<![CDATA[Tube Amp Forum: The Ultimate Tone

<![CDATA[Tube Amp Forum: The Ultimate Tone - Power Scaling]]> https://theultimatetone.com/ Mon, 11 May 2026 12:57:16 +0000 MyBB <![CDATA[Low-SPL Control Resolution]]> https://theultimatetone.com/Thread-Low-SPL-Control-Resolution Thu, 26 Feb 2026 17:00:51 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Low-SPL-Control-Resolution
In the Precision Power Scale thread I discussed using a "build-out" resistor to improve the control resolution at the quiet end of the Power Scale sweep. This method can be applied to all previous Power Scale installations. Ideally, we want the "audible transition" of sound-to-silence to occur exactly at the end of the PS sweep. The value required depends on the supply voltage in the amp, the kit type, and how quiet you wish the minimum loudness to be.

Classic-PS and the SB-series both have a build-out resistor already. The main purpose of the inclusion of this resistance was to keep the mosfets 'on' at the full-CCW end of the Power Scale pot sweep. However, if the sound disappears before full-CCW, then adding a bit more resistance in series with the pot-0 lead will be effective. In these Power Scale forms, it may only take a few kiloOhms, maybe up to 10k, to achieve this AT alignment with the pot sweep.

There were quite a few DC-PSK variations, and then the SF-series. The DC-PSK may require a build-out resistor up to 33k.

The Super Value SV-series followed, with many variations as I tried to use component values within the kit to eliminate dead spots of the pot sweep. Build-out resistor values of 10k to 33k are typical. Dead sweep at the CW or CCW pot ends shift with the supply voltage, requiring a tweak of two resistor values in the kit to optimise it for a given B+.

The Precision Power Scale Zh also needs a build-out resistor of 10k to 33k. I was too quick ordering these Smile

Zh has no loud-end dead spot but will have a quiet-end dead spot without the build-out R.

The Zp Precision Power Scale circuit has an on-board AT pot allowing the installer to set the audible transition EXACTLY. Zp has no dead spots and I believe it represents the ultimate performance in Power Scale kits.

The Precision Power Scale kits are still labeled as SVn but with a Zh or Zp suffix, as SVn-Zh and SVn-Zp. The SV2-Z forms incorporate VCK, since VCK is always needed in Power Scaled cathode-biased amplifiers. VCK is still available separately as it has other applications.]]> <![CDATA[Bias regulator output proportion]]> https://theultimatetone.com/Thread-Bias-regulator-output-proportion Tue, 17 Feb 2026 19:16:02 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Bias-regulator-output-proportion
The bias regulator in SV1 and TBS Tracking Bias Supply have a feedback resistor that controls the proportion of bias voltage to screen voltage. In absolute numerical terms, we describe this as a percentage, where the bias voltage available to the bias pot is expressed as a percentage of the screen voltage at the tube.

Most large bottle power tubes (and 6V6) used in musical instrument amplifiers have about the same voltage gain and end up with very similar grid-voltage to screen voltage proportion, where |-Vb| is around 10% of Vs. This is the "target" control voltage. Some amplifiers are designed to only have this much bias voltage, for example Hiwatt, and there is no leeway to run the tubes cooler or to accommodate tubes with higher transconductance - these will red plate. As TUTs recommend, the bias pot should have closer to 15% at its 'cold' end to be able to turn off most tube samples.

In our bias regulator, the feedback works against 330k, so we get these percentages for different Rfb values:
56k2 provides 15%
47k5 provides 13%
36k0 provides 10%
30k1 provides 8.5%

The lower values are suitable for EL-84 and 8417.

RBX Raw Bias Auxiliary Supply has a limited output, which is still higher than most stock bias supplies, but its applicability is reduced with higher percentage bias range combined with higher Vs, as follows:
56k2 limits RBX to amps with Vs=560V
47k5 limits RBX to amps with Vs=640V
36k0 limits RBX to amps with Vs=840V
30k1 limits RBX to amps with Vs=1kV]]>
The bias regulator in SV1 and TBS Tracking Bias Supply have a feedback resistor that controls the proportion of bias voltage to screen voltage. In absolute numerical terms, we describe this as a percentage, where the bias voltage available to the bias pot is expressed as a percentage of the screen voltage at the tube.

Most large bottle power tubes (and 6V6) used in musical instrument amplifiers have about the same voltage gain and end up with very similar grid-voltage to screen voltage proportion, where |-Vb| is around 10% of Vs. This is the "target" control voltage. Some amplifiers are designed to only have this much bias voltage, for example Hiwatt, and there is no leeway to run the tubes cooler or to accommodate tubes with higher transconductance - these will red plate. As TUTs recommend, the bias pot should have closer to 15% at its 'cold' end to be able to turn off most tube samples.

In our bias regulator, the feedback works against 330k, so we get these percentages for different Rfb values:
56k2 provides 15%
47k5 provides 13%
36k0 provides 10%
30k1 provides 8.5%

The lower values are suitable for EL-84 and 8417.

RBX Raw Bias Auxiliary Supply has a limited output, which is still higher than most stock bias supplies, but its applicability is reduced with higher percentage bias range combined with higher Vs, as follows:
56k2 limits RBX to amps with Vs=560V
47k5 limits RBX to amps with Vs=640V
36k0 limits RBX to amps with Vs=840V
30k1 limits RBX to amps with Vs=1kV]]> <![CDATA[How much power do you need for guitar? bass?]]> https://theultimatetone.com/Thread-How-much-power-do-you-need-for-guitar-bass Fri, 13 Feb 2026 01:50:35 +0000 nauta]]> https://theultimatetone.com/Thread-How-much-power-do-you-need-for-guitar-bass
I was wondering how much power you think is good for guitar?
how much for bass?

A couple of bass players I know think 100W is good enough for jamming and small gigs. I also know a guy who insists he needs his SVT. Ugh all these guys are using old ampeg tube heads but the 100watt guys have way lighter amps than the SVT. I can see why they like the lower power just on the basis of weight lol

bass seems to need a lot o power to sound as lloud as a guitar with a lot less power.

i was at this outdoor party last summer. The band had all small amps one guitar had a 20W openback tweedy lookin thing but you could hear him a couple of km away. Not sure how much power the bass had but he was using a tiny cab and everyone was keeping up with the drums which fortunately werent too loud.

My 100w Marshall was way to freakin loud so I got power scaling kits from KOC and now it is a sweet amp to play. sweet like rippin but not rippin my head off. I havent tried to measure how much power I use but it probably isnt even 20W with a drummer. just on my own its probably way on the down low man

So whaddaya think/ how many watts do you have?

peace (so we can hear the power chords)]]>
I was wondering how much power you think is good for guitar?
how much for bass?

A couple of bass players I know think 100W is good enough for jamming and small gigs. I also know a guy who insists he needs his SVT. Ugh all these guys are using old ampeg tube heads but the 100watt guys have way lighter amps than the SVT. I can see why they like the lower power just on the basis of weight lol

bass seems to need a lot o power to sound as lloud as a guitar with a lot less power.

i was at this outdoor party last summer. The band had all small amps one guitar had a 20W openback tweedy lookin thing but you could hear him a couple of km away. Not sure how much power the bass had but he was using a tiny cab and everyone was keeping up with the drums which fortunately werent too loud.

My 100w Marshall was way to freakin loud so I got power scaling kits from KOC and now it is a sweet amp to play. sweet like rippin but not rippin my head off. I havent tried to measure how much power I use but it probably isnt even 20W with a drummer. just on my own its probably way on the down low man

So whaddaya think/ how many watts do you have?

peace (so we can hear the power chords)]]> <![CDATA[Alternatives to Power Scaling?]]> https://theultimatetone.com/Thread-Alternatives-to-Power-Scaling Sat, 27 Dec 2025 00:03:55 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Alternatives-to-Power-Scaling
Power Scaling has been around for decades and has undergone continuous improvement and change as far as the kits go, but the goal and the performance overall has always been the same. We have been forced by parts availability issues to make wholesale circuit changes, such as the switch from the SB Super Budget series that used an expensive mil-spec pot and a very simple circuit, to the SV Super Versatile kit series, when the special pot became exceedingly expensive. So, we designed it out and the kit became a little more complex to accommodate its absence. We tried to use the budget of the old pot as a guide for the new circuitry cost.

Along the way, many amp builders, techs and hobbyists have tried to copy what we do. Most have since disappeared. One tech, Dana Hall, saw the original Classic-PS circuit in an amp he was to design a PCB for and decided to market his own kit. He called it "vvr" for variable voltage regulator. It is a Human foible that when we see how something is done, we inevitably say, "Oh... I knew that". One might have been aware of the basic circuit yet never did they apply it how we did.

Dana made his circuit simpler and changed the active current clamp to one that is hit and miss, depending on an unpredictable mosfet specification. It would have saved a component to leave this feature out. He also made a design choice that we described in TUT4 (The Ultimate Tone volume 4) as being "not preferred", because in some cases indirect control is preferred over direct control. For Dana this allowed another simplification and he sold his kit for a very low price benefiting a lot of players who wanted to make their amps quieter. He also sold his kits to a few amp builders who incorporated it into their products.

One of the problems with Dana's interpretation of Power Scaling, was that in most cases the whole amp is controlled. There are a few problems with doing this. One is that the amp tone changes with the power setting. This is distinctly not Power Scaling. The other problem is that every Volume pot that connects to a tube grid becomes "scratchy" when rotated. This is due to the changed DC current through the pot. The fix is to add a coupling cap to isolate the pot from DC, but this further requires that a grid-leak resistor be added for the tube grid as most guitar amps use the pot for that function.

The scratchy pot problem extends all the way back to the input where the guitar is plugged in. Most tube guitar amps do not have a coupling cap at the input; rather, they have a direct DC connection. The guitar pot is suddenly behaving quite rudely!

With Classic-PS as offered originally, none of those issues existed. Only the output stage was Power Scaled. Classic-PS had different issues that mostly made installation a little trickier, but once that was done correctly the tone stayed the same as one dialed the controls down. For the player, the main imposition was that there were two panel controls, Power Scale and Drive Compensation. Both had to be set about the same to retain the amp tone. They could be used independently to achieve three alternate performance ranges. Dana did not include the Drive Compensation control either out of further simplification or of simply not knowing why it was needed?

Other things, such as Power Dampening, were a copy of Mesa-Boggie's Limit control from a specific bass amp model. This varied the bias to the Schmitt splitter and thus limited drive to the output stage and subsequently of output power. This approach has the scratchy pot problem which we fixed in our SL-MV Splitter Limit Master Volume kit. We added three components to the existing one, quadrupling complexity, but making the approach actually useful for anyone that might need to change the control setting more than once per performance.

Marshall introduced a two-thirds form of Power Scaling on its Slash and Yngwie models. They followed the concepts presented in SSH Secrets & Secret Holders, but made an interpretive error in the execution. Their error is pretty common for techs and engineers not used to dealing with mosfets in power control positions in tube amplifiers for musical instruments. Yorkville Sound made the same error, although not in a variable power circuit, rather, in an active hum filter. In the Marshall amps, techs reported that if they disconnected the "Electronic Power Attenuator" that the amp sounded as it should, but once reconnected the amp sounded "stifled". Marshall combined the power control and drive compensation on a single control and that part worked inasmuch as the stifled sound was consistent over the loudness sweep.

In electronics there are countless ways to achieve the same goal and every tech, engineer or hobbyist will try to re-invent everything to put their own mark on whatever they are attempting. Sometimes, the best and/or easiest ways have been found. It was said once with respect to our Power Scaling kits, "Kevin O'Connor likes complicated circuits". It is not that I like them so much as I believe in Einstein's wisdom: "A thing must be made simple enough to achieve the goal, but no simpler" So, for me I do not want to sacrifice performance niceties, such as "smoothness of control" or player ergonomics just to save pennies, or to have an aesthetically simpler circuit.

All of the above is explained in much greater detail in TUT4 and TUT6.]]>
Power Scaling has been around for decades and has undergone continuous improvement and change as far as the kits go, but the goal and the performance overall has always been the same. We have been forced by parts availability issues to make wholesale circuit changes, such as the switch from the SB Super Budget series that used an expensive mil-spec pot and a very simple circuit, to the SV Super Versatile kit series, when the special pot became exceedingly expensive. So, we designed it out and the kit became a little more complex to accommodate its absence. We tried to use the budget of the old pot as a guide for the new circuitry cost.

Along the way, many amp builders, techs and hobbyists have tried to copy what we do. Most have since disappeared. One tech, Dana Hall, saw the original Classic-PS circuit in an amp he was to design a PCB for and decided to market his own kit. He called it "vvr" for variable voltage regulator. It is a Human foible that when we see how something is done, we inevitably say, "Oh... I knew that". One might have been aware of the basic circuit yet never did they apply it how we did.

Dana made his circuit simpler and changed the active current clamp to one that is hit and miss, depending on an unpredictable mosfet specification. It would have saved a component to leave this feature out. He also made a design choice that we described in TUT4 (The Ultimate Tone volume 4) as being "not preferred", because in some cases indirect control is preferred over direct control. For Dana this allowed another simplification and he sold his kit for a very low price benefiting a lot of players who wanted to make their amps quieter. He also sold his kits to a few amp builders who incorporated it into their products.

One of the problems with Dana's interpretation of Power Scaling, was that in most cases the whole amp is controlled. There are a few problems with doing this. One is that the amp tone changes with the power setting. This is distinctly not Power Scaling. The other problem is that every Volume pot that connects to a tube grid becomes "scratchy" when rotated. This is due to the changed DC current through the pot. The fix is to add a coupling cap to isolate the pot from DC, but this further requires that a grid-leak resistor be added for the tube grid as most guitar amps use the pot for that function.

The scratchy pot problem extends all the way back to the input where the guitar is plugged in. Most tube guitar amps do not have a coupling cap at the input; rather, they have a direct DC connection. The guitar pot is suddenly behaving quite rudely!

With Classic-PS as offered originally, none of those issues existed. Only the output stage was Power Scaled. Classic-PS had different issues that mostly made installation a little trickier, but once that was done correctly the tone stayed the same as one dialed the controls down. For the player, the main imposition was that there were two panel controls, Power Scale and Drive Compensation. Both had to be set about the same to retain the amp tone. They could be used independently to achieve three alternate performance ranges. Dana did not include the Drive Compensation control either out of further simplification or of simply not knowing why it was needed?

Other things, such as Power Dampening, were a copy of Mesa-Boggie's Limit control from a specific bass amp model. This varied the bias to the Schmitt splitter and thus limited drive to the output stage and subsequently of output power. This approach has the scratchy pot problem which we fixed in our SL-MV Splitter Limit Master Volume kit. We added three components to the existing one, quadrupling complexity, but making the approach actually useful for anyone that might need to change the control setting more than once per performance.

Marshall introduced a two-thirds form of Power Scaling on its Slash and Yngwie models. They followed the concepts presented in SSH Secrets & Secret Holders, but made an interpretive error in the execution. Their error is pretty common for techs and engineers not used to dealing with mosfets in power control positions in tube amplifiers for musical instruments. Yorkville Sound made the same error, although not in a variable power circuit, rather, in an active hum filter. In the Marshall amps, techs reported that if they disconnected the "Electronic Power Attenuator" that the amp sounded as it should, but once reconnected the amp sounded "stifled". Marshall combined the power control and drive compensation on a single control and that part worked inasmuch as the stifled sound was consistent over the loudness sweep.

In electronics there are countless ways to achieve the same goal and every tech, engineer or hobbyist will try to re-invent everything to put their own mark on whatever they are attempting. Sometimes, the best and/or easiest ways have been found. It was said once with respect to our Power Scaling kits, "Kevin O'Connor likes complicated circuits". It is not that I like them so much as I believe in Einstein's wisdom: "A thing must be made simple enough to achieve the goal, but no simpler" So, for me I do not want to sacrifice performance niceties, such as "smoothness of control" or player ergonomics just to save pennies, or to have an aesthetically simpler circuit.

All of the above is explained in much greater detail in TUT4 and TUT6.]]> <![CDATA[Applicability of RBX Raw Bias Auxiliary Supply]]> https://theultimatetone.com/Thread-Applicability-of-RBX-Raw-Bias-Auxiliary-Supply Thu, 11 Dec 2025 17:34:53 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Applicability-of-RBX-Raw-Bias-Auxiliary-Supply
The bias regulator used in our Power Scaling kits requires a higher raw bias supply voltage than many stock bias supplies provide AND a lower impedance. Bias supplies derived from the plate supply through high-value resistors DO allow an outrageous voltage to be attained, up to the absolute value of the B+. However this is at high-impedance. To make the bias regulator happy, the dropping resistor has to be made much lower in value, usually by paralleling many more resistors. This leads to an excess of heat in the chassis. The output of this supply needs a zener clamp as a minimum to protect the pass element of the bias regulator.

Other situations allow the stock bias winding to be separated from ground and a voltage doubler circuit to be implemented. This works reasonably well in Hiwatts, some Fenders and some Marshall amps.

Using an auxiliary transformer wired backwards and powered by the heater supply eliminates all of the concerns with the other methods, while providing a high-voltage at medium- to low-impedance. Our RBX Raw Bias Auxiliary Supply kit provides -84V with a typical bias-set network attached to the bias regulator output. With no load its output can pop up to -110V, but this is okay for the BJT in the regulator.

Readers of the TUTs (The Ultimate Tone series books), this forum and our FAQ will know that it is always a good idea to have excess sweep of the bias controls to allow complete turn-off of every tube sample. Usually, having 15% of the absolute screen voltage is sufficient. For example, if Vs=500V then -Vb should extend to -75V; for 400Vs, -60Vb, and so on. The target bias voltage for large-bottle tubes and 6V6 is around 10%, but we do not want to make the mistake of some amp companies of ONLY providing that amount of bias voltage as some tubes will red-plate and some will be stone cold.

From the numbers above we can see that there is a limit of compatible B+ for RBX. The bias regulator BJT can go to having zero volts across itself with -84V output. This would correspond to a screen voltage of 560Vs. For screen voltages higher than 560V RBX is inadequate for the task and the tubes will be overbiased and likely red-plate. The solution is to use a larger PT in the RBX format, i.e. go from 6VA to 12VA, then the output voltage will rise to almost double. An amp with 750Vs needs -113Vb with standard tubes to assure a proper control range of the tubes. Even at 600Vs, as in a Marshall Major, we need -90V for adequate controls, so RBX is not quite good enough there.

Note that the RBX PCB is sized only for 6VA transformers, which is suitable for the majority of amplifiers. For Vs>560V you would need a separate PT and RBX-LT with higher-voltage caps than usually provided.

Going to the larger auxiliary PT allows us to reconsider the decision of wiring the new PT forwards or backwards.The forward wiring has no loss per se and the output voltage will be as one would expect, allowing the use of the same 6VA PT provided this will support the bias-set network et al. We would need a 115Vac or so secondary. The primary can be a single or duals - with the 229-series from Hammond every PT has dual primaries and secondaries out of necessity of the design. The primaries are wired in series or parallel, as required for your mains, and the secondaries are wired in parallel. We now have enough bias voltage for Vs=800V. For higher Vs we wire the secondaries in series and regulate it down to a reasonable range less than 200V so as to protect the bias regulator BJT

Have fun]]>
The bias regulator used in our Power Scaling kits requires a higher raw bias supply voltage than many stock bias supplies provide AND a lower impedance. Bias supplies derived from the plate supply through high-value resistors DO allow an outrageous voltage to be attained, up to the absolute value of the B+. However this is at high-impedance. To make the bias regulator happy, the dropping resistor has to be made much lower in value, usually by paralleling many more resistors. This leads to an excess of heat in the chassis. The output of this supply needs a zener clamp as a minimum to protect the pass element of the bias regulator.

Other situations allow the stock bias winding to be separated from ground and a voltage doubler circuit to be implemented. This works reasonably well in Hiwatts, some Fenders and some Marshall amps.

Using an auxiliary transformer wired backwards and powered by the heater supply eliminates all of the concerns with the other methods, while providing a high-voltage at medium- to low-impedance. Our RBX Raw Bias Auxiliary Supply kit provides -84V with a typical bias-set network attached to the bias regulator output. With no load its output can pop up to -110V, but this is okay for the BJT in the regulator.

Readers of the TUTs (The Ultimate Tone series books), this forum and our FAQ will know that it is always a good idea to have excess sweep of the bias controls to allow complete turn-off of every tube sample. Usually, having 15% of the absolute screen voltage is sufficient. For example, if Vs=500V then -Vb should extend to -75V; for 400Vs, -60Vb, and so on. The target bias voltage for large-bottle tubes and 6V6 is around 10%, but we do not want to make the mistake of some amp companies of ONLY providing that amount of bias voltage as some tubes will red-plate and some will be stone cold.

From the numbers above we can see that there is a limit of compatible B+ for RBX. The bias regulator BJT can go to having zero volts across itself with -84V output. This would correspond to a screen voltage of 560Vs. For screen voltages higher than 560V RBX is inadequate for the task and the tubes will be overbiased and likely red-plate. The solution is to use a larger PT in the RBX format, i.e. go from 6VA to 12VA, then the output voltage will rise to almost double. An amp with 750Vs needs -113Vb with standard tubes to assure a proper control range of the tubes. Even at 600Vs, as in a Marshall Major, we need -90V for adequate controls, so RBX is not quite good enough there.

Note that the RBX PCB is sized only for 6VA transformers, which is suitable for the majority of amplifiers. For Vs>560V you would need a separate PT and RBX-LT with higher-voltage caps than usually provided.

Going to the larger auxiliary PT allows us to reconsider the decision of wiring the new PT forwards or backwards.The forward wiring has no loss per se and the output voltage will be as one would expect, allowing the use of the same 6VA PT provided this will support the bias-set network et al. We would need a 115Vac or so secondary. The primary can be a single or duals - with the 229-series from Hammond every PT has dual primaries and secondaries out of necessity of the design. The primaries are wired in series or parallel, as required for your mains, and the secondaries are wired in parallel. We now have enough bias voltage for Vs=800V. For higher Vs we wire the secondaries in series and regulate it down to a reasonable range less than 200V so as to protect the bias regulator BJT

Have fun]]> <![CDATA[Precision Power Scale Circuit]]> https://theultimatetone.com/Thread-Precision-Power-Scale-Circuit Thu, 11 Dec 2025 16:53:47 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Precision-Power-Scale-Circuit
In the quest towards a Power Scale circuit with "ideal" control, I developed some alternative circuits, some similar to the current kits and others that are quite different.

As discussed in the "Power Scale pot sweep" thread, the nominal issues are to avoid having dead spots in the sweep at the ends of the pot rotation, and to have reasonable resolution of control at the quiet end. One would suppose that the ideal solution would be to use an opamp with a tailored response to potentially allow the use of a common llinear pot but achieve a modified log control shape? Opamps are wonderful devices and there are lots of low-power types available. However, they introduce their own problems and would still require interfacing to the high-voltage circuitry. Feedback would definitely be incorporated, and would have to be fancy to achieve the response mentioned, as well as be stable over the entire range of input voltages. The kit afterall has to accommodate the whole range of supply voltages of all the guitar and bass amps on the market.

Of course, it is simple enough to build a discrete high-voltage opamp with flexible input voltage capability, and that is one of the paths investigated. Over a supply voltage range of 200V up to 880V it worked perfectly after some refinements were added. In this design, both NPN and PNP transistors are used and even from the same manufacturer they are not available with the same voltage capabilities. Placing devices in series is a necessity - just like the current kits have - but three devices in cascode for the NPNs and two devices for the PNPs. This is only a minor inconvenience. For precision, all of the resistors are metal-film 1% 600mW types wired in parallel or series to achieve the required net values and to accommodate the heat dissipation at the highest input voltage. All of this adds up to a lot of components, so a "sandwich" board assembly is required, that is two PCBs that plug together and are parallel to each other.

Overall, the high-voltage opamp approach can be made quite small. I laid out the first version using cordwood style resistor installation. The solder pads were too small and too tightly spaced and I could not see myself soldering it, nor the average hobbyist. Someone with fantastic soldering skills and/or a magnifier, and/or maybe someone who does some surface-mount soldering would have no problem. Anyway, I laid out a second then third version to make it more "buildable", which has all the Rs lying down (SVn-TH, not built or released).

One of the amendments added to the circuit allowed "programmable" output voltage at the quiet end of the pot sweep. Four DIP switches allow this selection. There was a 4-way rotary switch alternative that turned out to be very low-quality. Overall, the control board began as slightly narrower than the present SV1 board width of 2.3" (59mm) and ended up slightly larger at 2.6" (66mm). The power board changes for SV1 and SV2 applications. The two boards plug together with 0.1" header pins and receptacles, then two bolts with lock nuts to secure them safely.

I found that the heart of the programmable amendment could be added to the present kit circuit and that this greatly enhanced the sweep of the Power Scale pot. The minimum voltage is set to a fixed value and the pot sweep is very good. The board size for the SV1 increased by 0.15" (4mm) width and the SV2 remained at its present width. This keeps everything familiar for techs who have installed a lot of the current kits. These new versions are still called SV1 and SV2 as common designators, but have a smaller designation of SV1-M36 and SV2-M36. The 'M' designates a change to 1M pots for Power Scale and Drive Compensation, and the '36' refers to the 36k resistor in parallel with the PS pot. These boards arrive before the Holidays.

One other amendment to the new kits is the use of 3W resistors in some positions. These are marked on the schematic and the PCB as there are still two positions using 1W of the same value. As always, it is best to sort all the parts before assembly.

Have fun]]>
In the quest towards a Power Scale circuit with "ideal" control, I developed some alternative circuits, some similar to the current kits and others that are quite different.

As discussed in the "Power Scale pot sweep" thread, the nominal issues are to avoid having dead spots in the sweep at the ends of the pot rotation, and to have reasonable resolution of control at the quiet end. One would suppose that the ideal solution would be to use an opamp with a tailored response to potentially allow the use of a common llinear pot but achieve a modified log control shape? Opamps are wonderful devices and there are lots of low-power types available. However, they introduce their own problems and would still require interfacing to the high-voltage circuitry. Feedback would definitely be incorporated, and would have to be fancy to achieve the response mentioned, as well as be stable over the entire range of input voltages. The kit afterall has to accommodate the whole range of supply voltages of all the guitar and bass amps on the market.

Of course, it is simple enough to build a discrete high-voltage opamp with flexible input voltage capability, and that is one of the paths investigated. Over a supply voltage range of 200V up to 880V it worked perfectly after some refinements were added. In this design, both NPN and PNP transistors are used and even from the same manufacturer they are not available with the same voltage capabilities. Placing devices in series is a necessity - just like the current kits have - but three devices in cascode for the NPNs and two devices for the PNPs. This is only a minor inconvenience. For precision, all of the resistors are metal-film 1% 600mW types wired in parallel or series to achieve the required net values and to accommodate the heat dissipation at the highest input voltage. All of this adds up to a lot of components, so a "sandwich" board assembly is required, that is two PCBs that plug together and are parallel to each other.

Overall, the high-voltage opamp approach can be made quite small. I laid out the first version using cordwood style resistor installation. The solder pads were too small and too tightly spaced and I could not see myself soldering it, nor the average hobbyist. Someone with fantastic soldering skills and/or a magnifier, and/or maybe someone who does some surface-mount soldering would have no problem. Anyway, I laid out a second then third version to make it more "buildable", which has all the Rs lying down (SVn-TH, not built or released).

One of the amendments added to the circuit allowed "programmable" output voltage at the quiet end of the pot sweep. Four DIP switches allow this selection. There was a 4-way rotary switch alternative that turned out to be very low-quality. Overall, the control board began as slightly narrower than the present SV1 board width of 2.3" (59mm) and ended up slightly larger at 2.6" (66mm). The power board changes for SV1 and SV2 applications. The two boards plug together with 0.1" header pins and receptacles, then two bolts with lock nuts to secure them safely.

I found that the heart of the programmable amendment could be added to the present kit circuit and that this greatly enhanced the sweep of the Power Scale pot. The minimum voltage is set to a fixed value and the pot sweep is very good. The board size for the SV1 increased by 0.15" (4mm) width and the SV2 remained at its present width. This keeps everything familiar for techs who have installed a lot of the current kits. These new versions are still called SV1 and SV2 as common designators, but have a smaller designation of SV1-M36 and SV2-M36. The 'M' designates a change to 1M pots for Power Scale and Drive Compensation, and the '36' refers to the 36k resistor in parallel with the PS pot. These boards arrive before the Holidays.

One other amendment to the new kits is the use of 3W resistors in some positions. These are marked on the schematic and the PCB as there are still two positions using 1W of the same value. As always, it is best to sort all the parts before assembly.

Have fun]]> <![CDATA[Selecting Speaker For Low Volumes??]]> https://theultimatetone.com/Thread-Selecting-Speaker-For-Low-Volumes Mon, 20 Oct 2025 00:21:34 +0000 makinrose]]> https://theultimatetone.com/Thread-Selecting-Speaker-For-Low-Volumes <![CDATA[SV1 and SV2 assembly]]> https://theultimatetone.com/Thread-SV1-and-SV2-assembly Tue, 28 May 2024 15:40:41 +0000 K O'Connor]]> https://theultimatetone.com/Thread-SV1-and-SV2-assembly
If you are assembling SV1-Zh, SV1-Zp, SV2-Zh or SV2-Zp read this post for general guidelines, but see post-2 for specifics regarding the SVn-Z details.

The new kits are pretty tight, so the assembly order is important. The new boards are marked as SV1-D and SV2-D. The D refers to a much earlier circuit used in my amps, which I refer to as SD-1 (Super Design for fixed-bias). SD and SV are part of the Japanese-style naming Smile

Overview
Cordwood construction is used to reduce the board size by about 30% from the previous layout. In this format, axial-lead components (mostly resistors) have one lead folded back to parallel the component body with both leads now pointing the same way. The silk-screening on the PCB shows how the formed part should be inserted to avoid having the folded leads touch each other.

The little transistors should be inserted by their SHAPE not by the written face. STM disregarded industry convention and labeled the BJTs on their backside (curved side). The middle-size transistor Q5 should be oriented by the side with writing on it facing into the board; ignore the embossed 'A' on the back.

Hopefully you have a 50-60W iron as a 25W type more easily leads to cold joints and overheated semiconductors. If you have a soldering station with variable temperature, select a hotter range 700-800F and use a conical tip. Also, use thin solder, say 1mm to 1.6mm, so solder connections can be made quickly. As always, use rosin-core solder meant for electronics, that is the 63-37 eutectic ratio, as this goes from solid to liquid without the intermediate plastic phase.

Assembly Details
When I assemble an SV1 or SV2, I install the big mosfets first so they can be lined up nicely. I use a roll of electrical tape to support the PCB when soldering. You can use other types of tape or anything that is a handy height that allows the mosfets to rest on the bench top. To install transistor packages, push the leads all the way through the holes then slightly splay the outside leads outward. Now push the device back up until the height is what you want and the device is snug in the holes. With the big mosfets you want to make sure the two devices are the same height and straight, then flip the board and solder each pin. Cut each pin individually.

For all the other transistors do the same. With the small devices (Q2,7,8,9), you have to hold the transistor at an angle to the board, insert the outside pin closest to the board, then gently push and angle the device so the middle pin goes into its hole, then angle and push to insert the third pin. With the small transistors on SV1 and SV2 we leave the leads full length, so just push through 1-2mm (1/16") then make sure it is straight. Q5 is mounted similarly.

Assembly Order
As I said, the big mosfets go in first. Be sure to ground yourself before opening the anti-static bag that holds the mosfets and thermopads.

Then I install all the diodes and the gate-stops (R9,13-1k) for the big mosfets to protect them from any static during the rest of the assembly.

Then all the small resistors that lie down go in.

Then I install the small transistors along the edge of the board (Q2,7,8,9) and then Q1. Q1 is usually shipped with formed leads and should grab onto the holes. Make sure it is straight.

Then R6 13k3 which is cordwood.

The rest of the resistors are cordwood, too, and I put them in and solder them one at a time.
R12 1k between the big mosfets.
R4, R21, R10, R22 150k-1Ws
R7, R5, R11, R14, R15 330k-1W working across the board.
Q6, then Q5, then C2 and C1.

C1 usually ships with formed leads, as well, as we want to avoid damaging the epoxy seal around the leads by simply splaying them.

Yes, you really must insert, solder and trim each of these parts one at a time to assure they are mounted straight and that the soldering is good.

Other Tips
When mounting radial-lead electrolytic caps, the leads can be inserted through the holes and the cap body pushed all the way to the board. Then either splay the leads outward or pinch them inward. The pinch makes the cap tight to board for soldering . In either case, after you trim the leads you may need to make a hot adjustment to make the cap perpendicular to the PCB.

It is rare that you ever have to trim a component's leads before insertion into a board, so rare that I cannot bring any situation to mind.

When you trim the leads on transistor packages it is convenient to cut all three leads at the same time using side-cutters. The cutter jaws squeeze the pin and create a small burr on the trimmed lead in the axis of the jaws. For a low-voltage circuit this reduction of space between the pins is of little concern. However, SV1 and SV2 may have over 400V between the collector lead and the adjacent lead. Therefore, it is preferred to snip each lead individually with the axis of the jaws perpendicular to the axis of the line of three leads. This creates the burrs on the face of the lead pointing forward and rearward instead of side-to-side.

The pots come with small PCBs as the pots have PC pins rather than solder lugs. It is the convention with all PCBs that the component mounts on the silk-screened side of the board. For the pot boards, this aligns the X and 0 with the correct CW and CCW rotation of the pot. If you have an automatic wire stripper, vise, or stiff pliers, you can insert the shaft of the pot into this tool with the pot pins pointing up. Drop the PCB over the pins with the silk-screen side down facing the pot. Solder the middle pin while propping the board up so it is perpendicular to the pot leads. Now solder the outside pins.

Installation
When installing the kit in the amp, dry fit the assembled unit where there is airflow over the outside of that chassis area. Mark and drill the mounting holes (3mm or 1/8") and deburr the holes on both the inside and outside of the chassis. Use a larger drill bit in hand to cut the burr and slightly chamfer the edges of the holes.

DO NOT FORGET THE THERMOPADS WHEN SECURING THE BOARD IN ITS FINAL POSITION.

It may be easiest to solder wires to the board while it is still free, but each installation is unique and the dexterity of the installer is a factor.

You can "fly" the board for testing without tubes, or for other trouble-shooting without tubes. Place a piece of cardboard or other insulating material under the wired kit so it does not short to anything else in the amp.

BE CAREFUL NOT TO TOUCH ANY OF THE MOSFET METAL BACKS OR BARE COMPONENT LEADS. It is easy to get a shock with a flying board that is live.

Have fun]]> <![CDATA[Power Scale pot sweep]]> https://theultimatetone.com/Thread-Power-Scale-pot-sweep Mon, 05 Feb 2024 17:28:58 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Power-Scale-pot-sweep
There is a compromise in the Power Scale pot sweep, depending on the specific Power Scale circuit, the supply voltage, and the amplifier.

With the older SB-style approach using the mil-spec pot, the original kit offering incorporated a linear pot. It is easy to see on a scope or meter the direct relationship between the pot setting and the output voltage. Due to the logarithmic nature of our hearing, a linear pot is not a good choice, so, we changed to a log pot. Linear is good when using the circuit for a bench supply. The power change follows a squared curve, and a log-squared curve, respectively, and the latter ended up providing better low-loudness resolution of control.

With the newer Power Scale kits (those using a noncritical pot), there is a compromise that causes a dead spot in the sweep at one end or the other, or at both ends. The dead spot at the top end means no voltage change occurs over that part of the sweep. This is not too critical for most players once they get used to the fact that you will not hear a loudness change until you get to the power level you are actually using - which is lower than they think. But... to the player this means the useful range of the pot is smaller.

A dead spot at the low end means the amp has gone completely quiet or off before the end of the pot sweep. How big this dead spot is is a bit more concerning from an ergonomic view. In the present SVn-D kits, R1,2,3 form a voltage divider with the Power Scale pot in parallel with R2. The divider values are chosen to give a 100x power reduction with the pot set to 12-o'clock, and to go to zero volts at full sweep. Most amps will be silent at some positive voltage out of SV, so getting to a true zero is not necessary, more aesthetic. With R2=47k5 and R3=1k the output can go to zero, but changing to R2=43k2 or 44k2 and R3=2k21 gives better sweep overall.

Have fun]]>
There is a compromise in the Power Scale pot sweep, depending on the specific Power Scale circuit, the supply voltage, and the amplifier.

With the older SB-style approach using the mil-spec pot, the original kit offering incorporated a linear pot. It is easy to see on a scope or meter the direct relationship between the pot setting and the output voltage. Due to the logarithmic nature of our hearing, a linear pot is not a good choice, so, we changed to a log pot. Linear is good when using the circuit for a bench supply. The power change follows a squared curve, and a log-squared curve, respectively, and the latter ended up providing better low-loudness resolution of control.

With the newer Power Scale kits (those using a noncritical pot), there is a compromise that causes a dead spot in the sweep at one end or the other, or at both ends. The dead spot at the top end means no voltage change occurs over that part of the sweep. This is not too critical for most players once they get used to the fact that you will not hear a loudness change until you get to the power level you are actually using - which is lower than they think. But... to the player this means the useful range of the pot is smaller.

A dead spot at the low end means the amp has gone completely quiet or off before the end of the pot sweep. How big this dead spot is is a bit more concerning from an ergonomic view. In the present SVn-D kits, R1,2,3 form a voltage divider with the Power Scale pot in parallel with R2. The divider values are chosen to give a 100x power reduction with the pot set to 12-o'clock, and to go to zero volts at full sweep. Most amps will be silent at some positive voltage out of SV, so getting to a true zero is not necessary, more aesthetic. With R2=47k5 and R3=1k the output can go to zero, but changing to R2=43k2 or 44k2 and R3=2k21 gives better sweep overall.

Have fun]]> <![CDATA[Upgrading the VHT "WATTS" Circuit]]> https://theultimatetone.com/Thread-Upgrading-the-VHT-WATTS-Circuit Thu, 01 Feb 2024 08:48:31 +0000 Looselectron]]> https://theultimatetone.com/Thread-Upgrading-the-VHT-WATTS-Circuit What do I do to an amp that has a MOSFET voltage controller for its power tubes that FAILED TWICE??

First of all, I ordered a replacement. At the same time, I ordered the SV2+VCK kit for my VHT Special 12/20RT head but the instructions say make sure it's working properly without power scaling. I think I have the amp almost working without the original voltage starvation circuit but need to compare the test voltages against my second VHT Special 12/20RT combo (for a stereo rig) that still works...for now.

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It occurred to me to try installing the VCK first in the working combo to see if it makes any difference with the existing WATTS circuit before the SV2 in the head I have on the bench. Interestingly these amps have a RETURN control on the effects loop which effectively works like a master volume so I think I can safely omit the Drive Compensation control for this installation.
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Screenshot 2024-02-01 at 17.40.20.png (Size: 216.79 KB / Downloads: 59)

Updates to follow...]]> What do I do to an amp that has a MOSFET voltage controller for its power tubes that FAILED TWICE??

First of all, I ordered a replacement. At the same time, I ordered the SV2+VCK kit for my VHT Special 12/20RT head but the instructions say make sure it's working properly without power scaling. I think I have the amp almost working without the original voltage starvation circuit but need to compare the test voltages against my second VHT Special 12/20RT combo (for a stereo rig) that still works...for now.

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Screenshot 2024-02-01 at 17.40.20.png (Size: 216.79 KB / Downloads: 59)

Updates to follow...]]> <![CDATA[Field-coil speakers with variable FC]]> https://theultimatetone.com/Thread-Field-coil-speakers-with-variable-FC Tue, 25 Apr 2023 04:10:48 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Field-coil-speakers-with-variable-FC
Modern loud speaker drivers use permanent magnets to create a standing or static magnet field for the voice coil and its signal-generated magnetic field to work against. Because the speaker is akin to a "linear motor", the fixed field is the "stator" while the moving part is the "armature".

In the past, permanent magnets were difficult to manufacture except in small sizes, and as electronic amplification evolved, a clever solution was to use an electromagnet for the speaker's stator. Actually the clever part was to incorporate this winding into the power supply of the tube amp. At the same time, high-value high-voltage capacitors were expensive to manufacture but transformers and chokes were not, so you see a lot more choke filtering in older PSUs and the field coil was used as one of these chokes.

Another aspect of the amplifier design at the same time was the use of cathode-bias. This means the amp draws a fairly constant amount of power all the time and the current through the field coil would be steady, as well. It is ideal for the static speaker field to be constant so the driver performance can be predictable and consistent.

FC speakers disappeared as magnet technology and economy improved, but there has been a recent revival of FC drivers both in hifi and MI. Speakers of this design are quite expensive due to their low production volumes and the target customers tend to be economically well-off. Adding to the cost in hifi is the rather superfluous use of tubes in the FC PSU. The application is purely aesthetic.

In MI, the FC driver lends itself to field weakening which reduces the driver output and therefore can act as a loudness control if the FC power supply is made variable.

Traditional FC driver field coils had a DC resistance of 1k or less and the voice coil power would typically be in the 10-25W range. If we assume a worst-case scenario that FC power should exceed or match maximum audio power, then we might have a situation like this:
audio power = 25W
FC power = 25W
FC resistance = 1k
FC V and I: 158Vdc at 158mAdc

If FC resistance was only 100-ohms, FC V and I would be 50V at 500mA. Since the field coil is physically fixed in place and does not have to move, its weight is of no concern and it can have many turns of light wire or fewer turns of heavy wire. Coil weight is only of concern in the voice coil, as in conventional drivers.

Either way, the power supply is not that big a deal and can be made using standard linear regulator circuits. The regulator can be made constant-voltage or constant-current and both can be made variable. There is little point in extending the control range to zero, but there likely is a minimum point that is reasonable simply to have any sound. A higher limit would be "where does the sound change?". This might be a much higher FC strength than the sonic cutoff and may in fact be not at an SPL anyone would call "quiet".

As a case in point, there is a company (Fluxtone) making drivers specifically for MI that admits that their reference tone is what they achieve at 32W of input to the driver. That is incredibly loud and the player will not be hearing the speaker output accurately unless he is a hundred feet away. Even an open-back cabinet achieves efficiencies around 90dB@1W/1m, so 32W would be 105dB. Were the more usual reference of 100dB@1W/1m achieved, then loudness with 32W would be 115dB. Both levels are well beyond the limit for the Human Scale of Loudness as defined in our books (see TUT4).

The company selling these drivers claim that the SPL range extends down to 86dB, which is still beyond the Human Scale for anyone wishing to retain their hearing into old age. It may be that the manufacturer considers any volume reduction below this loudness using their field coil technology simply changes the tone and is therefore ineffective? For some players these products may provide the small amount of loudness reduction desired, but...

To take advantage of this technology you have to change your speaker to theirs. Considering the product range, this will be fine if you use low-power Jensen drivers or similar. At least you will be playing quieter.

At those high SPLs, your aural compression is being invoked quite heavily and the illusory sound in your head has little to do with the actual sound in front of the driver. Assessing the tone over 80dB is "optimistic" at the very least.

Note that an SPL change of 10dB corresponds to a power change of 10-times and a loudness perception change of 2. So, going from 100dB to 90dB will seem "half as loud" and requires a power change of 1W to 100mW. Similarly, going from 80dB to 90dB is a sonic perception of "twice as loud" and an increase of power from 10mW to 100mW. (100dB@1W/1m driver reference)]]>
Modern loud speaker drivers use permanent magnets to create a standing or static magnet field for the voice coil and its signal-generated magnetic field to work against. Because the speaker is akin to a "linear motor", the fixed field is the "stator" while the moving part is the "armature".

In the past, permanent magnets were difficult to manufacture except in small sizes, and as electronic amplification evolved, a clever solution was to use an electromagnet for the speaker's stator. Actually the clever part was to incorporate this winding into the power supply of the tube amp. At the same time, high-value high-voltage capacitors were expensive to manufacture but transformers and chokes were not, so you see a lot more choke filtering in older PSUs and the field coil was used as one of these chokes.

Another aspect of the amplifier design at the same time was the use of cathode-bias. This means the amp draws a fairly constant amount of power all the time and the current through the field coil would be steady, as well. It is ideal for the static speaker field to be constant so the driver performance can be predictable and consistent.

FC speakers disappeared as magnet technology and economy improved, but there has been a recent revival of FC drivers both in hifi and MI. Speakers of this design are quite expensive due to their low production volumes and the target customers tend to be economically well-off. Adding to the cost in hifi is the rather superfluous use of tubes in the FC PSU. The application is purely aesthetic.

In MI, the FC driver lends itself to field weakening which reduces the driver output and therefore can act as a loudness control if the FC power supply is made variable.

Traditional FC driver field coils had a DC resistance of 1k or less and the voice coil power would typically be in the 10-25W range. If we assume a worst-case scenario that FC power should exceed or match maximum audio power, then we might have a situation like this:
audio power = 25W
FC power = 25W
FC resistance = 1k
FC V and I: 158Vdc at 158mAdc

If FC resistance was only 100-ohms, FC V and I would be 50V at 500mA. Since the field coil is physically fixed in place and does not have to move, its weight is of no concern and it can have many turns of light wire or fewer turns of heavy wire. Coil weight is only of concern in the voice coil, as in conventional drivers.

Either way, the power supply is not that big a deal and can be made using standard linear regulator circuits. The regulator can be made constant-voltage or constant-current and both can be made variable. There is little point in extending the control range to zero, but there likely is a minimum point that is reasonable simply to have any sound. A higher limit would be "where does the sound change?". This might be a much higher FC strength than the sonic cutoff and may in fact be not at an SPL anyone would call "quiet".

As a case in point, there is a company (Fluxtone) making drivers specifically for MI that admits that their reference tone is what they achieve at 32W of input to the driver. That is incredibly loud and the player will not be hearing the speaker output accurately unless he is a hundred feet away. Even an open-back cabinet achieves efficiencies around 90dB@1W/1m, so 32W would be 105dB. Were the more usual reference of 100dB@1W/1m achieved, then loudness with 32W would be 115dB. Both levels are well beyond the limit for the Human Scale of Loudness as defined in our books (see TUT4).

The company selling these drivers claim that the SPL range extends down to 86dB, which is still beyond the Human Scale for anyone wishing to retain their hearing into old age. It may be that the manufacturer considers any volume reduction below this loudness using their field coil technology simply changes the tone and is therefore ineffective? For some players these products may provide the small amount of loudness reduction desired, but...

To take advantage of this technology you have to change your speaker to theirs. Considering the product range, this will be fine if you use low-power Jensen drivers or similar. At least you will be playing quieter.

At those high SPLs, your aural compression is being invoked quite heavily and the illusory sound in your head has little to do with the actual sound in front of the driver. Assessing the tone over 80dB is "optimistic" at the very least.

Note that an SPL change of 10dB corresponds to a power change of 10-times and a loudness perception change of 2. So, going from 100dB to 90dB will seem "half as loud" and requires a power change of 1W to 100mW. Similarly, going from 80dB to 90dB is a sonic perception of "twice as loud" and an increase of power from 10mW to 100mW. (100dB@1W/1m driver reference)]]> <![CDATA[Variacs and other things]]> https://theultimatetone.com/Thread-Variacs-and-other-things Tue, 25 Apr 2023 03:26:20 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Variacs-and-other-things
Everyone has heard the story that Eddie van Halen used a variac to dial the mains voltage down a bit so his amp would overdrive at a lower loudness. Does this work?

Short answer: yes.

Long answer: Only to a point.

Drops in the mains voltage effect ALL the internal voltages in a typical amp that does not have active voltage regulation. This means the heater voltage will drop, leading to slightly cooler cathodes and reduced emission of electrons. There is a 10% tolerance listed for heater voltage, beyond which the heater itself may not work but certainly beyond which the cathode stops functioning as it should.

When the plate supply voltage drops, the tube stages generally keep working as they should provided that bias supply tracks the plate change for fixed-bias amps. The tone will change and become a bit softer, rounder, and easier to overdrive. The power output may not change radically enough to attain "quiet" SPLs, but it might be enough to make the excess loudness tolerable. In any case, the amp tone changes with this type of power control.

In the refined version of the variac approach, a separate filament transformer is added to maintain heater and cathode temperatures. The variac now changes plate and bias voltages for the entire amp. The loudness control range may be extended but again tone changes as you dial down. Another issue arises that pots become "scratchy" even on your guitar !! This is a problem users of vvr encounter, and they have to add DC blocking caps to the input of the amp and where pots interface directly with tube grids. The latter requires also adding grid-leak resistors to maintain the tube bias and normal operation.]]>
Everyone has heard the story that Eddie van Halen used a variac to dial the mains voltage down a bit so his amp would overdrive at a lower loudness. Does this work?

Short answer: yes.

Long answer: Only to a point.

Drops in the mains voltage effect ALL the internal voltages in a typical amp that does not have active voltage regulation. This means the heater voltage will drop, leading to slightly cooler cathodes and reduced emission of electrons. There is a 10% tolerance listed for heater voltage, beyond which the heater itself may not work but certainly beyond which the cathode stops functioning as it should.

When the plate supply voltage drops, the tube stages generally keep working as they should provided that bias supply tracks the plate change for fixed-bias amps. The tone will change and become a bit softer, rounder, and easier to overdrive. The power output may not change radically enough to attain "quiet" SPLs, but it might be enough to make the excess loudness tolerable. In any case, the amp tone changes with this type of power control.

In the refined version of the variac approach, a separate filament transformer is added to maintain heater and cathode temperatures. The variac now changes plate and bias voltages for the entire amp. The loudness control range may be extended but again tone changes as you dial down. Another issue arises that pots become "scratchy" even on your guitar !! This is a problem users of vvr encounter, and they have to add DC blocking caps to the input of the amp and where pots interface directly with tube grids. The latter requires also adding grid-leak resistors to maintain the tube bias and normal operation.]]> <![CDATA[Single-knob Power Scaling]]> https://theultimatetone.com/Thread-Single-knob-Power-Scaling Tue, 25 Apr 2023 03:06:50 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Single-knob-Power-Scaling
The London Power Power Scaling kits come with two controls for the front panel: Power Scale and Drive Compensation. These two controls allow four ways to use the amp, one of which is Power Scaling where the tone is retained with loudness changes. The other modes are simply versatilities made possible.

Many players still want a 1-knob Power Scale solution, and there are situations where this is possible with the kits. Power Scale is a single-section pot where Drive Comp is supplied as a dual-pot and this is what you use for the single-knob solutions below.

If the amp has a master-volume preamp, such as the original "800" does, then the dual pot can replace the stock MV and the new Power Scale control. The Power Scaling goal is achieved as drive is in step with Power Scale and tone is maintained.

A Vox amp may come to mind here as those with an MV use a single-section pot. However, the MV is the cross-line type, the worst configuration possible, which would be greatly improved in function were it replaced by a 2-section pot wired in the more usual manner. This negates using the supplied DriveComp pot as a 1-knob Power Scale control. ***Edit: On later thought, a single-knob can work here. The MV section will just do what it always did and the Power Scale section will control output power ***

Amps that do not use a Schmitt splitter and instead have a gain stage followed by a concertina splitter are candidates for a 1-knob Power Scale solution. In this case, one section of the pot is placed between the gain stage and the concertina, with the other section used as the Power Scale control. It does not matter if feedback is wrapped around the PA.

If the MV is a post-PI type that is typically a dual-pot itself, then the above solution does not work. One would need a 3-section pot, which are available, but it is easier to find 4-sectiuon pots that are fairly common now for home theatre applications.

In fixed-biased amps, you still need bias controls per usual, mounted on the tube plane of the chassis and set ONLY at full power. These are not performance pots; rather, set and forget with each tube change or new PS installation.]]>
The London Power Power Scaling kits come with two controls for the front panel: Power Scale and Drive Compensation. These two controls allow four ways to use the amp, one of which is Power Scaling where the tone is retained with loudness changes. The other modes are simply versatilities made possible.

Many players still want a 1-knob Power Scale solution, and there are situations where this is possible with the kits. Power Scale is a single-section pot where Drive Comp is supplied as a dual-pot and this is what you use for the single-knob solutions below.

If the amp has a master-volume preamp, such as the original "800" does, then the dual pot can replace the stock MV and the new Power Scale control. The Power Scaling goal is achieved as drive is in step with Power Scale and tone is maintained.

A Vox amp may come to mind here as those with an MV use a single-section pot. However, the MV is the cross-line type, the worst configuration possible, which would be greatly improved in function were it replaced by a 2-section pot wired in the more usual manner. This negates using the supplied DriveComp pot as a 1-knob Power Scale control. ***Edit: On later thought, a single-knob can work here. The MV section will just do what it always did and the Power Scale section will control output power ***

Amps that do not use a Schmitt splitter and instead have a gain stage followed by a concertina splitter are candidates for a 1-knob Power Scale solution. In this case, one section of the pot is placed between the gain stage and the concertina, with the other section used as the Power Scale control. It does not matter if feedback is wrapped around the PA.

If the MV is a post-PI type that is typically a dual-pot itself, then the above solution does not work. One would need a 3-section pot, which are available, but it is easier to find 4-sectiuon pots that are fairly common now for home theatre applications.

In fixed-biased amps, you still need bias controls per usual, mounted on the tube plane of the chassis and set ONLY at full power. These are not performance pots; rather, set and forget with each tube change or new PS installation.]]> <![CDATA[SV1, SV2, SV-TT update]]> https://theultimatetone.com/Thread-SV1-SV2-SV-TT-update Mon, 12 Jul 2021 17:20:42 +0000 K O'Connor]]> https://theultimatetone.com/Thread-SV1-SV2-SV-TT-update
Note that this thread refers to older versions of the Power Scling kits, which are installed in a lot of amps. SV-TT is no longer offered.

Back when I designed SV1, SV2 and SV-TT it seemed like "a good thing" to add some tiny caps to reduce the bandwidth of the otherwise wide-bandwidth regulator sections. The idea was that noise on the supply line that might be impressed across the Power Scale control could be amplified through the regulators and be heard through the amp. Of course, C1 across the pot output would clean this up all on its own.

In most installations these caps do not present a problem and everything works fine. However, some installations will show some oscillation and removing the small feedback cap 56-68pF will eliminate the oscillation. In SV1 and SV-TT, remove the 270pF from the bias regulator. Do this in TBS if there are issues. The bias regulator cap can cause a secondary oscillation.

Over the life of these kits, the 330k-1W feeding the Power Scale pot was split into 150k + 180k and a filter cap to ground was added at their junction. This cap helps reduce hum, ripple or other noise getting to the pot. Some installers found that C1 could be removed without any audible effect, although the main purpose for C1 is to keep DC-related pot-scratchy sounds from happening.

Techs report that he removal of these caps makes the amp sound better. It should, since these are very wide-bandwidth regulators which present a low-impedance voltage source to the amp allowing any frequency restrictions to be only those imposed by the amp itself.

I annotated the kit notes but if you have old notes or ones without the update, at least the information is accessible here on the forum.]]>
Note that this thread refers to older versions of the Power Scling kits, which are installed in a lot of amps. SV-TT is no longer offered.

Back when I designed SV1, SV2 and SV-TT it seemed like "a good thing" to add some tiny caps to reduce the bandwidth of the otherwise wide-bandwidth regulator sections. The idea was that noise on the supply line that might be impressed across the Power Scale control could be amplified through the regulators and be heard through the amp. Of course, C1 across the pot output would clean this up all on its own.

In most installations these caps do not present a problem and everything works fine. However, some installations will show some oscillation and removing the small feedback cap 56-68pF will eliminate the oscillation. In SV1 and SV-TT, remove the 270pF from the bias regulator. Do this in TBS if there are issues. The bias regulator cap can cause a secondary oscillation.

Over the life of these kits, the 330k-1W feeding the Power Scale pot was split into 150k + 180k and a filter cap to ground was added at their junction. This cap helps reduce hum, ripple or other noise getting to the pot. Some installers found that C1 could be removed without any audible effect, although the main purpose for C1 is to keep DC-related pot-scratchy sounds from happening.

Techs report that he removal of these caps makes the amp sound better. It should, since these are very wide-bandwidth regulators which present a low-impedance voltage source to the amp allowing any frequency restrictions to be only those imposed by the amp itself.

I annotated the kit notes but if you have old notes or ones without the update, at least the information is accessible here on the forum.]]> <![CDATA[Question about filament transformer for bias supply in a Power Scaled Amp]]> https://theultimatetone.com/Thread-Question-about-filament-transformer-for-bias-supply-in-a-Power-Scaled-Amp Thu, 06 Feb 2020 17:05:35 +0000 makinrose]]> https://theultimatetone.com/Thread-Question-about-filament-transformer-for-bias-supply-in-a-Power-Scaled-Amp ]]> ]]>