London Power’s Amplifier Technologies – Evolution to Revolution

Invention. Innovation. Refinement.

London Power has been designing new ways to do things since we began in the 1990s. Some of our methods evolve from old techniques with thoughtful innovation, while other techniques are simply revolutionary! Designers who believe there is nothing new in guitar amplifier electronics should look more closely – and more widely. Through our publishing arm, Power Press Publishing, these methods have been made available to professional and amateur amp builders around the world. We have license agreements with many builders of all sizes, as each has found one or more of our technologies to be beneficial to their own bottom line and to their customers.


Power Scaling technology and methodologies have been around since before there was an internet. Developed by London Power for studio applications, the first production amplifier to incorporate Power Scaling was London Power’s Studio model, released in 1995.

Power Scaling, when implemented correctly, will reduce amplifier loudness by reducing the power generated. This has the added benefit of extending tube life while retaining “cranked” amp tone at any loudness level. Power Scaling can be applied to any tube amplifier, regardless of bias method or push-pull versus single-ended.

London Power has developed Power Scaling Kits to suit your needs. Our kits represent the very best way to control amplifier power without altering tone; simple without being ‘too simple, as that would not be best.


Power Scaling-TT is “two-thirds Power Scaling“. With full Power Scaling, the Power Scale regulator dissipates the heat that would otherwise be handled by the tubes. This heat output from the regulator can be regarded as a direct indication of tube life extension. Many engineers copying Power Scaling shy away from the demands of managing this heat and attempt to create a more economical solution. However, their choices introduce errors of design that impair dynamic qualities of the amplifier sound even at full output. London Power sets the standard for proper implementation of PS-TT.

London Power’s line of Power Scale kits can be wired for Full-PS or for PS-TT. We also have a dedicated SV-TT “Super Versatile Two-thirds Power Scaling” kit that is universally applicable and can be used in ANY production tube guitar or bass amp ever made.


Super Scaling preceded Power Scaling, and was invented in 1985 by Kevin O’Connor. Super Scaling allows the tone and character of a small amp to be retained while driving a speaker to much higher power levels. The original methodologies employed allowed both power boost and power reduction, while later implementations, such as used in London Power’s Stage amps of 1997, were just power boosters. Since that time, Super Scaling is primarily used for boost only.

Super Scalers can be all-tube, solid-state or hybrid. Some Super Scaler circuits look like they are from the 1940s – and indeed evaluation of those circuits identifies them as early Super Scalers, or at least as circuits that meet the criteria in such application.


GmX technology evolved during the 1990s and became a mature and uniquely new approach in 2002. The crude forms of GmX were presented in The Ultimate Tone Vol. 3 (TUT3), with the advanced forms described in The Ultimate Tone Vol. 4 (TUT4).

GmX allows an amp to sound and feel “muscular” with “effortless bass” and an “unstrained” sound. A small amplifier can sound much larger; a pair of EL-84s can sound like their big brother EL-34, or even like KT-88s – or a sea of KT-88s!

GmX technology can also be used to increase output power while retaining tube tone – a form of embedded Super Scaling. This can provide an immense economic saving when designing or building a high-power tube amp.


RmX technology has been around for decades – we merely quantified it and demonstrated uses for it in MI. RmX methods allow active circuitry to behave like an electronically controlled resistance. Although sometimes you can simply use a resistor or potentiometer for this purpose, there are many situations where you cannot – maybe because the voltage is too high, or the power level is too high, or the resistance must float at a high voltage, or multiple resistances must change simultaneously and/or in different directions.

An obvious application for RmX is as a tube rectifier simulator. We can readily design a “rectifier” with different modes, say from full solid-state, to 5U4, to 5AR4, to 5Y3, to 6X4 and beyond.

Another application is for sweepable fixed bias to cathode bias.


ZmX technology takes RmX methodology into the audio frequency range. Variable impedance with frequency is possible, with limitless frequency response variations and profiles. ZmX technology allows active loads to be created for an amp; or active attenuation that is “tunable”; or “powerless” attenuation with frequency shaping.

Z-B-X & ZBX2

Z-B-X amplifiers combine high-speed solid-state circuitry with traditional tube amplification to allow for extreme performance. A single power tube can be used to create a push-pull output – and not by relying on a dual-element tube. Where a pair of, say, 6L6s, might have been needed to produce 80W of output, a single 6L6 will do in a Z-B-X amp.

ZBX2 takes this approach “sideways”. Two tubes are used, simplifying some of the circuitry and allowing the primary audio path to be more conventional. The ZBX function allows these tubes to produce their full power with half the voltage stress of a conventional push-pull circuit. An added bonus is zero output noise at idle.


Master volume (MV) controls have been around in various forms since 1959 – as applied to guitar amplifiers. Most MVs suffer from tone change versus control sweep. We demonstrated improvements to standard circuits in the first volume of our The Ultimate Tone (TUT) book series. Two volumes later, in TUT3, we illustrated the “bootstrapped-MV”, and techs around the world began installing them in their own amps and those of customers.

The LP-MV is the improved bootstrap MV and can be applied to 99% of guitar and bass amps on the market. London Power offers an LP-MV kit.


An alternative to master volumes is the splitter current limit. Most amps fitted with such circuits disappoint the player for the same reasons each time – noise versus control sweep and a change of tone over the range of the control. There is a way to do this properly, and London Power has it in a kit form (SL-MV). If this approach appeals to a player, tech or amp builder for reducing output signal, he or she now has a solution that works without pot noise and with greater dynamic range than previously offered.


London Power has always offered options to its customers. Why be limited to one tube type in the output stage when so many other types will “fit the hole” with compatible wiring? And why be constrained to using pairs of similar tubes when it is easy to mix them?

These capabilities are inherent in every fixed-biased amplifier, yet manufacturers have been slow and even reticent to empower their musician customers with such choice. London Power respects the intelligence of the player and gives him the tools and options to make his own decision about which tubes to use. Even “how many” tubes to use is an option in London Power’s amps. This is innovation in the name of player ergonomics.


Effects loops became fashionable in the 1990s, and most manufacturers fitted crude series loops to at least some of their models. These dollar-driven designs typically altered the tone of the amp when used, and so many players came to believe that this was to be expected with all effects loops.

With the release of our book, The Ultimate Tone, amp builders and hobbyists could finally see the truth about effects loop design and the compromises that had been “inflicted” upon the music world up to that time. Again, thoughtful innovation resulted in a loop that does not change tone, and yet is all-tube. Tubes are capable of extremely hi-fi performance, and this is an application in the instrument amplifier market where a hi-fi approach pays off. Boutique amp builders who were inspired by TUT to get into the business, were also inspired to add “correct” loop circuits, just as the “big name” builders who have our books have done.

See our page, “Effects Loop Truths,” for more detail.