Power Scaling vs. Variable Power

Electronics often allows for the accomplishment of a particular goal in many different ways. For guitar amp users, controlling how loud or quiet their amp is while achieving a “cranked” sound has always been difficult. Attenuators can be placed between the speaker and amplifier to provide some means of control, but they tend to alter the sound and shorten tube life. The overdriven amp will clip its output continuously at its full rating.

Controlling Power

Power control methods have been around longer than electronics itself, but electronic control of power provides the greatest versatility of control attributes combined with low cost and fine resolution of control. Electronic methods had their beginnings with had active gain elements, such as vacuum tubes. With semi-conductor technology, power control is much easier to implement, using very low-cost devices and minimal space.

Scaling Power: Player’s Perspective & Electronic Perspective

Back to our guitar player with cranked tone. His goal is to get that same sound in the same way, at reduced volume, BUT without having to get a whole new smaller-quieter amp. The goal, then, is to “Power Scale the amp”, which will achieve “Power Scaling the sound”. The mission, then, is Power Scaling.

To top it off, how we get there (electronically speaking) is also Power Scaling.

If we maintain the “sound” of the amp while reducing its power output, we have by definition Power Scaled it. So, Power Scaling is the “methodology” we use to attain the sonic goal of Power Scaling. It is not overblown semantics to say that both the goal and how we get there are the same thing – Power Scaling. Both are intertwined: the goal defines the method, and the method achieves the goal.

Foundations: Transfer Curves

The amplifier has a certain “response” to the input signal. It produces an output with characteristics specific to the amplifier design and component choice. If we swap tube types, it sounds a little different, because now it is effectively a different amp with a new response. That response is referred to as a “transfer curve”. This is just a way of relating the output to the input. If we maintain the shape of the transfer curve, then we will maintain the sound, even if we make that curve “look” smaller. This is what Power Scaling achieves.

Transfer curves are generally considered on a single-stage basis in traditional tube electronic analysis, but I have extended the application here. An entire block of circuitry has a net transfer curve, as does the entire amplifier. For the most part, when we implement Power Scaling, it is not necessary to alter or control the entire amplifier circuit, but just a portion of it. Usually, this control is confined to the output stage, or possibly the whole power amp. This is the circuit area in which the greatest signal dynamics occur, and where signal processing from preceding stages is “swamped” by, say, hard clipping of the power stage.

Reducing the power output of the power stage *without altering* how it handles signals is quite simple, and any approach used to achieve this IS Power Scaling. There are also simple ways to control power output which result in *alternate tones* as we dial down; these are simply “Variable Power”.

There are yet other things that achieve variable output power and are labeled as such, but have nothing to do with either Power Scaling or variable-power methods.

Examples of Power Scaling

The following are examples of Power Scaled products and approaches: the techniques presented in our The Ultimate Tone volumes – particularly volumes 4, 5 and 6; London Power’s amplifiers; London Power’s former PSK-, DCPSK-, SB- and SF- kits, and current SV- Power Scaling kits representing preferred methods. There are countless circuit variations that will achieve this goal. The licensed amp products using the Power Scale trade names, of course, are Power Scaled amps and achieve the desired sonic goal. Unlicensed implementations are also plentiful, and those companies choose to call the control something else, like “Voltage”, “Power”, “Variac”, and others. Whether a company uses the Power Scale name or not, if they use our circuitry or variations of it they must pay licencing. Claret’s and Trentino’s approaches can be implemented well or poorly, achieving either Power Scaling or just variable power. Zimmerman’s approach achieves Power Scaling down to a still too-loud level.

Examples of Variable Power

Examples of “variable power” include: any implementations in which the sound changes as one dials down, but reduced loudness is nonetheless achieved; Mesa’s D-180 Limit control using a variable current-source for the splitter (as does Mojave’s copy of it); Carvin’s power reduction switches; Marshall’s “virtual” power reduction; Moore Amplification’s Power control; and many others.

Examples of “Something Else”

Things that are neither Power Scaling nor “variable power” include: all master volumes. Seymour-Duncan’s “Juice” control is an electronic post-splitter MV; most controls labeled “variac”, which are simply variable current sources for the splitter; and, the use of light bulbs to restrict power gives limited effect while “browning” the sound. One of the benefits of the preferred Power Scale approach is extended tube life. One can still achieve the sonic goal of Power Scaling without this benefit – but why would you want to?


Note that a “trade mark” and a “trade name” do not have to be registered to be recognized and protected. “Power Scale” and “Power Scaling” are trade names and trade marks of London Power and Kevin O’Connor and are legally protected, but are not registered with the US Patent Office. That organization would like you to believe that unregistered marks are without merit and protection. The only mark that is not legal to use is the circle-R for “registered”. The “TM” symbol can be used for both registered and nonregistered marks alike.

Guitar amps are built to give us fun, and we should have fun building them, too!

See also: Power Scaling Q&A