Power Scaling for Tube Amplifiers – Q&A

What is Power Scaling?

Power Scale Control by London PowerPower Scaling’s goal is to achieve the same tone as one’s preferred “loud sound,” but at a much lower volume. The method can involve one of over sixty distinct approaches, each with many variations.

Power Scaling™ is a methodology developed by Kevin O’Connor of London Power.

Is Power Scaling simply power reduction?

• No.
Earlier designers made attempts to achieve effective scaling of power, but never quite got there. London Power refined and fully developed the technology to allow the maximum power of an amplifier to be dialed down to whatever level a player needs. It was first used in the amazing London Power STUDIO amp.

Isn’t this just like a speaker load box?

• Not at all. Speaker load boxes, speaker emulators, and speaker attenuators are all forms of attenuation interposed between the power amp’s output and the speaker. They work for some people, but are notorious for sounding “buzzy” at high attenuations.

A speaker attenuator forces your amp to be run flat out, producing its full power all the time. The power that is not needed is thrown away as heat, with only the required power going to the speaker. It is quieter than full-tilt, but now the speaker is isolated from the amp and cannot interact with it, so some tone is lost.

Power Scaling is none of these.

The key to Power Scaling is that it is applied to the power output tube stage itself, and so comes before the output transformer. Power Scaling allows a dynamic power range over 40dB. Most speaker attenuators alter the tone before they reach 8dB reduction. Minus 8dB is just a little bit quieter than full blast; minus 40dB is literally a whisper.

Wouldn’t a master volume do the same thing?

• Only in specific situations. If you only play clean or you only use preamp overdrive or distortion tones, then a master volume will satisfy you.

Power Scaling is the best solution for players who incorporate some output stage “effect” in their sound.

This effect can be light or heavy clipping, or just that cusp of compression you get in a tube power amp approaching clipping. Power Scaling allows you to live at that cusp or beyond, but at ANY loudness you need.

So, Power Scaling will help my overdrive sounds. How clean will a Power Scaled amp play?

• All London Power Power Scaling amps are designed to provide smooth, sweet, clean sounds up to their limit of power. Set to full power, Power Scaling will not alter the clean sound of your amp. If you set Power Scale to a lower setting, the amp now behaves as a lower-power amplifier with a lower maximum loudness for clean sounds.

How does Power Scaling affect tube life?

• With the POWER SCALE dial set to any setting less than maximum, tube life will actually be extended. In accelerated tests, power tube life is as long as that of a preamp tube … up to 100,000 hours if the tube is not mechanically upset. This is a great attribute for players using NOS tubes (new-old-stock).

Output transformer life is also extended, since it is subject to much lower voltage stress even with fully squared output signals and unexpected load disconnection.

Can I run a Power Scaled amp without a load, right into a mixer?

• Yes. It is perfectly safe to do this, although you lose the benefit of frequency shaping provided by the speaker and the interaction of the output stage with the speaker.

The POWER SCALE control reduces voltage and current stress on the output tube, so even at a fully saturated distortion output, the tube is under less stress than it would be in, say, a 3W amp.

Couldn’t a low-power tube amp do the same thing?

• No. A low-watt amp only has one compression point, one cusp of distortion and one maximum loudness level through a speaker.

Power Scaling amps can play both louder and quieter than amps of less nominal power.

And, the compression point stays in the same position relative to the cusp point for all settings, allowing the touch responsiveness to remain consistent.

How is this possible? Is the circuitry complex or expensive?

• In technical terms, all that must be accomplished is to keep the “transfer curve” of the amplifier the same. The transfer curve is simply the relationship between the input and output signals, but as we know, tube amps respond differently to different-size signals. This is because the transfer is not straight and not uniformly curved.

Think of the transfer curve as a mirror.

A flat mirror, parallel to you, will reflect your image perfectly and full size. If you move the mirror away, the image is smaller but still perfect. A tube’s transfer curve is like a slightly curved or rippled mirror. In this case the image is slightly distorted, but this is exactly what we want – it is why you chose a tube amp in the first place. So, moving the mirror farther away reduces the size of the image, but it is still perfectly imperfect.

Electronically, it is very simple and inexpensive to achieve this goal. The diversity of electronic circuits allows countless approaches by different designers, with greater expense added or bulkier components used. In the end, it is all Power Scaling.

Does it affect the output impedance of the amplifier? People on the web say Power Scaling changes the tube plate resistance.

• No. Because the shape of the transfer curve is maintained, the plate resistance of the output tubes is also maintained. So, the output impedance of the amp does not change even though much less power is available once you dial down.

Can Power Scaling be added to any tube amp? Is the circuitry large?

The circuits are simple and small, and are easily retrofitted into existing tube amps. But… the tech performing the installation must be very good at mods and not just a good repair tech – as these are two different skills.

Depending on the amp, some heatsinking or a fan might be required to cool the Power Scale circuit.

Is the Power Scale circuit just converting the unwanted audio power into heat?

• No. If you only need 3W of power, only 3W is produced; if you need 29W, then you get 29W. The simplest Power Scale circuits are soft regulators, and as such, they divide the voltage available from the raw power supply between the amplifier and itself. In that voltage sharing, there will be some waste heat.

At POWER SCALE control settings between fully clockwise and about 12-o’clock, there will be waste heat from the Power Scale regulator. At settings between minimum (counter-clockwise) and 12-o’clock, the regulator runs cool. Meanwhile, as audio power is reduced, waste power in the power tubes goes down in direct proportion, which increases their reliability.

How does Power Scaling compare to Maven Peal’s Sag circuit?

Power Scaling allows the player to dial the sound down to whisper levels – actually unusably quiet. If you have a detuned speaker cabinet or any other design that extends dynamic linearity, you can play below a normal speaking level. With London Power’s Power Scaling design approach, the player can choose how quiet to play, with typically 44dB of dynamic range – that is 100W down to less than 0.01W.

Maven Peal’s designer chose the half-watt lower power limit for their WATTAGE control, so that the player would not encounter the point where the matching open-back combo’s speaker loses its tone at lower volumes. To put this into perspective, most guitar speakers produce 90-100dB of sound, with just one watt of input. Half as much input power only reduces loudness by 3dB, so you are at 87-97dB of sound, which is a loud party level.

So, even though Maven Peal has amps that go from 100W down to 1W (20dB range), and 20W down to 0.5W (16dB range), those lower power levels are still fairly loud. To some players, they are “quieter” or “quiet enough”. But it is obvious that a power range that is smaller than these – say, 100W down to 5W – is even less useful at just 13dB dynamic range. The quietest level is still too loud for most players.

“Sag” is an inherently signal-dependent effect exhibited by all amplifiers with conventional power supplies. A regulated supply with a very beefy transformer will exhibit much less sag. Sag is merely a voltage drop under loading, which affects the attack of a note. Once the supply sags, its stays sagged and then power-limits the entire signal.

Maven Peal products use stiffly regulated power supplies to reduce noise, and thus their amps have no inherent sag effect. Sag must then be added by letting the signal modulate the supply reference. Despite the potential of the system, the limits imposed by the designer restrict the range of sounds, both in the sag effect and the possible power reduction.

Power Scaling supplies, in contrast, have a soft-rectifier sound and do not tighten up as you dial down. Diode noise is inherently filtered, and you can play as quietly as you want. “Sustain” is the sonic quality of sag, and London Power uses various Sustain circuits to increase this tonal characteristic.

Must I buy a London Power amp to get Power Scaling?

• No. There are many amp brands licensed to use our technology and trademarks. Each builder begins with traditional or original audio circuit ideas enhanced by Power Scaling. You can also have your own an amp fitted with Power Scaling by a recommended installer.

London Power offers a range of Power Scaling Kits suitable for all tube guitar amps on the market, including vintage amps and new designs.

Where can I get help with Power Scaling?

If you purchase Power Scaling technology licensing or a Power Scaling kit from London Power, technical assistance is included.

The quiet revolution starts here.


See also:
About Our Power Scaling Kits
Selecting a Power Scaling Kit
Licensed Power Scaling Builders and Installers


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.

POP Table of Contents

Principles of Power

Table of Contents and List of Appendices

Chapter 1: The Power Imperative

Chapter 2: Vacuum Tubes
Early History
Vacuum Tube Diodes
Vacuum Triodes
Beam Power Pentodes
Multi-Element Tube Standards
Multiple Tubes

Chapter 3: Power Supplies
AC Power
Power Transformers
Half-Wave Rectifiers
Full-Wave Rectifiers
Power Supply Filtering
Capacitive Filters
Choke Filters
Power Supply Energy Storage
Capacitor Care and Safety
Bias Supplies
Filament Supplies
Turn-On Surge Protection
Screen Supplies
High-Voltage Regulators
Ground-Busses, Stars and Planes
Stand-By Switches
Cathode Switching
Screen Switching

Chapter 4: Power Amp Essentials
Voltage Amplification
Cathode Followers
Grounded-Grid Amplifier
Phase Splitters
Transformer-Coupled Splitter
Vacuum Phase-Inverters
Concertina Splitter
See-Saw Inverter
Schmitt Splitter
Differential Splitter
Schmitt Splitter
Differential Splitter
Driver Stages
Differential Driver
Cathode Drivers
Transformer Drive
Output Stages
Single-Ended Outputs
Push-Pull Outputs
Ultralinear and Unity-Coupled Outputs
Bias Networks
Cathode-Bias: Single-Ended
Cathode-Bias: Push-Pull
Fixed-Bias: Single-Ended
Fixed-Bias: Push-Pull
Operating Classes
Open-Loop vs. Closed-Loop Performance
Noise: What It Is and Where It Comes From
Noise in Tubes

Chapter 5: Design Pool
Dissipation by Design
Circuit Influence
Tube Influence
Bias Adjustment Procedure
Single-Ended Design Procedure
Limiting Class-A Push-Pull Design Procedure
Class-A2 Design Procedure
Class-AB Design Procedure
Class-B Design Procedure
Transformer-Based Design Procedure
SE Transformers
Push-Pull Transformers
Interstage Transformers
Custom Transformer Specification
Output Transformers
Power Transformers
Transformer Shapes
Transformer Orientation
Design Examples
15 Watt Hammond 125E
25 Watt Hammond 1627SE
25 Watt Hammond 1628SE
25 Watt Hammond 1640SE
25 Watt Hammond 1650F
40 Watt Hammond 1650H
50 Watt Hammond 1650K
60 Watt Hammond 1650N
60 Watt Hammond 1650P
100 Watt Hammond 1650R
120 Watt Hammond 1650T
280 Watt Hammond 1650W

Chapter 6: Toroidal Designs
Balancing the DC Idle Currents
Four Toroidal Designs
PAT 4002 Amp
PAT 4004 Amp
PAT 4006 Amp
PAT 4008 Amp
High-Performance Diff-Splitter

Chapter 7: OTL Valve Amplifiers
OTL Principles
Output Impedance of OTLs vs.
Transformer-Coupled Amplifiers
Differential Amplifiers

Chapter 8: Hybrid Amps
Hybrid Amp Overview
Designing the Tube Front-End
Designing the Solid-State Output
The Mosfet Solution
Biasing the Output Stage
Power Supply for the Hybrid Amp
Tube Plate Supply
Heater Supply
Output Stage Supply

Chapter 9: Tube Amplifier Protection
Power Transformer Protection
Output Transformer Protection
Current-Sense Resistors as Fuses
Electronic Protection
Mosfets as Cathode Switches
Creating a Latching Circuit Breaker
Annunciating the Fault Condition
Other Do’s and Don’ts

Appendix A: Tube Data
5AR4, GZ34
5U4GB, GZ32
5Y3GT, 5Y4GT
6BQ5, EL84
6CA7, EL34
6CG7, 6FQ7
6L6GC, 5881
12AT7, 6201, ECC81
12AX7A, 7025, ECC83
6080WA, 6080, 6AS7G
Other Devices
6K11, 6D10
6N7/GT, 12N7/GT
6SL7/GT, 12SL7/GT
6SQ7/GT, 12SQ7/GT

Appendix B: Plitron Toroidal Transformer Data
Plitron Ultra-Wide Bandwidth Output Transformers
Plitron Output Transformers

Appendix C: Hammond Transformer Information
Hammond 125 Series – Universal Output Transformers
Hammond 1650 Series
Hammond Potted 1650 Series
Hammond SE Series Single-Ended Output Designs
Hammond 1600 Series Push-Pull Output Designs
Hammond 263 to 282 Series Replacement Power Transformers
Hammond 150/190 Series Filter Chokes

RBX Raw Bias Auxiliary Supply – When Do You Need It?

London Power Raw Bias Auxiliary Supply Kit
London Power’s RBX Raw Bias Auxiliary Supply Kit

Here is why some amps will need London Power’s RBX Raw Bias Auxiliary Supply along with a Power Scaling kit.

Most guitar amps that are fixed biased have very poorly designed bias supplies, barely adequate to support the tube grids connected to them. Few have enough control range to actually turn the tubes ‘off’.

Two Problems With Stock Bias Supplies

First, they are usually “high impedance”, which means they are derived from the plate winding through very high value resistances (100-220k) or through capacitors. A high-impedance bias supply cannot support proper bias-set networks nor will it support a bias regulator.

The second issue is that they lack enough voltage range to properly control all samples of tube that may be plugged into the amp. Resistively derived bias supplies can have this range, but will lack the current needed for a bias regulator. Decreasing the series resistance creates a high amount of waste heat, and the bias regulator could be damaged by excess voltage to its input. Capacitively coupled bias supplies are inherently limited in both their voltage range and current output.

A separate bias winding has the potential to be “low impedance” and also of high enough voltage to properly support a bias regulator. Marshall’s 1959 and 1992 models use bias windings of sufficient voltage to properly support a bias regulator, provided the stock series resistances in the supply are reduced to 470R each. Although many Fender amps have separate bias windings, these are all too low in voltage to support a Power Scale circuit. Hiwatt’s bias winding will work if the bias supply is rewired as a voltage doubler, which requires lifting the grounded end of the winding. See our book, The Ultimate Tone, Vol. 2 (TUT2) for details.

So, most fixed-biased amps will require RBX Raw Bias Auxiliary Supply to achieve proper performance from the SV1 or SV-TT Power Scaling kits.

Note that the very earliest Power Scaling kits included RBX along with a BMK Bias Mod Kit, but these add-ons are not required to achieve Power Scaling, nor are they universally missing or required with newer amps, so the kits were separated so that installers could buy just the parts they needed. This also made the basic Power Scaling kits much less expensive.

Rack-Mount Chassis from Hammond


London Power offers chassis to house your projects!

These rack-mount chassis are manufactured in Canada by Hammond Mfg., and come as “knock down” pieces that bolt together with four screws per panel. The rack mounting ears can be left off if the unit is to sit on a shelf instead of being mounted in an equipment rack. The external components are thick aluminum, with four steel corner inserts for support and attachment of the panels to each other.

Aluminum is easy to drill and cut holes into, so it is a perfect choice for hobbyists and amp builders requiring a cost-effective and versatile chassis choice.

1U rack-mount chassis

The top, bottom, sides and rack ears are finished in black textured semi-gloss powder-coat paint. Top and bottom panels are interchangeable and are ventilated. The front and rear are also interchangeable, one having smooth black paint and the other having smooth white paint.

The manufacturer produces six chassis heights, from 1U to 6U (we currently offer 1U and 2U), and three chassis depths (front to back). The standard depth is 13″ (330mm), but the 1U and 2U are available in a shallower depth of 8″ (203mm). The 1U to 2U range is the most useful to hobbyists and small builders, but contact us if you are interested in 3U to 6U models and we may be able to assist.

1U-8 – Single Rack Space Chassis 8″ deep
1U-13 – Single Rack Space Chassis 13″ deep
2U-8 – Two Rack Space Chassis 8″ deep
2U-13 – Two Rack Space Chassis 13″ deep

For application diagrams, see Hammond Chassis Applications.

Selecting a London Power Preamp Kit

Build a Stand-Alone Tube Preamp for Guitar or Bass

The London Power Preamp Kits series allows hobbyists to build a stand-alone tube preamp for guitar or bass. The PSU-PRE power supply supports up to four tubes and allows multiple preamp kits to be combined to create a multi-voice system. London Power’s Galactic Ground is used for low noise and low hum. For elaborate preamps incorporating up to 21 tubes, use the PSU-10 actively regulated power supply.

Each kit uses a silk-screened fibreglass-epoxy printed circuit board to make assembly quick. Tube sockets are high-quality ceramic. Kits include instructions, the printed circuit board (PCB), socket(s) and electronic components. Component packages include metal-film resistors, plastic and electrolytic capacitors and 16mm pots that accept push-on knobs.

All boards and assemblies will fit within a 1U rack-mount chassis height (we have these here). Tubes and PCBs can be mounted in any orientation in larger chassis.

Tubes, wire and chassis are not included, as each builder will have his own preferences and aesthetic requirements.

Pssst!… We now offer a variety of chassis suitable for preamp kits and other projects – see our Rack-Mount Chassis page, or view them under Building Supplies.

The Preamp Series Kits include:

London Power Preamp Kits
Preamp Kits by London Power

PSU-PRE – Power Supply for Tube Preamp

REV – LP-Style Preamp Loop

LINE – Dual Output Buffer

D-PRE – Mini-Marshmallow Preamp

F-PRE – American-Style Preamp

LP-PRE – London Power 2-Channel Guitar Preamp

M-PRE – British-Style Preamp

S-PRE – London Power Spectrum Bass Preamp

V-PRE – Vox-Style Preamp

Z-PRE – High-Gain Preamp Kit

Combine a tube preamp with PSU-PRE to make a stand-alone preamp.

Please also review the Safety page on this site.

Independent Reviews of The Ultimate Tone Book Series

The following are independent reviews of our The Ultimate Tone series of books by Kevin O’Connor. Our thanks to the reviewers for sharing!

“The Ultimate Tone – A Book Review of the Best DIY Guitar Tube Amplifier Series”

Published March 29, 2010
by Mark Douglas Roberts, tubenexus.com

See original review of The Ultimate Tone Series

This article is a book review on a series of six books that is the most comprehensive and lucid explanation of guitar tube amp architecture, circuits, tone, components and construction technique for DIYers I have ever found. The review deals with each book separately and recommends purchasing them in a specific order to grow your knowledge in step with your building experience.

Kevin O’Connor of London Power has created a series of books under the main title of “The Ultimate Tone.” These books are truly unique and carefully tailored for the Do-It-Yourself (DIY) guitar tube amp hobbyist and boutique amplifier builder. The books have a home-made ‘feel’ as well… all the illustrations are done by hand and the books are photocopy-printed on 8.5″x11″ paper and bound with plastic spines and clear plastic covers. There are six books in the series now with the most recent being released in the late spring of 2008.

You may want to buy the entire series all at once and get a modest savings, but I think you should consider buying them one at a time and digest as you go, building projects along the way. A key point though… you don’t necessarily want to buy them in numerical order. I recommend the following sequence:

The Ultimate Tone Volume 3 – Generations of Tone
The Ultimate Tone Volume 5 – Tone Capture
The Ultimate Tone Volume 2 – Systems Approach to Stage Sound Nirvana
The Ultimate Tone Volume 4 – Advanced Techniques for Modern Guitar Amp Design
The Ultimate Tone – Modifying and Custom Building Tube Guitar Amps
The Ultimate Tone Volume 6 – Timeless Tone Built for the Future Today

The Ultimate Tone Volume 3 – Generations of Tone

This is the most important book in the series for the beginning tube amp builder.

Chapters 2 through 4 lay down the foundation of good DIY tube amp construction, filling you in on good electrical connections, grounding technique, lead dress and other wiring techniques, and mechanical layout including assembly methods like terminal strips, turret and eyelet boards.

The subsequent chapters each pick a particular ‘iconic’ amplifier, each iconic amplifier being a prototypical example of amplifiers of its class, and they are examined in detail as to the circuit topology, peculiar tonal characteristics that result and fatal flaws that the product is notorious for.

Kevin provides the original schematics and then shows how you can apply the techniques detailed in the earlier chapters to improve the performance and reliability of the icon without harming the tone. The schematics are redrawn, layouts are provided and mechanical solutions are worked out to make each chapter a complete, self-contained, build-it-yourself amp project.

This process is repeated for several variations of the Champ in chapter 5 (this chapter greatly influenced my own single-ended amp project), then in quick succession: the Bassman, Plexi, 800, Bull Dog, AC-30, Portaflex, SVT, Bass Master, Custom Special, Guitar Mate, Herzog and Laney amplifiers are dealt with.

If you can only buy one book for your DIY guitar amp hobby I’d heartily recommend this one.

The Ultimate Tone Volume 5 – Tone Capture

Volume 5 picks up where Volume 3 left off, with a project-oriented approach and some sophisticated DIY tube amp solutions for guitar and bass.

The book starts off with a chapter overviewing vacuum tube operation called ‘tube tone,’ followed up by a chapter on guitar electronics and pickup characteristics.

The next two chapters are small projects: Sigma for effects switching and Triple-X for amplifier switching.

Chapters 5 & 6 are on transformers… important components but it made me yawn, sorry.

Starting with chapter 7, all the stops are pulled out and you are in project heaven… Major (200W), Soma 84 (EL84 amp), Standard (the London Power Standard Preamp from 1995 coupled to a 50W amp using four power tubes), Doppelsonde (mixing power tube types), AX84 (discussion on the original goal of a very low output power amp), Kelly (50W from 4 6V6s), and several other projects of lesser scope.

One favorite project I did was based on Kevin’s reworking of the HotBox tube preamp pedal from Matchless in chapter 16. I built this pedal in a truly “true point-to-point style” (meaning terminal strips) in a tube pedal enclosure from Doug Hoffman, substituting a Baxandall tone stack and reworking the preamp values to be more Dumble-esqe (non-HRM type).

What would you do to match an amplifier to Yngwie Malmsteen’s style? See chapter 18, ‘Swede.’

The Ultimate Tone Volume 2 – Systems Approach to Stage Sound Nirvana

Volume 2 is not project oriented. The bulk of the book, chapters 2 through 5, deals with power supply tricks and a comprehensive overview of power amplifiers, including tube, solid-state and hybrid power amps. Chapter 3, on tube power amplifiers, has some very practical information on mods and fixes to Marshall and Fender bias circuits.

I like the 1st and last chapters of Volume 2 the best. The first chapter is a short discussion of sound stages and how you might setup your gear on stage for the best audience/band experience. The last, chapter 6, is called “Pillars of Tone” and in this chapter the major contributors tone at the block-level of a guitar tube amplifier system design are discussed one by one and Kevin provides some very valuable insight into tone shaping throughout the preamp/amplifier.

The Ultimate Tone Volume 4 – Advanced Techniques for Modern Guitar Amp Design

This is the book you’ll want to buy if you feel the need to get deeply involved with the power scaling technology that Kevin has developed. Power Scaling, coined and trademarked by Kevin, is the way you can get aspects of power amp distortion (as opposed to preamp distortion) into your tone at bedroom volume levels. Volume 4 is not DIY project oriented but explores the issues, including attenuation, power scaling (both down and up), sag, and power management, tackled by modern guitar tube amp designers.

That said, the second-to-last chapter in Volume 4 might be important for a broader group of enthusiast builders… design philosophy. In this chapter Kevin provides a hierarchical design process that could be used to make key decisions on how you approach your next project.

The Ultimate Tone – Modifying and Custom Building Tube Guitar Amps

There is no volume number in the title of this book, it was the 1st. Personally, I bought it for completeness. I specifically wanted to have the ‘perfect effects loop’ information, although the loop itself is incorporated into a project in Volume 5. TUT also has some excellent material on reverbs and signal switching methods that is not explained in the other volumes. The first half of TUT introduces/overviews tube amp systems, power supplies & grounds then focuses on preamp and power amp modifications to commercial amplifiers (e.g. Marshall / Fender)… if you are totally new to tube electronics you may want to buy this 1st volume at the same time as Volume 3.

The Ultimate Tone Volume 6 – Timeless Tone Built for the Future Today

In many ways, Volume 6 is a continuation and extension of the material in Volume 4, where Power Scaling is introduced. In Volume 6 a new ‘direct control’ version of Powerscaling is featured which was introduced in Vol 4 but flushed out with comprehensive circuits and applied to ‘sag’ and sustain control as well in Vol 6. The new scaling circuits have many advantages for a DIY builder like greater noise immunity and less sensitivity to layout, etc.

I applied the new DC Power Scaling to a Trainwreck clone project and was really impressed with the improvement in ‘playability’ at lower volumes… the unmodified Trainwreck Express circuit is just too loud for domestic use, needing to be cranked to get the sweet tones it is renowned for.

One of the chapters in Volume 6 is dedicated to the Dumble amplifiers… something I was really looking forward to since many of my hobby projects focus on those circuits. I found this short chapter to be a good introduction to the overall architecture of the Dumble amps, written from the point of view of the evolution from the early modified standard amps that Alexander Dumble started out doing, but I felt the chapter fell short in discussing some of the more important subtleties of the later Dumble models.

Volume 6 also has lots of other material in it, including a great tutorial on designing really high output power amplifiers and a great chapter on high gain amplifier designs with real-world circuits referenced and detailed.

In Summary…

Kevin’s books have a very empirical approach. He encourages you to set aside convention in some instances or not be afraid to try combinations of tubes or even pulling tubes and in all cases clearly explains why it is o.k. and points out any reasons why it wouldn’t be o.k. All the examples in the books are very practical and he certainly has the DIYer in mind as he is writing.

Kevin’s body of work is truly encyclopedic in nature, and considering that, one feature sorely lacking from his books is any kind of indexing… this is aggravated by the fact that Kevin constantly refers to previous writings rather than repeat himself in a new volume, and it is very difficult to put your finger on the reference even with the other book in hand. Perhaps search engine technology, like Google’s ability to search protected content, could be put to good use in this case and provide a kind of ‘auto-index’ on the web of all of Kevin’s books without actually giving away the book itself. Or better yet, how about an e-book format of Kevin’s entire collection of TUT books… I think all of the e-book readers include searching capabilities… and Kevin’s hand-drawn schematics would probably scale adequately and be very readable on the e-paper displays these devices feature.

Meanwhile, how do you get Kevin’s books today? The best way to get the books is to directly order them from London Power Press. They now have a shopping cart on https://londonpower.com.