https://theultimatetone.com/ Sun, 26 Jul 2026 09:48:01 +0000 MyBB https://theultimatetone.com/Thread-PT-Filament-current-test Mon, 29 Jun 2026 14:13:41 +0000 notabot]]> https://theultimatetone.com/Thread-PT-Filament-current-test https://theultimatetone.com/Thread-Rectification-basics Wed, 26 Jun 2024 14:42:35 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Rectification-basics
Rectification is the process of converting AC into DC.

AC by definition is "alternating current", which means that for half the cycle current moves in one direction and then for the second half of the cycle the current reverses direction.

DC is "direct current", which moves only in one direction, as with a battery power source.

Transformers must have alternating or at least pulsing current. Mains AC is a nice clean sine wave as generated, although noise may be added along the way, but we will consider the ideal for now. A linear supply uses the mains frequency AC at 50Hz or 60Hz. In a switching supply, the transformer uses chopped DC applied either to one end of the PT winding or to both ends out of phase, at a high frequency which allows the PT to be smaller than the low-frequency PT. In both cases, the PT isolates the mains from the audio circuit to be powered and can provide output voltages of widely differing values if required using multiple secondary windings. The rectification of the output has the same basic requirement and uses the same circuits regardless of the PT operating frequency.
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If you have a transformer winding with one end grounded and a single diode as a rectifier, that diode blocks half the wave and conducts for half the wave. During conduction, the diode is 'on' and has practically zero resistance, so can pass current from the PT to the load. This is obviously called "half-wave rectification". The diode can be oriented to produce a positive or negative output.
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If we configure four diodes into the standard "bridge" arrangement, the winding can no longer be tied to ground. The AC terminals of the bridge tie to the ends of the winding; the DC terminals of the bridge provide current to the load for both halves of the AC wave, so we have "full-wave rectification" AND we are using a "full bridge".

Inside the bridge, two diodes conduct at a time and you could look at this as being an "overlap" of four half-wave rectifiers. There are two negative-output half-wave rectifiers and two that are positive-output. Each end of the winding has a positive-output half-wave rectifier and a negative half-wave rectifier tied to it, with the positive DC outputs tied together and the negative DC outputs tied together. For each half of a cycle the positive-output rectifier at one end of the winding works with the negative-output rectifier at the opposite end of the winding, so two pairs of half-wave rectifiers working alternately to steer the current from the winding to produce full-wave DC.
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We can have a winding with a centretap and use a single diode connected to each end with their outputs joined and the CT grounded. For one half of the AC cycle, one diode conducts current to the load. For the other half cycle the second diode conducts current to the load. The load receives current over the full AC cycle, so we have full-wave rectification again BUT we have a "half-bridge rectifier".
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In the case of a tube amp that requires a high-voltage positive plate supply and a moderate-voltage negative bias supply, we can add a half-wave rectifier to one end of the CTed winding with its diode reversed. This produces a negative DC voltage output BUT uses only half the AC wave. TUT readers know that this is far from ideal, and we can add another half-wave reversed-diode from the other end of the winding and join the two negative outputs to achieve a bias supply derived from full-wave AC. The bias supply is now more reliable and easier to filter.

We now have a CTed winding with a centretap and a full-bridge rectifier producing plus and minus DC rails. This is the most common type of supply for solid-state amplifiers.
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With any of these connections, the DC output is neither negative nor positive until we decide to ground one end or the other, as RSG explains.]]>
Rectification is the process of converting AC into DC.

AC by definition is "alternating current", which means that for half the cycle current moves in one direction and then for the second half of the cycle the current reverses direction.

DC is "direct current", which moves only in one direction, as with a battery power source.

Transformers must have alternating or at least pulsing current. Mains AC is a nice clean sine wave as generated, although noise may be added along the way, but we will consider the ideal for now. A linear supply uses the mains frequency AC at 50Hz or 60Hz. In a switching supply, the transformer uses chopped DC applied either to one end of the PT winding or to both ends out of phase, at a high frequency which allows the PT to be smaller than the low-frequency PT. In both cases, the PT isolates the mains from the audio circuit to be powered and can provide output voltages of widely differing values if required using multiple secondary windings. The rectification of the output has the same basic requirement and uses the same circuits regardless of the PT operating frequency.
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If you have a transformer winding with one end grounded and a single diode as a rectifier, that diode blocks half the wave and conducts for half the wave. During conduction, the diode is 'on' and has practically zero resistance, so can pass current from the PT to the load. This is obviously called "half-wave rectification". The diode can be oriented to produce a positive or negative output.
------
If we configure four diodes into the standard "bridge" arrangement, the winding can no longer be tied to ground. The AC terminals of the bridge tie to the ends of the winding; the DC terminals of the bridge provide current to the load for both halves of the AC wave, so we have "full-wave rectification" AND we are using a "full bridge".

Inside the bridge, two diodes conduct at a time and you could look at this as being an "overlap" of four half-wave rectifiers. There are two negative-output half-wave rectifiers and two that are positive-output. Each end of the winding has a positive-output half-wave rectifier and a negative half-wave rectifier tied to it, with the positive DC outputs tied together and the negative DC outputs tied together. For each half of a cycle the positive-output rectifier at one end of the winding works with the negative-output rectifier at the opposite end of the winding, so two pairs of half-wave rectifiers working alternately to steer the current from the winding to produce full-wave DC.
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We can have a winding with a centretap and use a single diode connected to each end with their outputs joined and the CT grounded. For one half of the AC cycle, one diode conducts current to the load. For the other half cycle the second diode conducts current to the load. The load receives current over the full AC cycle, so we have full-wave rectification again BUT we have a "half-bridge rectifier".
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In the case of a tube amp that requires a high-voltage positive plate supply and a moderate-voltage negative bias supply, we can add a half-wave rectifier to one end of the CTed winding with its diode reversed. This produces a negative DC voltage output BUT uses only half the AC wave. TUT readers know that this is far from ideal, and we can add another half-wave reversed-diode from the other end of the winding and join the two negative outputs to achieve a bias supply derived from full-wave AC. The bias supply is now more reliable and easier to filter.

We now have a CTed winding with a centretap and a full-bridge rectifier producing plus and minus DC rails. This is the most common type of supply for solid-state amplifiers.
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With any of these connections, the DC output is neither negative nor positive until we decide to ground one end or the other, as RSG explains.]]> https://theultimatetone.com/Thread-Reverse-PT-for-auxiliary-supply Tue, 25 Jun 2024 13:51:22 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Reverse-PT-for-auxiliary-supply
There are situations where we need an extra voltage that we cannot derive directly from the main power transformer, such as when adding a bias supply to a cathode-biased amplifier to give it switchable bias, or if we are adding a tube to a solid-state circuit, or if we need power for relays or opamps. For many of the tube-based applications we need a voltage step-up: and this may require wiring the new PT backwards. Since its secondary is tied to the secondary of the existing PT this is referred to as a"back-to-back" connection.

The reverse-wired auxiliary PT is usually low-VA which results in a voltage loss of twice the regulation rating. Tiny PTs are a compromise of design and even when used with forward connections, the loss is significant and certain adaptations are made to get useful throughput of energy.

In the case of a bias supply in a tube power amp, the aux-PT is powered from the AC heater voltage, which can be tied to ground, floating, or on a DC-stand-off. The new transformer provides galvanic isolation to its output, which can be rectified and filtered and used how we wish. For a bias supply, the mains windings are now the secondary and we build a DC power supply off of this winding and tie the positive end to ground. Most amps have 6Vac heater, so we use a PT rated for 120V:6V, or 120V+120V:6V. We want the output to be nominally 120V., so for countries outside of Canada,Japan and USA, you must use a PT with dual primaries wired in parallel. This will produce about 84V rather than 169V, which we discuss below.

In the case where the bias supply is feed a Power Scaling circuit, the 84V output from works for B+ up nearl 600V. For 600V and higher (to the 800V limit of SV1) one could wire the 120V windings in series to achieve 169V of raw bias. Add a heatsink to the bias regulator transistor.

The aux-PT may be for other uses, such as to support switching circuits, relays, opamps, or whatever else. In these cases, you would look at the voltage you require and the heater voltage, then select a PT with a similar voltage ratio. For example, if you want 24Vdc for use with 24V relays, this is about four-times the heater voltage. If there are not many relays and the prospective VA of the new PT is 6VA or less, then we need to double the ratio to eight. We won't find a 48V to 6V PT, but we can find 120V to 30V.

Similarly, if we have a solid-state circuit to wish to add a tube AND the stock supply can support the heater current, then we can use the aux-PT to generate the plate voltage for the tube. The reverse connection is more lossy and is very useful in some cases to achieve a more controlled output - see below. For the plate supply, the series-connected primaries of the aux-PT will produce about 200Vdc instead of the expected  320Vdc.This is at least into the accepted "tube range" of preamp tube B+.

The reduced output of the backwards PT is only an issue with small PTs, say 7VA or less. If you use a higher-VA part even though you do not need the extra power, the effective loss is reduced because the basic design of higher-VA PTs is less compromised than for low-VA devices. We can use this factor to our advantage. For example, say we use a  small PT with dual primaries and dual secondaries and all windings are rated for 115Vac.We will power this from the mains to demonstrate the difference between forward and backward use. We wish to make a tube plate supply, so the output windings are series connected. If we use a forward connection for the 6VA part, the output is way in excess of the 320Vdc expected, due to the high regulation figure of 30% for this size device. We get  over 400Vdc unloaded and may need a regulator to rein things in. If we reverse the PT, we get half as much voltage. Of course, because the regulation is so poor, loading the PT will quickly pull the raw output down.

The back-to-back connection requires that the main PT that is tied to the mains be able to handle the extra power needed by the new PT and circuit. Generally, the main PT will be used to generate the low voltages required, say for opamp supplies, and to support the tube heater. For example, say the main-PT output is 12-0-12Vac. We can rectify and filter this to produce about +/-16-19Vdc. Best to use it to support regulators making clean DC of lower value and allow margin for line drops. If we are using 12V heater tubes, then the heaters can be connected to +/-12Vdc or be tied directly to the 12Vac.

The reverse-wired PT must have its secondary windings tied to the AC secondary of the main-PT, since transformers only work with AC.

Ideally, the main-PT has each primary fused, and then each secondary fused. The aux-PT only needs one fuse between it and the main-PT and then one or two fuses on the output depending on how the transformer is used. For a bias or plate supply, a single fuse can protect the output. We have four fuses around the main-PT and two for the aux-PT, so six fuses in this example. If the main-PT is only generating a single DC output apart from supporting the aux-PT, then we have one less fuse. One of the main reasons to use the B2B connection is to avoid dealing with mains voltages and wiring.

Compare the example of B2B PTs to using two forward-wired PTs. In the latter, each PT must have both primaries fused, then their respective outputs fused as needed. In the case of +/-12Vdc plus a plate supply, we have four mains fuses plus three secondary fuses, so seven total. However, depending on how much power we need for the various supplies, the compromise of one more fuse may allow us to use devices that are available, and certainly we can use a main-PT that is slightly lower-VA.]]>
There are situations where we need an extra voltage that we cannot derive directly from the main power transformer, such as when adding a bias supply to a cathode-biased amplifier to give it switchable bias, or if we are adding a tube to a solid-state circuit, or if we need power for relays or opamps. For many of the tube-based applications we need a voltage step-up: and this may require wiring the new PT backwards. Since its secondary is tied to the secondary of the existing PT this is referred to as a"back-to-back" connection.

The reverse-wired auxiliary PT is usually low-VA which results in a voltage loss of twice the regulation rating. Tiny PTs are a compromise of design and even when used with forward connections, the loss is significant and certain adaptations are made to get useful throughput of energy.

In the case of a bias supply in a tube power amp, the aux-PT is powered from the AC heater voltage, which can be tied to ground, floating, or on a DC-stand-off. The new transformer provides galvanic isolation to its output, which can be rectified and filtered and used how we wish. For a bias supply, the mains windings are now the secondary and we build a DC power supply off of this winding and tie the positive end to ground. Most amps have 6Vac heater, so we use a PT rated for 120V:6V, or 120V+120V:6V. We want the output to be nominally 120V., so for countries outside of Canada,Japan and USA, you must use a PT with dual primaries wired in parallel. This will produce about 84V rather than 169V, which we discuss below.

In the case where the bias supply is feed a Power Scaling circuit, the 84V output from works for B+ up nearl 600V. For 600V and higher (to the 800V limit of SV1) one could wire the 120V windings in series to achieve 169V of raw bias. Add a heatsink to the bias regulator transistor.

The aux-PT may be for other uses, such as to support switching circuits, relays, opamps, or whatever else. In these cases, you would look at the voltage you require and the heater voltage, then select a PT with a similar voltage ratio. For example, if you want 24Vdc for use with 24V relays, this is about four-times the heater voltage. If there are not many relays and the prospective VA of the new PT is 6VA or less, then we need to double the ratio to eight. We won't find a 48V to 6V PT, but we can find 120V to 30V.

Similarly, if we have a solid-state circuit to wish to add a tube AND the stock supply can support the heater current, then we can use the aux-PT to generate the plate voltage for the tube. The reverse connection is more lossy and is very useful in some cases to achieve a more controlled output - see below. For the plate supply, the series-connected primaries of the aux-PT will produce about 200Vdc instead of the expected  320Vdc.This is at least into the accepted "tube range" of preamp tube B+.

The reduced output of the backwards PT is only an issue with small PTs, say 7VA or less. If you use a higher-VA part even though you do not need the extra power, the effective loss is reduced because the basic design of higher-VA PTs is less compromised than for low-VA devices. We can use this factor to our advantage. For example, say we use a  small PT with dual primaries and dual secondaries and all windings are rated for 115Vac.We will power this from the mains to demonstrate the difference between forward and backward use. We wish to make a tube plate supply, so the output windings are series connected. If we use a forward connection for the 6VA part, the output is way in excess of the 320Vdc expected, due to the high regulation figure of 30% for this size device. We get  over 400Vdc unloaded and may need a regulator to rein things in. If we reverse the PT, we get half as much voltage. Of course, because the regulation is so poor, loading the PT will quickly pull the raw output down.

The back-to-back connection requires that the main PT that is tied to the mains be able to handle the extra power needed by the new PT and circuit. Generally, the main PT will be used to generate the low voltages required, say for opamp supplies, and to support the tube heater. For example, say the main-PT output is 12-0-12Vac. We can rectify and filter this to produce about +/-16-19Vdc. Best to use it to support regulators making clean DC of lower value and allow margin for line drops. If we are using 12V heater tubes, then the heaters can be connected to +/-12Vdc or be tied directly to the 12Vac.

The reverse-wired PT must have its secondary windings tied to the AC secondary of the main-PT, since transformers only work with AC.

Ideally, the main-PT has each primary fused, and then each secondary fused. The aux-PT only needs one fuse between it and the main-PT and then one or two fuses on the output depending on how the transformer is used. For a bias or plate supply, a single fuse can protect the output. We have four fuses around the main-PT and two for the aux-PT, so six fuses in this example. If the main-PT is only generating a single DC output apart from supporting the aux-PT, then we have one less fuse. One of the main reasons to use the B2B connection is to avoid dealing with mains voltages and wiring.

Compare the example of B2B PTs to using two forward-wired PTs. In the latter, each PT must have both primaries fused, then their respective outputs fused as needed. In the case of +/-12Vdc plus a plate supply, we have four mains fuses plus three secondary fuses, so seven total. However, depending on how much power we need for the various supplies, the compromise of one more fuse may allow us to use devices that are available, and certainly we can use a main-PT that is slightly lower-VA.]]> https://theultimatetone.com/Thread-Integrated-bridge-used-for-plate-winding Tue, 25 Jun 2024 13:22:36 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Integrated-bridge-used-for-plate-winding
With a traditional CTed plate winding on a PT designed to support a tube amp, there are two options when using an integrated bridge rectifier.

The first option is basic and the AC terminals of the bridge merely tie to the ends of the winding. The bridge has to be rated for the full winding voltage. For example, if the winding is 300-0-300Vac, that is 600Vac total which produces a peak voltage of 848V. The rated voltage is at full load and the regulation rating for the PT will suggest what the unloaded voltage will be, where lower-VA devices have worse regulation and their unloaded voltage will be higher. Say it is 20%, then the unloaded peak is now just over 1kV - 1,060V - so the bridge must be rated for >1kV.

In this example, because the CT is tied to ground, the transformer produces +/-420Vdc (loaded; up to 470Vdc unloaded0, once filter caps are added, with the usual bleeder resistors to ground from each DC output. Of course, you are unlikely to need such a high negative voltage although you now have full-wave pulsating DC to generate a bias voltage from.

Further with this same example, if there is a bias tap on the winding, it will still look like its AC value, usually around 50Vac, and can be half-wave rectified for bias or other uses.

In a cathode-biased amp, the negative end of the bridge simply ties to ground through a bleeder resistor to protect the bridge. No cap.

The alternate bridge wiring would be for the bridge to span from the CT to one end of the winding. The free winding end is insulated and stored. The DC output of the bridge is handled in the usual way with a cap and bleeder resistor. In the case of a plate supply, the negative end of this supply ties to ground. Bias should be derived from a separate PT if needed, or the winding and bridge can be rewired as above.

With regard to an aux winding or PT generating bias: There is no worry about which voltage rises faster. None of the voltages used in typical guitar amps are high enough to cause cathode stripping, as TUTs state, so this is a non-worry. Even were B+ to rise ahead of C- the tubes are not warmed enough for current to flow. Usually the supplies rise at about the same rate despite their seemingly different impedances.

Have fun]]>
With a traditional CTed plate winding on a PT designed to support a tube amp, there are two options when using an integrated bridge rectifier.

The first option is basic and the AC terminals of the bridge merely tie to the ends of the winding. The bridge has to be rated for the full winding voltage. For example, if the winding is 300-0-300Vac, that is 600Vac total which produces a peak voltage of 848V. The rated voltage is at full load and the regulation rating for the PT will suggest what the unloaded voltage will be, where lower-VA devices have worse regulation and their unloaded voltage will be higher. Say it is 20%, then the unloaded peak is now just over 1kV - 1,060V - so the bridge must be rated for >1kV.

In this example, because the CT is tied to ground, the transformer produces +/-420Vdc (loaded; up to 470Vdc unloaded0, once filter caps are added, with the usual bleeder resistors to ground from each DC output. Of course, you are unlikely to need such a high negative voltage although you now have full-wave pulsating DC to generate a bias voltage from.

Further with this same example, if there is a bias tap on the winding, it will still look like its AC value, usually around 50Vac, and can be half-wave rectified for bias or other uses.

In a cathode-biased amp, the negative end of the bridge simply ties to ground through a bleeder resistor to protect the bridge. No cap.

The alternate bridge wiring would be for the bridge to span from the CT to one end of the winding. The free winding end is insulated and stored. The DC output of the bridge is handled in the usual way with a cap and bleeder resistor. In the case of a plate supply, the negative end of this supply ties to ground. Bias should be derived from a separate PT if needed, or the winding and bridge can be rewired as above.

With regard to an aux winding or PT generating bias: There is no worry about which voltage rises faster. None of the voltages used in typical guitar amps are high enough to cause cathode stripping, as TUTs state, so this is a non-worry. Even were B+ to rise ahead of C- the tubes are not warmed enough for current to flow. Usually the supplies rise at about the same rate despite their seemingly different impedances.

Have fun]]> https://theultimatetone.com/Thread-First-filter-cap-value-selection Mon, 27 Nov 2023 17:58:44 +0000 K O'Connor]]> https://theultimatetone.com/Thread-First-filter-cap-value-selection
If you browse through a lot of schematics for audio gear, you see certain common values for the main filter cap immediately after the rectifier. How much if this is by design? How much is just copied from similar equipment? How do you know what the "right" value is?

Let's use our ubiquitous 50W push-pull amp model with these specs:
output transformer: 4kaa
100pk requires 316Vpk at 316mApk
tube saturation voltage is 60V
minimum plate voltage at full load: 316Vpk + 60V = 376V, call it 380Vdc

The effective load is then 380V / 316mA = 1k2

Assuming the plate winding on the power transformer has the usual 20% regulation, unloaded Va is then about 1.25 x 380V = 475Vdc. This would be derived from 330Vac. At full load, the winding voltage has fallen by 20% to about 270Vac and its 380Vpk is the relevant value to use.

We always use full-wave rectification for ALL windings, so the ripple frequency is twice the mains frequency, making the ripple 100Hz or 120Hz depending on where we live.

The approximate ripple voltage from a rectifier can be calculated using the ripple frequency, the peak voltage from the rectifier, and the filter cap value. Say we use 100Hz ripple for 50Hz mains and a 100uF cap to begin with.

Vripple = (Vpk x Tau) / (C x R-load)
where Tau is the reciprocal of frequency >> 1 / 100Hz = 0.01s, or 10ms,
then

Vripple = (380Vpk x 10ms) / (100uF x 1k2)
          = 31V7ac

Truly, this is going to be marginal with respect to not having the output signal being modulated by AC ripple.

If we double the filter cap value the ripple will be half - 16Vac - a straight linear relationship BUT with diminishing returns if we go too large. 1mf (1,000uF) would cut the load ripple to just over 3V and this is what hifi guys do in their tube amps, often followed by a massive choke into another 1mF cap Great if you do not want to add solid-state hum filters.

In the past, high-voltage capacitors of this value range were not available and series-parallel banks would be needed. The cost would be prohibitive and the bulk of components needed to be managed. Now that snap-mount caps are affordable and of tremendous quality, there is no excuse for under-filtered power supplies EXCEPT for equipment that is never expected to be driven to its maximum capability. A music amp used to be such a device, although nowadays there is so much compression used in recording that peak-versus-average levels have narrowed. In a guitar amp, under-filtering is historic and thus copied widely. This means that any amp driven to clipping is also likely to exhibit a lot of hum - witness Komet amps, Tranwrecks, some plexis, and all their clones.

A very reasonable observation is to use a PT with better regulation, or at the very least use one rated for a bit higher voltage so that the sagged voltage will not modulate the output even at clipping. In TUT5's Stentorian chapter, you may recall that polypropylene caps were used as supply filter. The caps were 47uF each and that with three caps the clipped signal output was hum-modulated but at four caps it was clean - 141uF changed to 188uF. This result would have been the same were the caps electrolytic.

When you go to lower voltages as found in solid-state amps, the cap values are necessarily much higher.

Say we have a 100W amp at 8-ohms, which requires a peak signal of 40V at 5A. The saturation voltage of BJTs and mosfets is quite low and we may only need a few volts. Let's say, this is 5V. We now have an effective load of 45V / 5A = 9-ohms. We will use the same 50Hz mains >> 100Hz ripple >> Tau = 10ms.

We are being thrifty and try 1,000uF first (1mF):
Vripple = (45V x 10ms) / (1mF x 9)
           =  450mV / 0.009
           = 50Vac ???

This is more ripple than AC voltage coming in to the rectifier, so this value must be ridiculous or we have a wonky equation? If we go to a more common value of 10mF, at least the ripple is more reasonable at 5Vac.

Since the solid-state amplifier is likely to use symmetric supplies, it is a common assumption that half the power comes from either side of the PT center-tap - indeed it does over the complete signal cycle, but each peak is carried fully by just one side at a time. The "averaging" assumption would have the effect of making the load seen by each side twice as high in value and potentially reduces the ripple voltage to 2V5. This assumption is often used in assessing the ripple current rating required for the filter caps.

In both the tube and solid-state examples, it is clear that it is highly beneficial to have an idle DC voltage quite a bit higher than the expected loaded value, not just to accommodate supply and mains regulation, but to accommodate insufficient ripple reduction.

In any case, the first filter capacitor is the main element in fighting ripple through the supply line, so skimping on its value is never a good idea.]]>
If you browse through a lot of schematics for audio gear, you see certain common values for the main filter cap immediately after the rectifier. How much if this is by design? How much is just copied from similar equipment? How do you know what the "right" value is?

Let's use our ubiquitous 50W push-pull amp model with these specs:
output transformer: 4kaa
100pk requires 316Vpk at 316mApk
tube saturation voltage is 60V
minimum plate voltage at full load: 316Vpk + 60V = 376V, call it 380Vdc

The effective load is then 380V / 316mA = 1k2

Assuming the plate winding on the power transformer has the usual 20% regulation, unloaded Va is then about 1.25 x 380V = 475Vdc. This would be derived from 330Vac. At full load, the winding voltage has fallen by 20% to about 270Vac and its 380Vpk is the relevant value to use.

We always use full-wave rectification for ALL windings, so the ripple frequency is twice the mains frequency, making the ripple 100Hz or 120Hz depending on where we live.

The approximate ripple voltage from a rectifier can be calculated using the ripple frequency, the peak voltage from the rectifier, and the filter cap value. Say we use 100Hz ripple for 50Hz mains and a 100uF cap to begin with.

Vripple = (Vpk x Tau) / (C x R-load)
where Tau is the reciprocal of frequency >> 1 / 100Hz = 0.01s, or 10ms,
then

Vripple = (380Vpk x 10ms) / (100uF x 1k2)
          = 31V7ac

Truly, this is going to be marginal with respect to not having the output signal being modulated by AC ripple.

If we double the filter cap value the ripple will be half - 16Vac - a straight linear relationship BUT with diminishing returns if we go too large. 1mf (1,000uF) would cut the load ripple to just over 3V and this is what hifi guys do in their tube amps, often followed by a massive choke into another 1mF cap Great if you do not want to add solid-state hum filters.

In the past, high-voltage capacitors of this value range were not available and series-parallel banks would be needed. The cost would be prohibitive and the bulk of components needed to be managed. Now that snap-mount caps are affordable and of tremendous quality, there is no excuse for under-filtered power supplies EXCEPT for equipment that is never expected to be driven to its maximum capability. A music amp used to be such a device, although nowadays there is so much compression used in recording that peak-versus-average levels have narrowed. In a guitar amp, under-filtering is historic and thus copied widely. This means that any amp driven to clipping is also likely to exhibit a lot of hum - witness Komet amps, Tranwrecks, some plexis, and all their clones.

A very reasonable observation is to use a PT with better regulation, or at the very least use one rated for a bit higher voltage so that the sagged voltage will not modulate the output even at clipping. In TUT5's Stentorian chapter, you may recall that polypropylene caps were used as supply filter. The caps were 47uF each and that with three caps the clipped signal output was hum-modulated but at four caps it was clean - 141uF changed to 188uF. This result would have been the same were the caps electrolytic.

When you go to lower voltages as found in solid-state amps, the cap values are necessarily much higher.

Say we have a 100W amp at 8-ohms, which requires a peak signal of 40V at 5A. The saturation voltage of BJTs and mosfets is quite low and we may only need a few volts. Let's say, this is 5V. We now have an effective load of 45V / 5A = 9-ohms. We will use the same 50Hz mains >> 100Hz ripple >> Tau = 10ms.

We are being thrifty and try 1,000uF first (1mF):
Vripple = (45V x 10ms) / (1mF x 9)
           =  450mV / 0.009
           = 50Vac ???

This is more ripple than AC voltage coming in to the rectifier, so this value must be ridiculous or we have a wonky equation? If we go to a more common value of 10mF, at least the ripple is more reasonable at 5Vac.

Since the solid-state amplifier is likely to use symmetric supplies, it is a common assumption that half the power comes from either side of the PT center-tap - indeed it does over the complete signal cycle, but each peak is carried fully by just one side at a time. The "averaging" assumption would have the effect of making the load seen by each side twice as high in value and potentially reduces the ripple voltage to 2V5. This assumption is often used in assessing the ripple current rating required for the filter caps.

In both the tube and solid-state examples, it is clear that it is highly beneficial to have an idle DC voltage quite a bit higher than the expected loaded value, not just to accommodate supply and mains regulation, but to accommodate insufficient ripple reduction.

In any case, the first filter capacitor is the main element in fighting ripple through the supply line, so skimping on its value is never a good idea.]]> https://theultimatetone.com/Thread-Question-about-Hybrid-Tube-SS-Bridge-Rectifier Fri, 18 Aug 2023 03:16:47 +0000 makinrose]]> https://theultimatetone.com/Thread-Question-about-Hybrid-Tube-SS-Bridge-Rectifier
Thank for the help!]]>
Thank for the help!]]> https://theultimatetone.com/Thread-Common-Mode-DC-Power-Filter Mon, 29 May 2023 04:24:46 +0000 mooreamps]]> https://theultimatetone.com/Thread-Common-Mode-DC-Power-Filter https://youtu.be/-HHfrJvo3aU]]> https://youtu.be/-HHfrJvo3aU]]> https://theultimatetone.com/Thread-Power-Entry-module-Twin-fused Wed, 03 May 2023 17:17:40 +0000 Champ81]]> https://theultimatetone.com/Thread-Power-Entry-module-Twin-fused
I was looking for a power entry module from digikey. There are fused PEMs which I was looking for to fit the chassis cutout.  There are ones with one fuse in the drawer. Now I see ones with "twin fuses" so the drawer in fact needs two. Any benefit or less benefit to using two vs one anyone know of?  There is definitely a reason to incorporate twin fuses of course which is why they manufacture them. But not sure why.

My thoughts process was that if there was a power surge any one of the terminals other than ground would blow one or both fuses. I heard they use these on medical equipment. But just curious if these would either be redundant, have more safety or less safety.

Here is one I see on digikey.

https://www.digikey.ca/en/products/detai...100/641415

Thanks.]]>
I was looking for a power entry module from digikey. There are fused PEMs which I was looking for to fit the chassis cutout.  There are ones with one fuse in the drawer. Now I see ones with "twin fuses" so the drawer in fact needs two. Any benefit or less benefit to using two vs one anyone know of?  There is definitely a reason to incorporate twin fuses of course which is why they manufacture them. But not sure why.

My thoughts process was that if there was a power surge any one of the terminals other than ground would blow one or both fuses. I heard they use these on medical equipment. But just curious if these would either be redundant, have more safety or less safety.

Here is one I see on digikey.

https://www.digikey.ca/en/products/detai...100/641415

Thanks.]]> https://theultimatetone.com/Thread-Voltage-Ratings-on-Coupling-Caps Fri, 31 Mar 2023 00:10:19 +0000 makinrose]]> https://theultimatetone.com/Thread-Voltage-Ratings-on-Coupling-Caps
Thanks for the help!]]>
Thanks for the help!]]> https://theultimatetone.com/Thread-Wire-Gauge-Size-For-Transformers Wed, 26 Oct 2022 14:15:52 +0000 ZeusMC]]> https://theultimatetone.com/Thread-Wire-Gauge-Size-For-Transformers
Thanks.]]>
Thanks.]]> https://theultimatetone.com/Thread-Wire-Size-For-Power-Output-Transformers Fri, 19 Aug 2022 08:50:29 +0000 ZeusMC]]> https://theultimatetone.com/Thread-Wire-Size-For-Power-Output-Transformers What size or sizes of wire would be used in making power and output transformers?
Thanks.]]> What size or sizes of wire would be used in making power and output transformers?
Thanks.]]> https://theultimatetone.com/Thread-Valve-Power-Transformer-testing-my-sanity Wed, 13 Apr 2022 20:17:52 +0000 fusepop]]> https://theultimatetone.com/Thread-Valve-Power-Transformer-testing-my-sanity
A friend brought over a vintage DIY amplifier that he purchased with a "bad" Power Transformer . He had a schematic as it was built from a magazine article and had previously heard it playing a year before.

Making  resistance checks, I unsoldering some wires and found that the primary of the PT read at 2.7 Ohms ! Wow ! that's close to a dead short ! Should be around 15-25 Ohms I'm thinking.   (Got to be the problem right ?)

Testing the HV secondaries  I found them to be 2.1 Ohms across both ends !(around 1 Ohm from each end to the CT) Should that not read somewhere around the 500-ish Ohms area?! (this is supposed to supply 400V each side of the CT !)

I found the filament windings and they measured 2.6 Ohms ?! Isn't that high ?!

Did this transformer implode? I have never seen this before.

 This is a Thorndarson 22R35, very popular but I have not been able to find winding resistance specs. Maybe I'm losing my marbles.

Wondering if this is not the original PT?

Any thoughts or advice greatly appreciated....

Rick

[font=Arial, Arial, Helvetica]You can also test a power transformer by measuring the resistance between leads. A healthy Hammond 270AX 240-0-240v power transformer measured: 14 ohms between primary leads, 223 ohms between the secondary center tap and HT wire 1 and 250 ohms from center tap to HT wire 2, 0.3 ohms between the 6.3v leads [/font]
[font=Arial, Arial, Helvetica]You can also test a power transformer by measuring the resistance between leads. A healthy Hammond 270AX 240-0-240v power transformer measured: 14 ohms between primary leads, 223 ohms between the secondary center tap and HT wire 1 and 250 ohms from center tap to HT wire 2, 0.3 ohms between the 6.3v leads [/font]]]>
A friend brought over a vintage DIY amplifier that he purchased with a "bad" Power Transformer . He had a schematic as it was built from a magazine article and had previously heard it playing a year before.

Making  resistance checks, I unsoldering some wires and found that the primary of the PT read at 2.7 Ohms ! Wow ! that's close to a dead short ! Should be around 15-25 Ohms I'm thinking.   (Got to be the problem right ?)

Testing the HV secondaries  I found them to be 2.1 Ohms across both ends !(around 1 Ohm from each end to the CT) Should that not read somewhere around the 500-ish Ohms area?! (this is supposed to supply 400V each side of the CT !)

I found the filament windings and they measured 2.6 Ohms ?! Isn't that high ?!

Did this transformer implode? I have never seen this before.

 This is a Thorndarson 22R35, very popular but I have not been able to find winding resistance specs. Maybe I'm losing my marbles.

Wondering if this is not the original PT?

Any thoughts or advice greatly appreciated....

Rick

[font=Arial, Arial, Helvetica]You can also test a power transformer by measuring the resistance between leads. A healthy Hammond 270AX 240-0-240v power transformer measured: 14 ohms between primary leads, 223 ohms between the secondary center tap and HT wire 1 and 250 ohms from center tap to HT wire 2, 0.3 ohms between the 6.3v leads [/font]
[font=Arial, Arial, Helvetica]You can also test a power transformer by measuring the resistance between leads. A healthy Hammond 270AX 240-0-240v power transformer measured: 14 ohms between primary leads, 223 ohms between the secondary center tap and HT wire 1 and 250 ohms from center tap to HT wire 2, 0.3 ohms between the 6.3v leads [/font]]]> https://theultimatetone.com/Thread-Hammond-290-series-Guitar-amp-PTs Mon, 10 Jan 2022 20:35:32 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Hammond-290-series-Guitar-amp-PTs
Hammond has reverse-engineered every useful guitar amp OT and PT and have a line of 1700-series output transformers and 290-series power transformers. These are all drop-in replacements, so if you have an amp with a dead OT or PT you can use these parts without having to drill any holes.

because the transformers are modelled after named amp models, hobbyists and builders can be assured of the tonal range possible assuming similar or identical circuits and tubes are used, too. In any case, if you want an amp with the power of the original Bassman, say, you can buy the matching OT and PT set. More adventurous builders may wish to mix and match these offerings with those of other brands. By this I mean, say, using the repro Bassman OT from Hammond but use a non-Hammond PT.

There is a critical point of interest regarding the PT specifications in the Hammond 290-series inasmuch as the plate winding uses an AC voltage but a DC current. This is the same as they did in the "Classic" transformer lines, and it will confuse most hobbyists and builders born in the sixties or later.

For example, the Bassman PT for a 50W amp has a 660Vct plate winding rated at 230mA. We can take this as 330Vac at 230mAdc given the half-bridge rectification.This results in about 470Vdc and 108W of power. The Bassman OT is 4kaa, representing a 1ka load. At 100Wpk audio power, there needs to be 316Vpk at 316mApk. Then we need about 60Vsat across the tube at full load and have a loaded supply of  about 380Vdc The sag is 90V, or almost 20%. If we look at the 316mA at 380V we have 120W, which is not too far off the 108W calculated previously, so we will say it is "close enough".

Most of Fender's amps evolved to use about the same supply voltages and a variation of OT impedances. Hammond has added the devices Fender should have switched to when tube rectifiers were dropped. That is, non-CT plate windings. This allows the use of a full bridge rectifier modules and requires a separate winding for bias. The latter helps to improve grounding, as TUT3 shows.

Power supply design defaults should always be:
full-wave rectification is always used
half-wave rectification is NEVER used especially for bias supplies
every supply has its own winding
dual primaries for world-wide use
electrostatic screen and/or core band for noise reduction

Have fun]]>
Hammond has reverse-engineered every useful guitar amp OT and PT and have a line of 1700-series output transformers and 290-series power transformers. These are all drop-in replacements, so if you have an amp with a dead OT or PT you can use these parts without having to drill any holes.

because the transformers are modelled after named amp models, hobbyists and builders can be assured of the tonal range possible assuming similar or identical circuits and tubes are used, too. In any case, if you want an amp with the power of the original Bassman, say, you can buy the matching OT and PT set. More adventurous builders may wish to mix and match these offerings with those of other brands. By this I mean, say, using the repro Bassman OT from Hammond but use a non-Hammond PT.

There is a critical point of interest regarding the PT specifications in the Hammond 290-series inasmuch as the plate winding uses an AC voltage but a DC current. This is the same as they did in the "Classic" transformer lines, and it will confuse most hobbyists and builders born in the sixties or later.

For example, the Bassman PT for a 50W amp has a 660Vct plate winding rated at 230mA. We can take this as 330Vac at 230mAdc given the half-bridge rectification.This results in about 470Vdc and 108W of power. The Bassman OT is 4kaa, representing a 1ka load. At 100Wpk audio power, there needs to be 316Vpk at 316mApk. Then we need about 60Vsat across the tube at full load and have a loaded supply of  about 380Vdc The sag is 90V, or almost 20%. If we look at the 316mA at 380V we have 120W, which is not too far off the 108W calculated previously, so we will say it is "close enough".

Most of Fender's amps evolved to use about the same supply voltages and a variation of OT impedances. Hammond has added the devices Fender should have switched to when tube rectifiers were dropped. That is, non-CT plate windings. This allows the use of a full bridge rectifier modules and requires a separate winding for bias. The latter helps to improve grounding, as TUT3 shows.

Power supply design defaults should always be:
full-wave rectification is always used
half-wave rectification is NEVER used especially for bias supplies
every supply has its own winding
dual primaries for world-wide use
electrostatic screen and/or core band for noise reduction

Have fun]]> https://theultimatetone.com/Thread-LP-Quiet-supplies-and-Bias-supply Wed, 27 Oct 2021 16:38:21 +0000 caveman]]> https://theultimatetone.com/Thread-LP-Quiet-supplies-and-Bias-supply
It seems that the London Power modules QS-HV and QS-LV do the trick but they are only described for "voltage up to" but don't really offer much information. Does that mean that these modules provide filtering and noise reduction only based on the input voltage or is it possible to control the voltage drop?

The RBX bias supply states what it does, provide bias voltage. It does not say how high the voltage output is. It refers to TUT 2 but I only have TUT1, 3 & 4.

Can anyone clarify this for me?

Thanks.]]>
It seems that the London Power modules QS-HV and QS-LV do the trick but they are only described for "voltage up to" but don't really offer much information. Does that mean that these modules provide filtering and noise reduction only based on the input voltage or is it possible to control the voltage drop?

The RBX bias supply states what it does, provide bias voltage. It does not say how high the voltage output is. It refers to TUT 2 but I only have TUT1, 3 & 4.

Can anyone clarify this for me?

Thanks.]]> https://theultimatetone.com/Thread-High-voltage-Super-Reverb Fri, 22 Oct 2021 08:59:42 +0000 Tomislaw]]> https://theultimatetone.com/Thread-High-voltage-Super-Reverb
I've got an interesting case of excess voltage in power supply on the bench right now - it's my pal's mistreated 1972 Super Reverb AA270. It's got a whole bunch of problems but the most interesting one is that it the voltage is way too high. When I first turned it on, the Chinese 6L6WGS were dissipating 26 watts with 540V on the plates! I had to modify the bias supply in the first place to get enough negative voltage to stop them from everheating, but they should probably be replaced with proper 30W 6L6GC's as well. Rectifier tube was GZ34 even though the tube chart calls for 5U4GB. I put one in and we are now at 500V which is probably still too much for those cheap tubes. A correct rectifer does help the voltage problem a bit, but there's 7,2V AC on heaters under full load and the voltage selector is already in the 240V position. I assume this gives away that the PT could have developed a small short in the primary winding and needs to be replaced. The owner is hesitant, though, as he's worried that the unique - as he say - sound of his amp could change.

Are silverface Fenders notorious for the voltage creep up problems? I know the network mains voltage is now higher than 50 years ago, but in case of this amp the difference is too big anyway, and it's not even running on the 230V selector setting either. It's obvious that voltages in old amps are usually higher than specified in the schematic, but I'm sure we're not talking about 540V on 6L6s' plates. 

On the side note, only the filter caps were replaced in this amp at some point in the past and probably the tech was aware of the higher voltage as they are now all doubled in series, not only the first one. No resistors across the converted ones, unfortunately. It's definitely the AA270 model judging by the bias supply arrangement and a few other details, but values of the resistors in the supply doesn't match the schematic which calls for 1K and 4,7K after the choke and I have 2,2K and 10K instead.
All other caps including filter caps in the bias supply are still original from 1972. I could observe the bias voltage go rapidly up or even disappear as I was lightly tapping the bias board - frightening, so I had to replace the associated caps immediately. The 12AT7 driver tube turns off and on by itself when it feels like it, which is always fun.

Cheers,
Tomi]]>
I've got an interesting case of excess voltage in power supply on the bench right now - it's my pal's mistreated 1972 Super Reverb AA270. It's got a whole bunch of problems but the most interesting one is that it the voltage is way too high. When I first turned it on, the Chinese 6L6WGS were dissipating 26 watts with 540V on the plates! I had to modify the bias supply in the first place to get enough negative voltage to stop them from everheating, but they should probably be replaced with proper 30W 6L6GC's as well. Rectifier tube was GZ34 even though the tube chart calls for 5U4GB. I put one in and we are now at 500V which is probably still too much for those cheap tubes. A correct rectifer does help the voltage problem a bit, but there's 7,2V AC on heaters under full load and the voltage selector is already in the 240V position. I assume this gives away that the PT could have developed a small short in the primary winding and needs to be replaced. The owner is hesitant, though, as he's worried that the unique - as he say - sound of his amp could change.

Are silverface Fenders notorious for the voltage creep up problems? I know the network mains voltage is now higher than 50 years ago, but in case of this amp the difference is too big anyway, and it's not even running on the 230V selector setting either. It's obvious that voltages in old amps are usually higher than specified in the schematic, but I'm sure we're not talking about 540V on 6L6s' plates. 

On the side note, only the filter caps were replaced in this amp at some point in the past and probably the tech was aware of the higher voltage as they are now all doubled in series, not only the first one. No resistors across the converted ones, unfortunately. It's definitely the AA270 model judging by the bias supply arrangement and a few other details, but values of the resistors in the supply doesn't match the schematic which calls for 1K and 4,7K after the choke and I have 2,2K and 10K instead.
All other caps including filter caps in the bias supply are still original from 1972. I could observe the bias voltage go rapidly up or even disappear as I was lightly tapping the bias board - frightening, so I had to replace the associated caps immediately. The 12AT7 driver tube turns off and on by itself when it feels like it, which is always fun.

Cheers,
Tomi]]>