortion (as with KTs or structural pentodes / tetrodes). Structural triodes and heater/cathode triodes have a tone similar to the beam-forming plate pentodes.

Ultralinear tone is somewhere between triode tone and the innate tones of the various tube types. UL connections are intended to reduce THD within the tube and the OT yet exhibits a compression effect of its own, representing a gain nonlinearity with signal swing. For most musicians, Fender's dabbling with UL connections turned many players and hobbyists away from this operating mode, but their sonic achievement resulted more from other factors of the rest of the circuit design.

In hifi, many of the "tubes with character" are revered, and this depends a lot on the circuit the tubes are used in and the interactions between the power amp in total with the speakers. Many hifi PAs use unstable circuits with too many gain stages within a single feedback loop. The classic Williamson falls into this category despite achieving good results for the day. A design that squashes the tube characteristics to achieve remarkable results is the MacIntosh Unity-Coupled OT design. Both Williamson and MacIntosh paid great attention to the winding of the output transformer, with the latter being a break from tradition combining both plate and cathode drive, with the bonus of bootstrapping the screens and the driver tube. Audio Research followed along in the plate + cathode drive and often used cathode feedback to the output tubes to provide better linearisation.

MI power amps using tubes fall into two basic designs based on the splitter circuit used. For both, the overall effect of a net low-gain means that the character of the power tubes is well out in the open, and substituting other tube types lends different textures to the amplifier. This means that the player does not have to be limited to the tone of the stock tube set, provided there is a means to adjust individual tube bias (as with our Bias Mod Kits).

As TUT3 demonstrates, the famous Ampeg SVT (Super Valve technology) and V9 amps used an unstable, overly-complex power amp circuit that required many band-aids to make stable. Fender copied this circuit for use in all of its current 300W tube bass amps. As is the usual result of such copying, all of these amps tend to "eat" power tubes unless we make the simple modification recommended throughout the TUT-series.]]>
Vacuum tubes aka valve, allowed the electronics age to begin and grow into something indispensable in our modern lives. For most things, newer technologies have taken over from tubes, but there are still places where tubes are either the only possible gain element to use, or they are simply preferred for their unique characteristics. For example, microwave ovens use a magnetron tube to generate the microwave energy used to stimulate food to cook it. Add a wave guide and a capacitor and this is the heart of every microwave oven regardless of what the front panel looks like. Very high-power radio transmitters are still tube-based although modular solid-state system are working their way into this realm.

For audio amplification, tubes are still used by many manufacturers, hobbyists and music enthusiasts even though dramatically lower distortion can be attained using solid-state circuits. Nominally, a playback system is not supposed to change the sound, but every listener assembles a system to his own liking, and this is because our individual hearing is unique as is our aesthetic sense of how things should sound. Tubes are considered to be almost "organic" in how they handle music, which can be explained in scientific terms of their structure and resulting nonlinearities. The varying transfer curve of what the output looks like for any given input level gives the tube a "pleasant" character, and makes it doubly useful for musical instrument applications where the amplifier is "the other half of the instrument".  MI is about tone creation, so we want the character of the tubes to shine through, and to be able to combine these characters to achieve our sonic goal.

Fortunately for both hifi and MI, there are a lot of power tube types that share a common base type and pin-out, making them more or less plug-and-play compatible - with the necessary check of the idle condition to assure safe operation. The sound of each tube is unique, but we can group them all based on their sonic texture, which strongly follows their internal structure.

The first broad strike of distinction of power tube tone is based on the use of beam-forming plates. This was an RCA development in the 1950s, and it resulted in a reduction of distortion contributed by the tube. Tubes using beam-forming plates are still called "pentodes" or "tetrodes" as there are still five or four elements within the tube, respectively, but the screen grid is replaced by plates. There is no need for a critical alignment of the various grids, and the electron stream is focused towards the plate.

Beam-forming plate tubes:
6CA7
6L6GB/GC
5881
6550
7027
7481
7491
8417
These tubes have a neutral tone with no specific emphasis of frequencies. Compared to the types below, this group may sound "clean", or "less bright", or "less harsh".

"Kinkless tetrodes" is Mullard's term for their power tetrodes with critically-aligned  grid wires. This group includes the famous KT-series:
KT-66
KT-77
KT-88
KT-100*
KT-120*
*Devised well after Mullard went out of business and the trade name was bought by Mike Matthews of Electroharmonix. So, the structure may not be true to the legacy?

Kinkless tetrodes tend to have a higher distortion than the beam-forming plate tubes, with this distortion being perceived as "thick", "rich", "brittle", or "harsh", "muddy". Personally I find this group to have a muddy tone, but it is a huge component of the distinctive Hiwatt sound.

Structural pentodes have grid wires for the control grid, screen and suppressor. As a power tube there is some critical alignment necessary. This group includes:
6BQ5
6V6
6973
EL-34
EL84
These tubes tend to have a "brittle", "bright" or "harsh" tone, except for 6V6 which is either  "creamy" or "muddy". Being a 9-pin miniature base type, the 6BQ5, EL-84 and 6973 have no substitutes per se, as all have the identical tone.

Each group has variants of many of the tube types within it and there are no doubt other tubes not mentioned here.

Note also that the tones described are for the wiring connection using the tube elements as generally perceived to be "standard", where signal is applied to the control grid, the screen is tied to a fixed voltage, and signal is taken from the plate through an output transformer. The character of each tube will change when alternate operating modes are used, such as triode wiring or the use of ultralinear taps on the OT.

Triode tone is generally perceived to be "clean", but can be deemed "dark" when compared to a reference tone that contains more distortion (as with KTs or structural pentodes / tetrodes). Structural triodes and heater/cathode triodes have a tone similar to the beam-forming plate pentodes.

Ultralinear tone is somewhere between triode tone and the innate tones of the various tube types. UL connections are intended to reduce THD within the tube and the OT yet exhibits a compression effect of its own, representing a gain nonlinearity with signal swing. For most musicians, Fender's dabbling with UL connections turned many players and hobbyists away from this operating mode, but their sonic achievement resulted more from other factors of the rest of the circuit design.

In hifi, many of the "tubes with character" are revered, and this depends a lot on the circuit the tubes are used in and the interactions between the power amp in total with the speakers. Many hifi PAs use unstable circuits with too many gain stages within a single feedback loop. The classic Williamson falls into this category despite achieving good results for the day. A design that squashes the tube characteristics to achieve remarkable results is the MacIntosh Unity-Coupled OT design. Both Williamson and MacIntosh paid great attention to the winding of the output transformer, with the latter being a break from tradition combining both plate and cathode drive, with the bonus of bootstrapping the screens and the driver tube. Audio Research followed along in the plate + cathode drive and often used cathode feedback to the output tubes to provide better linearisation.

MI power amps using tubes fall into two basic designs based on the splitter circuit used. For both, the overall effect of a net low-gain means that the character of the power tubes is well out in the open, and substituting other tube types lends different textures to the amplifier. This means that the player does not have to be limited to the tone of the stock tube set, provided there is a means to adjust individual tube bias (as with our Bias Mod Kits).

As TUT3 demonstrates, the famous Ampeg SVT (Super Valve technology) and V9 amps used an unstable, overly-complex power amp circuit that required many band-aids to make stable. Fender copied this circuit for use in all of its current 300W tube bass amps. As is the usual result of such copying, all of these amps tend to "eat" power tubes unless we make the simple modification recommended throughout the TUT-series.]]> https://theultimatetone.com/Thread-Power-Amp-using-preamp-tube Fri, 17 Jan 2025 10:58:00 +0000 Strelok]]> https://theultimatetone.com/Thread-Power-Amp-using-preamp-tube
I would like to build a Power Amp using a preamp tube.
Also because power tubes are getting too expensive for me.
And I am very curious about the sound of course.

I looked at the AX-84 site but like Kevin said there is only one design, the Moonlight.
But it uses a custom transformer.

In TUT5 there is a chapter about AX-84.
There a 125A is used.
As far as I can see, the primaries use colors, like in the schematic.
But the secondaries use numbers.
I don't see them in the schematic.

Wiring should be done in a way that reflects the highest Z.

With an 8 Ohm speaker, it would be 2 & 4 on the secondaries, the highest Z would be 22,500 Ohms then.
Is this correct?

For the Herzog secondaries 1 & 4 are used, giving 11,600 back.


And another question: just for testing, can I use that replacment Reverb TX for our friends over at NewSensor?
I have a few around here.

I will be using an 12AT7.

Kind regards,

Strelok]]>
I would like to build a Power Amp using a preamp tube.
Also because power tubes are getting too expensive for me.
And I am very curious about the sound of course.

I looked at the AX-84 site but like Kevin said there is only one design, the Moonlight.
But it uses a custom transformer.

In TUT5 there is a chapter about AX-84.
There a 125A is used.
As far as I can see, the primaries use colors, like in the schematic.
But the secondaries use numbers.
I don't see them in the schematic.

Wiring should be done in a way that reflects the highest Z.

With an 8 Ohm speaker, it would be 2 & 4 on the secondaries, the highest Z would be 22,500 Ohms then.
Is this correct?

For the Herzog secondaries 1 & 4 are used, giving 11,600 back.


And another question: just for testing, can I use that replacment Reverb TX for our friends over at NewSensor?
I have a few around here.

I will be using an 12AT7.

Kind regards,

Strelok]]> https://theultimatetone.com/Thread-SEM-Mod-on-a-5f6-A Tue, 28 May 2024 03:12:33 +0000 harpdog123]]> https://theultimatetone.com/Thread-SEM-Mod-on-a-5f6-A
Best Regards,
David Pearce]]>
Best Regards,
David Pearce]]> https://theultimatetone.com/Thread-Setting-bias-with-multiple-pots-and-tube Wed, 07 Feb 2024 21:17:22 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Setting-bias-with-multiple-pots-and-tube
You see many amps that are fixed-biased built with a single bias pot for the output stage regardless of how many tubes are present. This is definitely poor design.

In TUT3 and TUT5, we see 2-tube output stages that have two bias pots, one for the 'push' side and one for the 'pull' side. In the 4-tube output stages of the Plexi, 800, Swede, Standard, Custom Special, and the 6-tube SVT, there are individual bias pots for each tube. It is easy to understand how to set the single pot, but how do we adjust multiple pots?

First, when setting bias in an amp, turn the Master Volume or Volume to zero so the readings will be just DC and not noise or signals. In a Power Scaled amp, also set Power Scale for maximum voltage. We set all the bias pots to minimum so that each tube ideally is 'off'.

For a 2-tube output stage with separate bias pots for 'push' and 'pull, which is essentially the same as individual bias pots per tube in this situation, we set the first pot by meter so we know the tube is biased within a safe range, then set the second pot by ear for lowest hum. If you measure the currents they will be slightly different. This is exactly what we expect considering how nearly all output transformers are wound. There are typically more turns of wire on one side of the primary centre-tap than the other but the manufacturer is trying to have the same inductance per side. Even with interleaving and other techniques, most OT primaries are "unbalanced" on a DC resistance basis.

With more tubes, which is generally equal numbers per circuit half, we have to alternate using a metered setting, then an ear setting. Say the first tube is on the 'push' side'. We set that tube by meter to a safe idle current. We go to the first tube on the 'pull' side and adjust it for minimum hum. At this point, only these two tubes are influencing how the OT behaves, whether there is hum or not.

Now we go to the second tube on the 'push' side and set it by meter. We may move the negative meter lead to do a "zero" between it and the first tube we set on this side. Then we set the second tube on the 'pull' side for minimum hum. If there are more tube pairs, we continue along on this procedure until all the tubes are adjusted.

What if there are an odd number of output tubes?

We have to make sure that both sides can be set to about the same net value of current. The side with only a single tube would be set first by meter. The first tube on the other side would also be set by meter to some value that allows the difference to be safely carried by the third tube. The third tube is set by ear for minimum hum.

How about dissimilar tubes?

For either side that has a small-bottle tube, set the small-bottle first to a safe value for itself. If the other side has a small-bottle tube, set that one by ear to cancel the small-bottle hum from the first side. Proceed with the other tubes. If it is just 2-tubes then set the small-bottle tube first by meter and the large-bottle tube by ear for minimum hum.]]>
You see many amps that are fixed-biased built with a single bias pot for the output stage regardless of how many tubes are present. This is definitely poor design.

In TUT3 and TUT5, we see 2-tube output stages that have two bias pots, one for the 'push' side and one for the 'pull' side. In the 4-tube output stages of the Plexi, 800, Swede, Standard, Custom Special, and the 6-tube SVT, there are individual bias pots for each tube. It is easy to understand how to set the single pot, but how do we adjust multiple pots?

First, when setting bias in an amp, turn the Master Volume or Volume to zero so the readings will be just DC and not noise or signals. In a Power Scaled amp, also set Power Scale for maximum voltage. We set all the bias pots to minimum so that each tube ideally is 'off'.

For a 2-tube output stage with separate bias pots for 'push' and 'pull, which is essentially the same as individual bias pots per tube in this situation, we set the first pot by meter so we know the tube is biased within a safe range, then set the second pot by ear for lowest hum. If you measure the currents they will be slightly different. This is exactly what we expect considering how nearly all output transformers are wound. There are typically more turns of wire on one side of the primary centre-tap than the other but the manufacturer is trying to have the same inductance per side. Even with interleaving and other techniques, most OT primaries are "unbalanced" on a DC resistance basis.

With more tubes, which is generally equal numbers per circuit half, we have to alternate using a metered setting, then an ear setting. Say the first tube is on the 'push' side'. We set that tube by meter to a safe idle current. We go to the first tube on the 'pull' side and adjust it for minimum hum. At this point, only these two tubes are influencing how the OT behaves, whether there is hum or not.

Now we go to the second tube on the 'push' side and set it by meter. We may move the negative meter lead to do a "zero" between it and the first tube we set on this side. Then we set the second tube on the 'pull' side for minimum hum. If there are more tube pairs, we continue along on this procedure until all the tubes are adjusted.

What if there are an odd number of output tubes?

We have to make sure that both sides can be set to about the same net value of current. The side with only a single tube would be set first by meter. The first tube on the other side would also be set by meter to some value that allows the difference to be safely carried by the third tube. The third tube is set by ear for minimum hum.

How about dissimilar tubes?

For either side that has a small-bottle tube, set the small-bottle first to a safe value for itself. If the other side has a small-bottle tube, set that one by ear to cancel the small-bottle hum from the first side. Proceed with the other tubes. If it is just 2-tubes then set the small-bottle tube first by meter and the large-bottle tube by ear for minimum hum.]]> https://theultimatetone.com/Thread-Trans-amp Thu, 22 Jun 2023 18:03:41 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Trans-amp
In TUT3 The Ultimate Tone vol.3 we introduced the notion of "transconductance multiplication" output stages. Simply put, this is just the use of paralleled output tubes. The sonic difference between what most guitar amp hobbyists and players might refer to as a "50W" output stage and a "100W" output stage are explained using the tube characteristics. Most players encounter this sonic difference in the 50W and 100W models within the same brand, say as Marshall plexi or 800 amps, or as a Fender Pro versus a Twin.

"Transconductance" (gm) is a characteristic relating the input stimulus of an active gain element (BJT, jfet, triode) to the resulting output, where the input and output quantities are different "domains". For example, transconductance for a tube is the relationship between the voltage at the control grid and the current at the cathode (equals plate current). Voltage and current are two different domains. A BJT used in a gain stage often has the opposite condition, of a current at the base and an output voltage at the collector.

With a standard tube output stage, we can view the voltages on a scope or measure them with a meter, but we can see a signal voltage at the grid causing a much larger signal voltage at the plate. This voltage gain is the result of the transconductance of the tube and the resistance or impedance of the load. If we double the transconductance by paralleling similar tubes, the voltage gain doubles. We can add more tubes, and use dissimilar tubes, in which case we add all the individual gm values and multiply by the load to find the new voltage gain.

If we place a feedback resistor from the plate to grid, we lose voltage gain and alter the circuit performance. For one thing, we must usually insert a DC blocking cap in series with the feedback resistor in order to keep the idle condition the same. The direct resistive link would otherwise cause the bias to shift dramatically positive and melt down the tube and OT. In many examples of this circuit form, the feedback resistor is taken to the input side of the grid coupling cap. which is often the plate output of the driver or splitter. In any case, the local feedback around the output tube decreases its output impedance and distortion contribution. The reduced drive impedance into the OT helps improve the OT frequency response and the OT parasitics (leakage inductance, coil capacitances) become less problematic.

The DC blocking cap introduces a problem of its own in that another RC constant is added to the circuit, which potentially decreases stability at low frequencies. The next post will show a very simple SE PA that overcomes the problems and illustrates the basic principles of the "trans-tube" amplifier.]]>
In TUT3 The Ultimate Tone vol.3 we introduced the notion of "transconductance multiplication" output stages. Simply put, this is just the use of paralleled output tubes. The sonic difference between what most guitar amp hobbyists and players might refer to as a "50W" output stage and a "100W" output stage are explained using the tube characteristics. Most players encounter this sonic difference in the 50W and 100W models within the same brand, say as Marshall plexi or 800 amps, or as a Fender Pro versus a Twin.

"Transconductance" (gm) is a characteristic relating the input stimulus of an active gain element (BJT, jfet, triode) to the resulting output, where the input and output quantities are different "domains". For example, transconductance for a tube is the relationship between the voltage at the control grid and the current at the cathode (equals plate current). Voltage and current are two different domains. A BJT used in a gain stage often has the opposite condition, of a current at the base and an output voltage at the collector.

With a standard tube output stage, we can view the voltages on a scope or measure them with a meter, but we can see a signal voltage at the grid causing a much larger signal voltage at the plate. This voltage gain is the result of the transconductance of the tube and the resistance or impedance of the load. If we double the transconductance by paralleling similar tubes, the voltage gain doubles. We can add more tubes, and use dissimilar tubes, in which case we add all the individual gm values and multiply by the load to find the new voltage gain.

If we place a feedback resistor from the plate to grid, we lose voltage gain and alter the circuit performance. For one thing, we must usually insert a DC blocking cap in series with the feedback resistor in order to keep the idle condition the same. The direct resistive link would otherwise cause the bias to shift dramatically positive and melt down the tube and OT. In many examples of this circuit form, the feedback resistor is taken to the input side of the grid coupling cap. which is often the plate output of the driver or splitter. In any case, the local feedback around the output tube decreases its output impedance and distortion contribution. The reduced drive impedance into the OT helps improve the OT frequency response and the OT parasitics (leakage inductance, coil capacitances) become less problematic.

The DC blocking cap introduces a problem of its own in that another RC constant is added to the circuit, which potentially decreases stability at low frequencies. The next post will show a very simple SE PA that overcomes the problems and illustrates the basic principles of the "trans-tube" amplifier.]]> https://theultimatetone.com/Thread-Help-Identifying-Tube-Amp-Chassis Mon, 12 Jun 2023 01:14:45 +0000 dtbradio]]> https://theultimatetone.com/Thread-Help-Identifying-Tube-Amp-Chassis
I picked up this amp chassis for $20 today. No tubes or documentation included. The only markings are a stamped circle with "32E" inside. The over-all circuit appears to be push-pull using some kind of octal power tubes like 6v6, 6l6, etc (pin 1's unused, pin 3's [plate] tied to one side of opt primary, pins 2 and 7 heater). Here's where the odd comes in. The screen grid pins (pin 4's) are tied to the cathodes (pin 8's), which are tied to what appears to be a filter capacitor positive voltage node. Both grids are tied together, and go to the 9-pin octal style socket. The power supply uses solid-state rectifiers, and appears normal enough. There appears to be both main B+ and a single-diode negative voltage source, and there is what looks like a bias POT with no guts which is NOT tied to the power tube grids. I can't figure out what exactly its purpose was. There doesn't appear to be any specific grounding scheme, as some grounds go to chassis, while others tie together in a semi-star style. My gut instinct is to just gut it and start a build from scratch. However, if this is a bit of a rare amp configuration worth a possible resurrection, I'd consider that undertaking.Any help/input appreciated!

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I picked up this amp chassis for $20 today. No tubes or documentation included. The only markings are a stamped circle with "32E" inside. The over-all circuit appears to be push-pull using some kind of octal power tubes like 6v6, 6l6, etc (pin 1's unused, pin 3's [plate] tied to one side of opt primary, pins 2 and 7 heater). Here's where the odd comes in. The screen grid pins (pin 4's) are tied to the cathodes (pin 8's), which are tied to what appears to be a filter capacitor positive voltage node. Both grids are tied together, and go to the 9-pin octal style socket. The power supply uses solid-state rectifiers, and appears normal enough. There appears to be both main B+ and a single-diode negative voltage source, and there is what looks like a bias POT with no guts which is NOT tied to the power tube grids. I can't figure out what exactly its purpose was. There doesn't appear to be any specific grounding scheme, as some grounds go to chassis, while others tie together in a semi-star style. My gut instinct is to just gut it and start a build from scratch. However, if this is a bit of a rare amp configuration worth a possible resurrection, I'd consider that undertaking.Any help/input appreciated!

  20230611_201628.jpg (Size: 298.96 KB / Downloads: 20)

  20230611_201633.jpg (Size: 327.46 KB / Downloads: 22)

  20230611_201652.jpg (Size: 277.92 KB / Downloads: 21)

  20230611_202355.jpg (Size: 558.9 KB / Downloads: 20)

  Image1.jpg (Size: 108.71 KB / Downloads: 21) ]]> https://theultimatetone.com/Thread-SE-power-amps-that-are-not Thu, 13 Apr 2023 21:30:32 +0000 K O'Connor]]> https://theultimatetone.com/Thread-SE-power-amps-that-are-not
A lot of guys build what they think are single-ended (SE, not Swedish) that are not. These are solid-state designs and it is is generally hifi builds that are in question.

BJT = bipolar junction transistor, the usual kind generically called "transistors"

MOSFET = metal-oxide semiconductor field-effect transistor

Both semiconductors come in two sexes as P or N; for BJTs this is PNP and NPN.

A single-ended amp gets its name from tube circuits and the connection between the output transformer (OT) and the power tube. One end of the OT primary winding  is tied to B+ while the other end is driven by the tube plate. The tube idles at a current that is a little bit higher than the peak signal current. During the positive half of the signal, the tube conducts more current, pulling the end of the winding towards ground. On the negative half-cycle, the tube conducts less current and the energy stored in the OT causes the plate voltage to swing well above B+. The current through the tube swings from approximately Ipk, to (2 x Ipk) to almost zero, averaging at Ipk.

In the tube circuit, the OT is driven at one end, thus "single-ended". A push-pull circuit uses a centre-tapped primary OT with a tube driving each end.

Where the OT is the usual means to match the high-voltage environment of the tube to the low-voltage requirement of the speaker, the tube can be choke-loaded and capacitively-coupled to the speaker provided the power output is low AND the speaker impedance is high. This is only done with headphones. A variation on this theme is to use a choke load capacitively-coupled to an OT that carries no idle current for the tube - the choke does that. In these latter cases, the choke acts just as the SE OT does except it does not have a secondary to match the speaker load. The variation uses a separate OT for that. because this OT does not have to be gapped to withstand the DC idle current, its performance can be greatly improved and its size reduced.

We can configure solid-state SE exactly like the tube circuits above, with a conventional but lower-impedance OT, with choke loading, and with the choke - cap - OT. But...

Part of the advantage of solid-state is that semiconductors can pass the heavy speaker currents directly. So, the first attempt is to replace the load with a current source. The current source is an active circuit that does as its name suggests, it supplies a constant current regardless of the voltage across itself. Technically, the circuit can be a current "source" or a current "sink" depending on which side of the signal-driving element it is on.

An SE BJT amp output stage looks pretty much like any push-pull BJT output stage EXCEPT that the signal is only applied to half of it. Suppose we have an NPN BJT for the upper element, C tied to V+, E tied to the load and B tied to the drive circuit which we do not care about here. A second NPN has its E tied to V- and C tied to the load, which is also the emitter of the signal transistor. The lower BJT is configured one of many ways to provide a steady current. Feedback MUST be taken from the junction of the two BJTs to control the DC voltage at the very least even if we use a coupling cap to the speaker.

Suppose the idle current is 1A and the load is 8R. Suppose the supply rails are 10V each. With no signal, each BJT has 10V across it and 1A through it, and thus each device dissipates 10W at idle.

For the positive signal half, the upper BJT is turned 'on' more until it conducts 2A peak. At that point, the upper transistor dissipates 2A x 2V = 4W.  Meanwhile, the lower BJT still has 1A but now has 18V across it and 18W of heat. The speaker is conducting the difference of current, at 1A and 8V with 8W peak power.

For the negative half-cycle, the upper transistor is driven 'off' until it is at zero current with 18V across it. The current source still has 1A through itself but now at 2V, so 2W. The 1A from the current source flows through the speaker producing the 1A 8W negative peak of the signal.

It is clear that the signal-driven BJT only contributes to one half of the signal while the current source makes up the other half. This is really an aesthetic SE that is actually push-pull. It should be clear that the current source is much more relevant to performance than most people believe and that the current source should be a "good" one rather than a simple buffered voltage reference. In audio, current sources should always be the feedback type, comprised of two same-sex devices, as two BJTs or as a MOSFET pass element and BJT control element.

Have fun]]>
A lot of guys build what they think are single-ended (SE, not Swedish) that are not. These are solid-state designs and it is is generally hifi builds that are in question.

BJT = bipolar junction transistor, the usual kind generically called "transistors"

MOSFET = metal-oxide semiconductor field-effect transistor

Both semiconductors come in two sexes as P or N; for BJTs this is PNP and NPN.

A single-ended amp gets its name from tube circuits and the connection between the output transformer (OT) and the power tube. One end of the OT primary winding  is tied to B+ while the other end is driven by the tube plate. The tube idles at a current that is a little bit higher than the peak signal current. During the positive half of the signal, the tube conducts more current, pulling the end of the winding towards ground. On the negative half-cycle, the tube conducts less current and the energy stored in the OT causes the plate voltage to swing well above B+. The current through the tube swings from approximately Ipk, to (2 x Ipk) to almost zero, averaging at Ipk.

In the tube circuit, the OT is driven at one end, thus "single-ended". A push-pull circuit uses a centre-tapped primary OT with a tube driving each end.

Where the OT is the usual means to match the high-voltage environment of the tube to the low-voltage requirement of the speaker, the tube can be choke-loaded and capacitively-coupled to the speaker provided the power output is low AND the speaker impedance is high. This is only done with headphones. A variation on this theme is to use a choke load capacitively-coupled to an OT that carries no idle current for the tube - the choke does that. In these latter cases, the choke acts just as the SE OT does except it does not have a secondary to match the speaker load. The variation uses a separate OT for that. because this OT does not have to be gapped to withstand the DC idle current, its performance can be greatly improved and its size reduced.

We can configure solid-state SE exactly like the tube circuits above, with a conventional but lower-impedance OT, with choke loading, and with the choke - cap - OT. But...

Part of the advantage of solid-state is that semiconductors can pass the heavy speaker currents directly. So, the first attempt is to replace the load with a current source. The current source is an active circuit that does as its name suggests, it supplies a constant current regardless of the voltage across itself. Technically, the circuit can be a current "source" or a current "sink" depending on which side of the signal-driving element it is on.

An SE BJT amp output stage looks pretty much like any push-pull BJT output stage EXCEPT that the signal is only applied to half of it. Suppose we have an NPN BJT for the upper element, C tied to V+, E tied to the load and B tied to the drive circuit which we do not care about here. A second NPN has its E tied to V- and C tied to the load, which is also the emitter of the signal transistor. The lower BJT is configured one of many ways to provide a steady current. Feedback MUST be taken from the junction of the two BJTs to control the DC voltage at the very least even if we use a coupling cap to the speaker.

Suppose the idle current is 1A and the load is 8R. Suppose the supply rails are 10V each. With no signal, each BJT has 10V across it and 1A through it, and thus each device dissipates 10W at idle.

For the positive signal half, the upper BJT is turned 'on' more until it conducts 2A peak. At that point, the upper transistor dissipates 2A x 2V = 4W.  Meanwhile, the lower BJT still has 1A but now has 18V across it and 18W of heat. The speaker is conducting the difference of current, at 1A and 8V with 8W peak power.

For the negative half-cycle, the upper transistor is driven 'off' until it is at zero current with 18V across it. The current source still has 1A through itself but now at 2V, so 2W. The 1A from the current source flows through the speaker producing the 1A 8W negative peak of the signal.

It is clear that the signal-driven BJT only contributes to one half of the signal while the current source makes up the other half. This is really an aesthetic SE that is actually push-pull. It should be clear that the current source is much more relevant to performance than most people believe and that the current source should be a "good" one rather than a simple buffered voltage reference. In audio, current sources should always be the feedback type, comprised of two same-sex devices, as two BJTs or as a MOSFET pass element and BJT control element.

Have fun]]> https://theultimatetone.com/Thread-Bias-class Thu, 13 Apr 2023 16:42:07 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Bias-class
The idle condition of a power amplifier output stage is referred to as its "operating class". It is important to note that the bias condition is signal-dependent and has nothing to do with how the output stage devices are controlled. Again, bias condition is a universally-applicable concept.

Class-A: All of the output devices contribute to the signal over the full audio cycle (360-degrees)

Class-A2: Tubes only, the "2" indicates that grid conduction occurs in the output devices. This is simply class-AB below with a low-impedance drive  circuit and very high idle current.

Limiting Class-A: The peak signal current never exceeds the total idle current. This term was common in tube days, but still applies universally although it may be considered redundant to the class-A definition above.

Sliding Class-A: Solid-state, a method of varying the idle condition so that neither half of the circuit ever turns off even though transference of signal control may occur.

Class-B: In push-pull, each half of the output stage contributes exactly half the output signal (180-degrees)

Class-B2: Tubes only, the "2" indicates grid conduction made possible by a low-impedance drive circuit.

Class-AB: In push-pull, each half of the output stage contributes to slightly more than half of the signal output. Most "class-B" output stages are actually biased this way, with a slight overlap of conduction between circuit halves.

Class-C: The output device conducts for only half the signal cycle (180-degrees) with a tuned load providing the remainder. Used in RF.

Class-D: Solid-state only, a method of using a nonlinear output stage where the devices switch 'on' and 'off' in a pulse-width-modulated (PWM) format, and the output signal is integrated using LC filters. This approach is highly load noncompliant inasmuch as the load should be of fixed value versus frequency (resistive rather than inductive or capacitive). Class-D allows cold operation of the output devices but is only suitable for driving subwoofers in audio.

Class-E: Solid-state, where parallel-driven output stages supported by different supply values contribute to the signal. The low-voltage stage amplifies the signal up to its limits with the high-voltage stage contributing higher amplitude signals as required. The low-voltage output stage can be biased class-A,-B or -AB.

Class-G: Solid-state, a multi-tier output stage uses multiple supply voltages,switching between them as the signal requires. The transition shifts the burden of output heat from the low-tier device to the next higher-tier device. Overall dissipation is generally reduced by the number of tiers.

Class-H: Solid-state, a multi-tier output stage supported by multiple supply voltages, switching between them as the signal requires. The supply switches turn 'on' hard and the burden of heat dissipation remains with the lowest-tier devices. Overall dissipation is reduced by the number of tiers.

Class-I: Similar to sliding class-A.

Class-T: A variation of class-D, with all of the same inherent issues.

Class-Z: A method of power transfer using saturable coils "steered" by tubes with output stage power provided by a switching supply. designed by Lundahl (SE) in the 1960s, then revised and patented by Berning (USA) in the 1990s.]]>
The idle condition of a power amplifier output stage is referred to as its "operating class". It is important to note that the bias condition is signal-dependent and has nothing to do with how the output stage devices are controlled. Again, bias condition is a universally-applicable concept.

Class-A: All of the output devices contribute to the signal over the full audio cycle (360-degrees)

Class-A2: Tubes only, the "2" indicates that grid conduction occurs in the output devices. This is simply class-AB below with a low-impedance drive  circuit and very high idle current.

Limiting Class-A: The peak signal current never exceeds the total idle current. This term was common in tube days, but still applies universally although it may be considered redundant to the class-A definition above.

Sliding Class-A: Solid-state, a method of varying the idle condition so that neither half of the circuit ever turns off even though transference of signal control may occur.

Class-B: In push-pull, each half of the output stage contributes exactly half the output signal (180-degrees)

Class-B2: Tubes only, the "2" indicates grid conduction made possible by a low-impedance drive circuit.

Class-AB: In push-pull, each half of the output stage contributes to slightly more than half of the signal output. Most "class-B" output stages are actually biased this way, with a slight overlap of conduction between circuit halves.

Class-C: The output device conducts for only half the signal cycle (180-degrees) with a tuned load providing the remainder. Used in RF.

Class-D: Solid-state only, a method of using a nonlinear output stage where the devices switch 'on' and 'off' in a pulse-width-modulated (PWM) format, and the output signal is integrated using LC filters. This approach is highly load noncompliant inasmuch as the load should be of fixed value versus frequency (resistive rather than inductive or capacitive). Class-D allows cold operation of the output devices but is only suitable for driving subwoofers in audio.

Class-E: Solid-state, where parallel-driven output stages supported by different supply values contribute to the signal. The low-voltage stage amplifies the signal up to its limits with the high-voltage stage contributing higher amplitude signals as required. The low-voltage output stage can be biased class-A,-B or -AB.

Class-G: Solid-state, a multi-tier output stage uses multiple supply voltages,switching between them as the signal requires. The transition shifts the burden of output heat from the low-tier device to the next higher-tier device. Overall dissipation is generally reduced by the number of tiers.

Class-H: Solid-state, a multi-tier output stage supported by multiple supply voltages, switching between them as the signal requires. The supply switches turn 'on' hard and the burden of heat dissipation remains with the lowest-tier devices. Overall dissipation is reduced by the number of tiers.

Class-I: Similar to sliding class-A.

Class-T: A variation of class-D, with all of the same inherent issues.

Class-Z: A method of power transfer using saturable coils "steered" by tubes with output stage power provided by a switching supply. designed by Lundahl (SE) in the 1960s, then revised and patented by Berning (USA) in the 1990s.]]> https://theultimatetone.com/Thread-Bias-controls-for-fixed-bias-tube-amps Thu, 13 Apr 2023 16:07:49 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Bias-controls-for-fixed-bias-tube-amps
Readers of our books will know that we follow a universally-applicable definition for bias conditions:

High bias = high heat caused by high current

Low-bias = low heat caused by low heat

A "fixed-bias" tube output stage can be confusing to some hobbyists and builders inasmuch as the bias should be adjustable. The "fixed" term is with respect to whether the idle point shifts with signal present. The first attempt to control this facet of performance was with cathode-biased amplifiers, where a very high-value bypass cap is added across the bias resistor. The typical 22uF to 50uF seen in guitar amps barely has an effect as anyone who measures Vk will know.

Cathode bias tends to be inefficient and has unnecessarily high heat at idle for higher-power applications. The fix is to ground the cathodes and apply negative voltage to the control grids. Remember, tubes are cathode-centric and to control them the grid voltage must be negative with respect to the cathode NOT negative with respect to ground - the tube does not know where ground is but it knows where its cathode is

The negative grid supply is referred to as the "fixed bias voltage". The circuit is equivalent to a cathode-biased circuit with an infinite Ck value, so this new bias method achieves the goal of "fixing" the idle condition regardless of the signal amplitude.

If Vg = Vk there is no control over idle current and the tube draws as much current from the supply as the circuit allows, destroying itself and the output transformer in the process. obviously, we should do what we can to keep this from happening. For self-bias, Rk is the limiter, but in fixed-bias, we have to restrict how close to ground Vg can get. This is achieved very simply by placing a "range resistor" in series with the bias adjustment pot-X end. This resistor is often left out in generic "concept" or "application" drawings, as the writer wants to make the drawing clear, or they are having the quite common issue that drawing software is not as easy to use as it should be. The result is that uninformed readers will take this simplified drawing as the recommended approach and build circuits lacking bias-restricting resistors.

Ironically, many people who should know better, present their information using the simplified circuit AND their own products use it too, making it possible for a user of either the product or the information to damage the tubes or expensive OT.

Some designers or builders believe that if they use a multi-turn trim pot for bias control, that there should never be such a situation as described above. However, this assumes that the person doing the biasing knows enough to preset the grid voltage as negatively as it can be right at the tube socket. Most people do not think of this and need to be directed.

Following our bias condition definition, we also approach the wiring of bias set-pots in a universal manner:

Fully-clockwise = hot = high current

Fully -counter-clockwise = cold = low current

In a tube amp, the raw bias voltage should be applied to pot-0 end (CCW) and the range resistor to ground ties to pot-X (CW). This gives the same response as a Level or EQ control, where clock-wise equals "more", which in this case, means "more current".

The range resistor value depends on the bias pot value and whether there are multiple bias pots in parallel for individual tube control or for control of the two halves of the push-pull circuit. Generally, the range resistor is about one-quarter the pot value, or one-quarter the net parallel pot value.

Ideally, there is a safety resistor across each bias pot from pot-0 to the wiper. This resistor is usually ten-times the pot value and comes into play if the wiper opens providing an alternate path for the bias voltage.

Obsolete texts will use a grid-voltage bias reference and thus present an incorrect notion of bias. Grid voltage is the most important voltage in a tube amp BUT its value is unimportant except that it should have sufficient range to properly control every tube sample.

All of the above info and much more can be found in the TUT volumes (The Ultimate Tone book series)

As TUTs show, single-ended amps can be fixed-biased, too.

Have fun]]>
Readers of our books will know that we follow a universally-applicable definition for bias conditions:

High bias = high heat caused by high current

Low-bias = low heat caused by low heat

A "fixed-bias" tube output stage can be confusing to some hobbyists and builders inasmuch as the bias should be adjustable. The "fixed" term is with respect to whether the idle point shifts with signal present. The first attempt to control this facet of performance was with cathode-biased amplifiers, where a very high-value bypass cap is added across the bias resistor. The typical 22uF to 50uF seen in guitar amps barely has an effect as anyone who measures Vk will know.

Cathode bias tends to be inefficient and has unnecessarily high heat at idle for higher-power applications. The fix is to ground the cathodes and apply negative voltage to the control grids. Remember, tubes are cathode-centric and to control them the grid voltage must be negative with respect to the cathode NOT negative with respect to ground - the tube does not know where ground is but it knows where its cathode is

The negative grid supply is referred to as the "fixed bias voltage". The circuit is equivalent to a cathode-biased circuit with an infinite Ck value, so this new bias method achieves the goal of "fixing" the idle condition regardless of the signal amplitude.

If Vg = Vk there is no control over idle current and the tube draws as much current from the supply as the circuit allows, destroying itself and the output transformer in the process. obviously, we should do what we can to keep this from happening. For self-bias, Rk is the limiter, but in fixed-bias, we have to restrict how close to ground Vg can get. This is achieved very simply by placing a "range resistor" in series with the bias adjustment pot-X end. This resistor is often left out in generic "concept" or "application" drawings, as the writer wants to make the drawing clear, or they are having the quite common issue that drawing software is not as easy to use as it should be. The result is that uninformed readers will take this simplified drawing as the recommended approach and build circuits lacking bias-restricting resistors.

Ironically, many people who should know better, present their information using the simplified circuit AND their own products use it too, making it possible for a user of either the product or the information to damage the tubes or expensive OT.

Some designers or builders believe that if they use a multi-turn trim pot for bias control, that there should never be such a situation as described above. However, this assumes that the person doing the biasing knows enough to preset the grid voltage as negatively as it can be right at the tube socket. Most people do not think of this and need to be directed.

Following our bias condition definition, we also approach the wiring of bias set-pots in a universal manner:

Fully-clockwise = hot = high current

Fully -counter-clockwise = cold = low current

In a tube amp, the raw bias voltage should be applied to pot-0 end (CCW) and the range resistor to ground ties to pot-X (CW). This gives the same response as a Level or EQ control, where clock-wise equals "more", which in this case, means "more current".

The range resistor value depends on the bias pot value and whether there are multiple bias pots in parallel for individual tube control or for control of the two halves of the push-pull circuit. Generally, the range resistor is about one-quarter the pot value, or one-quarter the net parallel pot value.

Ideally, there is a safety resistor across each bias pot from pot-0 to the wiper. This resistor is usually ten-times the pot value and comes into play if the wiper opens providing an alternate path for the bias voltage.

Obsolete texts will use a grid-voltage bias reference and thus present an incorrect notion of bias. Grid voltage is the most important voltage in a tube amp BUT its value is unimportant except that it should have sufficient range to properly control every tube sample.

All of the above info and much more can be found in the TUT volumes (The Ultimate Tone book series)

As TUTs show, single-ended amps can be fixed-biased, too.

Have fun]]>