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+--- Thread: Alternatives to Power Scaling? (/thread-358.html)
Alternatives to Power Scaling? - K O'Connor - 12-26-2025
Hi Guys
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.
RE: Alternatives to Power Scaling? - makinrose - 12-27-2025
Great write up! Having serviced many amps with various "power controls" and power reduction switches almost none of them have true Power Scaling but instead have major tone changes as you dial down. Many of the major companies that offer some sort of power reduction don't seem very concerned with transparency.
I think this has led to a great deal of confusion among players many of whom treat volume reduction with trepidation because of past experiences. It takes patience and time to explain the differences between those types of power reduction and Power Scaling when I am talking to potential amp customers.
RE: Alternatives to Power Scaling? - K O'Connor - 12-27-2025
Hi Guys
Before Marshall introduced EPA, they tried a different approach called VPR, Virtual Power Reduction. Kind of a silly name begging the question as to whether there is power reduction or not? This was actually a post-PI MV with a few fixed settings. Reducing drive from the splitter to the output stage will reduce output power but not with any of the benefits that Power Scaling offers.
Apart from the loudness reduction without tone change, Power Scaling increases tube life by reducing waste heat off the tubes when the Power Scale control is set low. vvr will do this, too, except with a tone change. VPR will not change the tube dissipation any more than turning any level control in the amp does. EPA and any two-thirds approach will reduce waste heat from the tubes but not to the extent full Power Scaling can.
Similar to the splitter bias method, a cathode-biased output stage can have its power output reduced by changing the bias resistor value. The intuitive form uses a rheostat in series with a fixed resistor. With the pot at zero resistance, there is just the fixed Rk and maximum heat and maximum power. With all of the rheostat's resistance in play, idle power is reduced and so might be output power. We say "might" because the presence of a bypass cap here will maintain maximum power even with the reduced idle current.