About Power Press’s Tube Guitar Amp Books (and more!)

Tube Guitar Amp Books (and more!) by Kevin O’Connor

Browse our Tube Guitar Amp Books

In electronics and audio design, many circuit methods and topologies are available for any given goal. Most books present a single solution to a design problem, but Kevin O’Connor’s books offer many possibilities.

Justified ApproachKevin O'Connor's "The Ultimate Tone" - open book

It’s nice for a builder or hobbyist to know the reasons for choosing one method over another. Like any electronics designer, Kevin O’Connor prefers certain circuits and methods. However, rather than simply giving instructions, Kevin explains why one method may be preferred over another. The selection of a method is up to you, and you can proceed on the basis of informed choice.

The Big Picture

Electronics is a very wide discipline; audio amplifier technology is just a small part of it. Approaches to audio design in the musical instrument (M.I.) field are very restricted – a limitation that the M.I. industry imposes upon itself, sometimes through simple lack of imagination. Fortunately, Kevin O’Connor’s books look past what is done to what can be done, offering the reader a glimpse beyond the state of the art.

Practical, Proven Solutions

Kevin O’Connor’s practical approach is based on real-world experience in repair, design and construction of all types of audio equipment. Tabled data in his books are the result of measurements of actual circuits. Design options are those used by Kevin over the past decades, and are “known-good” circuits.

All 11 books by Kevin O'ConorEasy Understanding

Electronics has its own language of schematics, symbols and mathematics. Hobbyists can find these intimidating. Kevin explains the essence of circuit operation in easy-to-understand language that conveys respect for the reader and the subject. We all learn together. Depending on where you are on your own path of enlightenment, Power Press has books that start right at schematic symbols; others that use text only; or most, that assume some schematic reading abilities.

Myths Debunked

Audio design is often clouded in “mystical secrecy” – particularly in musical instrument amplifier design circles. Kevin O’Connor’s books “clear the smoke away” and reveal the simplicity of most designs and modifications. The Ultimate Tone (TUT1) is the bible for boutique amp builders. Many have gained ideas and their courage from TUT1. A number of “reference” websites appeared with information from TUT repeated nearly verbatim, on the heels of TUT‘s release. The Ultimate Tone Vol. 2 (TUT2) is the unsung hero in Glass Audio magazine’s ‘Glass Shards’ column. Principles of Power (POP) is a popular reference cited in many audio power amplifier projects in Glass Audio (this magazine is now titled Audio XPress).

Continued Revelation

Kevin continues to write and publish information for hobbyists and amp builders alike. The open-ended TUT series continues. Specialty publications such as Secrets and Secret Holders (SSH) will continue. Kevin’s DIY Speaker Design for Musical Instrument Applications (SPKR) booklet allows individuals to build their own cabinets for guitar, bass or keyboard.

Bench-friendly Books, Packed with Information

Kevin O’Connor’s books are intended to be used as well as read. The reader is encouraged to build circuits from the books and “do-while-reading”. Each publication has protective clear covers and is cerlox-bound to lie flat on your workbench. Our pages have no large margins to waste space. There are no photos of amps, as these do not teach you how they work. There are no large advertisements for the author’s own products. There are no repeated legal disclaimers occupying valuable schematic space. There is just information – lots and lots of information! – in both text and schematic form.


Read a recent review of Kevin’s books by Mark Douglas Roberts of tubenexus.com.

Which Volume Should You Start With?

Check our page: Selecting a TUT Book.

Read.  Learn.  Enjoy.

Power Scaling Kit Selection

The Cutting Edge of Power Scaling Technology from the Inventors of Power Scaling

London Power's SV1 Power Scaling KitSelection Guidelines

The first selection parameter is how the amplifier is biased: cathode bias or fixed bias. In general, this resolves to the following kit requirements:

Fixed bias: SV1 + RBX

Cathode bias: SV2 + VCK

Switchable fixed/cathode bias: SV1 + RBX

The above selection covers 99% of all guitar amps, but there are a few exceptions listed below:

1 – Cathode biased with bias-modulated tremolo:

2 – Fixed biased with bias-modulated tremolo:

3 – Cathode biased but single-node supply:
SV84 + VCK

4 – If your amp is a Univox or Sound City:

5 – If your amp is MusicMan RD-series or Peavey TX-series:
SV2 with a modification to R12

6 – If your amp is Ampeg V9 or SVT, or Fender PS-series:
SV1 + SV3

7 – If your amp is a Sound City, or Marshall 250-475W, or Trace Elliot 300-400W:
SV-TT + SV3 for Full-PS

8 – If your amp is a Gibson G5, RV-series, or similar:
SV84 + VCK

9 – If your amp is actually a preamp:
SV84 or SV2

*Most bias supplies in fixed biased amplifiers are too low in voltage and/or too high in impedance to fully support the Power Scale regulators. For these amps, our RBX Raw Bias Auxiliary Supply should be used. It is never a mistake to use RBX. See the note below.

Note that the older products SB-1, SB-2 and SB-84 PCBs are still available for licensed users of these products.

When Do You Need the RBX Raw Bias Supply Kit?

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

There are two problems with these 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.

Second, these bias supplies 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, TUT2, for details.

So, unfortunately most fixed biased amps will also require RBX Raw Bias Auxiliary Supply to attain proper performance from the SV1 or SV-TT Power Scale kits. 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 only the parts they needed. This also made the basic Power Scaling kit much less expensive.

For a detailed selection procedure, see:
About Our Power Scaling Kits

For more information about Power Scaling, see:
Power Scaling Q&A

Who can install Power Scaling for you?
Licensed Power Scaling Builders and Installers

Power Scaling Kits: About

Learn about Power Scaling from Its Inventors

Browse our Power Scaling products.

The newest term in Power Scaling is “Super Versatile” (SV).

Our SV-range of Power Scaling kits represents a breakthrough in performance and value, combining the sophistication of our former “Super Flexible” kits with the cost-performance of the “Super Budget” kits – so SV also means “Super Value”.

The majority of guitar and bass amplifiers (>99%) have nearly identical plate (Va) and screen (Vs) voltages that are separately filtered and most operate at <550Vdc – with the bulk of these below 500V. These amps can be described as having “dual-node” output stages as far as their power supplies are concerned, and they require two paths of high-voltage control to Power Scale them. Most of these amps will be fixed biased, but the high-voltage description here also covers most cathode biased amps.

The voltage limits for the SV-kits are:

SV1 <700V Va and Vs
SV2 <550V Va and Vs; can be supplied with 700V limit
SV84 < 450V Va
SV-TT <900V Vs
SV3 <900V Va and Va-Vs

There are dual-node supplies generated by voltage doublers, with Va being twice as high as Vs. These can be described as “disparate-node” supplies. Lower-wattage amps (50-120W) like the MusicMan RD-series, Sound City and most Univox amps have Vs of 250-350V and Va of 500-700V. Larger amps like Ampeg’s SVT and V-9 (300W), and Fender’s PS-series (300-450W) have Vs of 350-450V and Va just under 800V. The smaller of these amps (<120W) requires the use of two kits to achieve full Power Scaling. Ironically, it will tend to be the lower-wattage amps that get Power Scaled. All production amps in this range are fixed biased.

There are tiny amps and simple amps that use the same supply node for both plate and screen. This group includes ultralinear-wired, triode-wired, and true-triode amps. These supplies are “single-node” and require a single, high-voltage regulator path. These amps may be fixed biased or cathode biased. SV84 is the choice for most of these amps, which tend to be <450V.

All of these supply arrangements and their families of amplifiers can be Power Scaled using London Power’s “Super Value Super Versatile” SV Power Scale kits.

The Two Classes of Power Scaling

London Power's SV1 Power Scaling KitSV1 Power Scaling Kit
Power Scaling itself can be divided into two classes: Full Power Scaling (Full-PS) and Two-Thirds Power Scaling (PS-TT). In Full-PS wiring, the Power Scale regulator takes up the heat that would otherwise have been dissipated by the tubes, extending tube life in the process. Many designers shy away from trying to manage this heat and attempt to attain power control without full control over the tubes. They introduce errors of design in their economic choice, eroding dynamic performance of the amp even at full output. London Power’s Power Scale circuitry retains dynamic response for both Full-PS and PS-TT methods. Any of the standard SV1, SV2 and SV84 Power Scale kits can be wired for either Full-PS or PS-TT, and can be used within their voltage limits. The SV-TT kit is optimised for high-voltages of 600-900V. Note that in a single-node amp, SV-TT can be wired for Full-PS or PS-TT operation. In very high-power amps, SV-TT can be combined with SV3 Tracking High-voltage Regulator for Full-PS wiring.

London Power's SV-TT Power Scaling KitSV-TT Power Scaling Kit
All kits require proper heat-sinking, usually accomplished by mounting the pass elements to the chassis. For PS-TT wiring, this is ample for amps up to 700W output. For Full-PS, high-bias amps and high-power amps will require extra heat sinking and/or fan cooling. All kits use a Drive Compensation control to assist in keeping tone exactly where the player needs it, which also allows four operating modes of the amp. The SV Power Scaling kits all use printed circuit boards – with small boards for the pots to make wire connections easy – and all pots are 16mm. For more information, our book TUT4 provides a complete tutorial on Power Scaling methods, while TUT6 expands on the discussion of DC Power Scaling.

Note that the SB-1, SB-2 and SB-84 PCBs are still available for licensed users of these products.

Like all of our other kits, London Power’s Power Scaling kits use high-quality, reliable components, including precision metal-film resistors, metal-oxide power resistors, long-life electrolytics, ceramic and plastic caps, all on a high-quality printed circuit board. Pots are 16mm non-critical types of standard values. Semiconductors are used well within their limits but proper heat sinking is required, as explained in the kit notes. Mounting hardware is included.

For more information about Power Scaling Kits, see also:

Selecting a Power Scaling Kit

Power Scaling Q&A

Who can install Power Scaling for you?
Licensed Power Scaling Builders and Installers

Other Retailers of Our Books

These fine businesses carry our books by Kevin O’Connor!


Antique Electronic Supply
6221 South Maple Avenue
Tempe, AZ
85283   USA
Tel: 480-820-5411
Website: www.tubesandmore.com



Bob Pearman Books
8 Ffolkes Place
Runcton Holme
Kings Lynn
PE33 0AH    UK
Tel: 01553 810673
Website: www.pearman-books.com



The Tube Store
120 Lancing Drive, Unit 8
Hamilton, ON
L8W 3A1
Tel: 905-570-0979 or toll-free in North America 1-877-570-0979
Website: www.thetubestore.com

Power Scaling Builders & Installers


These amplifier manufacturers are licensed by London Power to incorporate Power Scaling into their amplifier products, and to use the terms “Power Scale” and “Power Scaling.

POB 176
Parksville, BC V9P 2G4
250-954-2292 10am-5pm M-F Pacific Time
Mark Stephenson was the first licensed Power Scaling amp builder. His line ranges from his Power Scaled 1W Stage Hog to 40W heads, and pedals like the FMD booster and an amp switcher.

British Columbia

Vancouver, British Columbia, Canada
email: info@oxygenamps.com
phone: (778) 808-587

Lake Elsinore, California
www.suhrguitars.com (951) 471-2334 (Tues-Friday 9-5)
The Suhr Badger amp has been exciting players for quite a while – check it out.

Cincinnati, OH

Tel: 808-292-8331

New Mexico
Carl Carrasco builds custom amps with Power Scaling

Rhode Island
Manny Bettencourt is a knowledgeable and friendly East-Coast amp builder who can repair, modify or build you an amp at a reasonable price. He is also a Power Scale installer.

Maywood, CA

North Highlands, CA

Martin Whelan at MJW Amplification in North Derbyshire is your source for service, modification and design of tube sound equipment, and for Power Scaling builds/installations.

Andy Dokken modifies Valve Jr amps, adding Power Scaling and other mods. He sells only through eBay.

Offers a kit/prebuilt model with stepped Power Scaling – a modest 10dB loudness reduction at the lowest setting.

EUROPE – Belgium
Geoffrey Herbrand rtb@gmail.com

EUROPE – Spain
Mario Punsola builds beautiful amps, cabinets and other valve equipment in Cardedeu, Barcelona, Spain, with models including Power Scaling. He also does repairs,maintenance and modifications.

EUROPE – Sweden
Stockholm, Sweden
Our friend Ola Lehnert produces wonderful Lehnert combo amps with Power Scaling – built with the player in mind. He’s also a Power Scaling installer.

Alvorada Rio Grande do Sul
Luciano Silva builds amps that capture the British vibe but with modern playing requirements, such as effects loops and Power Scaling. He also does repairs and mods.


Looking for someone to install Power Scaling in your amp? Here is a list of recommended Power Scaling installers.

Cory Davies (Davies Amplification)
British Columbia

Brian Ohman (Tube Amp Technologies)
Central Ontario
Tel: 705-833-1234

Rick Hayward (Hayward Amps)
St. John’s, Newfoundland

Mike Putnam (Putnam Guitars Inc.)
New York

Manny Bettencourt (Circuitrix)
Rhode Island

Brad Weeks,
NW Chicago, IL
Friendly, honest, professional mods and Power Scale installation

John Suhr (Suhr Guitars)

Albert Rivera (ACR Amplification)
Tel: 808-292-8331

Frank Lamarra (Amp Lounge)

Kevin Silva (Uncle Albert’s amp repair)
Tel: 800-416-2444 unclealberts2@juno.com

Europe – UK
Shane Dolman
West Midlands, Great Britain

Martin Whelan (MJW Amplification)
Derbyshire, England

Andy Dokken (Rat Amps)

EUROPE – Spain
Mario Punsola (MPF-Sound)
Cardedeu, Barcelona, Spain

EUROPE – The Netherlands
Ed Thoen (Ed’s Custom Shop)
Maarssen, The Netherlands

EUROPE – Sweden
Ola Lehnert (Lehnert Amps)
Stockholm, Sweden

Martin Kuhn (Kuhnamps)
Järna, Sweden

Kalle Norman (LH Musik & Audio AB)

Alvorada Rio Grande do Sul
Luciano Silva builds amps that capture the British vibe but with modern playing requirements, such as effects loops and Power Scaling. He also does repairs and mods.

Would you like to be part of this list? Contact London Power.

Sag/Sustain Kit Selection

What is Power Supply “Sag”?

“Sag” is a well-known power supply effect caused by resistance in the supply and variable current loading. As an amplifier pulls more current from the supply, the supply voltage drops, or sags, and this can cause a compression effect. The transient or attack portion of a note is “squashed” or limited by the falling supply voltage, and thus the attack part of the signal envelope blends with the sustained portion. Sonicly, we perceive this as a rounding or smoothing of the tone as the attack portion of the signal envelope has been cut down to be the same level as the sustain portion of the envelope. Since the attack is no longer abrupt, you might feel that the “edge” has been removed.

SUS-1 Sustain Enhancing Kit for AmplifierIn compression terms, this is “limiting” or “soft compression”.

If you push the amp further, so that the whole signal is clipped as the amp runs out of headroom, the supply stays sagged and the signal is “hard limited”. This sounds brighter than the soft limiting but is a different tone than the original unlimited sound from the non-sagged amp and supply.

Note that the sustain effect is subtle.

If you have Power Scaling installed in the amp, then the Power Scale controls will let you dial the amp down to its compression point quite easily. In a non-Power Scaled amp, or in a preamp, the sustain effect can be attained using London Power’s  SUS-kits, which provide an identical sound using a compression side-chain circuit that samples voltage instead of current.

London Power’s SUS-1, SUS-2 & SUS-3 Sustain-Enhancing Kits

These sustain-enhancing kits use non-critical 16mm pots and other components to achieve the desired compression effect. An amplifier with any power rating can be fitted with one of the SUS kits.

SUS can be used with any non-Power Scaled fixed-biased amplifier. Our RBX Raw Bias Auxiliary Supply Kit may be required for proper functioning.

SUS-2 can be used with any cathode-biased amplifier, regardless of whether the amp is Power Scaled, and can also be used in preamp circuits.

SUS-3 is for fixed-biased amplifiers fitted with Power Scaling, or a fixed-power amp that has our TBS Tracking Bias Supply Kit for Fixed Bias installed.

For Power Scaled amplifiers:
If fitted with our SV1, use SUS-3 to add sustain.
If fitted with our SV2 or SF-2, use SUS-2 to add sustain.
If switchable between fixed-bias and cathode bias, use SV1, plus SUS-3 for sustain.

For non-Power Scaled amplifiers with our TBS Tracking Bias Supply Kit for Fixed Bias installed:
Use SUS-3 and RBX For non-Power Scaled amplifiers without TBS: Use SUS-1 and RBX.

Note: Proper functioning of the SUS kit with the fixed-bias Power Scale kits or TBS may require the addition of the RBX to assure function of SUS all the way to full output power. RBX provides a much higher raw bias voltage than most stock bias supplies, and at a much lower impedance. Both attributes are important for proper functioning of the SV1 and TBS, but becomes more important with the addition of sustain.

About Our Switching Kits

Switching Kits by London Power – Human-Scale Technology

London Power‘s range of switching kits allows the player, hobbyist or builder to access amplifier channels, amp features and system configurations, using simple circuitry. All of our switching circuits are discrete so they can be repaired by anyone with average skill, and can be maintained for decades of use. There are no computers to go obsolete and no software that can be “buggy” or need updating. Just simple circuits built with noncritical components for which there will always be a compatible sub in the future.

The actual switch elements used in the audio path can be n-channel jfets, relays, BJTs, mosfets or CMOS gates. We have left out p-channel jfets as their availability and performance for switching is limited.

Most switching applications are very simple: turn a reverb or effects loop ‘on’/’off’; select a gain boost element; select an alternate ‘volume’ setting, and then there are complex requirements. For the latter, it is actually a simple matter to use jumpers, diodes or DIP switches to provide easy “programming” that can be changed at a later date. The DIP-switch option that we provide is a high-quality type rated for 10,000 cycles – ten times the usual rating for such switches.

System Voices

Preamp channels are generally used “exclusively”, meaning only one at a time. Features built into an amp, such as reverb or an effects loop, can be preset as always-on, or have channel-synchronized control. Features might be used in combination with each other, as well. The net combination of a preamp channel and other features is a “voice”.

Poorly designed switching systems tend to be of the type where each feature has its own switch along with a switch to select the channels. This is a nightmare for the performing or recording musician, as he may have to hit multiple footswitches to go from one voice to another. Ideally, each voice would have some sort of “programming” or “synchronization” of the channels and features, settable by the player. In the ideal case, hitting a single switch selects the chosen channel and chosen features all at once. Hitting a different switch would enable a different combination of channels and features, and so on.

In a guitar system, even one that is just a single amplifier and speaker, the number of “voices” will determine the number of switches required at the player’s feet. A single-channel amp has one voice and nothing to switch. Add a reverb to this amp and we have one thing to switch, so there is one footswitch to select the reverb status. The amplifier now has two voices: the dry amp sound and the amp sound with reverb.

A 2-ch amp has two voices and only requires one switch to select which voice is live. Mechanical switches have two states, being ‘open’ or ‘closed’, the same as ‘on’ and ‘off’. Those two conditions are “exclusive” as they cannot occur at the same time. Just as the reverb can only be on or off, or only one of the two channels can be on at a time, the exclusive nature of the switch allows us to use a single switch to go between two choices.

If we add reverb to the 2-ch amp, we suddenly have four choices: ch-A, ch-A with reverb, ch-B, ch-B with reverb. Having just a channel switch and a reverb switch as most designers and amp manufacturers provide is not the best approach for the musician. There will be times when the player has to hit both buttons at the same time to go from one sound to another. Here, we need a separate switch for each of the four voices. This allows direct-random-access and only one button needs to be hit to go from any one voice to any other voice.

A three-channel amp requires three switches to allow selection of any voice in a single switch transaction. From here on we invoke “direct random access” as an attribute of the switching system that makes player ergonomics simple.

Two-way Communication

The amplifier should normally be able to work independently without requiring a footswitch be plugged in. However, there are many three-channel amps on the market that cannot operate this way, where only two channels can be selected using front-panel switches. In these amps, to access all three channels the player must use the footswitch. This is not exactly good ergonomic design, and is entirely an economic choice made by the designer.

Most amps that do have footswitch capabilities and a properly capable front panel miss out on another ergonomic detail: “two-way communication”. In typical amps like this, the footswitch is configured to over-ride the panel controls. LEDs will show what has been selected at both the footswitch end and the amp end, but only the footswitch controls anything. In the rare case where the amp’s panel switches are still active with the footswitch plugged in, the LEDs on the amp may not indicate the same amp status as the LEDs at the footswitch if panel switches are used to make the selection.

Two-way communication between the amp and the footswitch assures that whatever is being indicated as being active at the footswitch end is the same indication at the amp end. Using a panel switch or a footswitch changes the indication at both ends of the system. Such a system can be expanded further to allow multiple footswitch locations, as might be required on a large stage. With 2-way communication, actuation of any switch at any location causes all locations to indicate the new system status.

None of this is rocket science, or even computer science; it was all being done before there were computers built into everything.

From the Simplest…

London Power offers two approaches for simple exclusive selection and switching: the RLY mechanical relay, and ERK Electronic Relay Kit.

RLY is simply a DPDT relay on a small board. The installer has to provide power for the relay coil and his own switch to actuate the relay. This is “stone age” technology that anyone can understand. There can be a LED on the footswitch or not, and a LED on the amp or not. There can be a panel switch or not, which can be over-ridden by the footswitch or not. You see, even with such a simple switch-plus-relay there are many options.

ERK is slightly more sophisticated using jfets wired for shunt switching as the switch elements. This means the jfets are tied to ground, which is the quietest switching form and is suitable for use with our LP-PRE 2-ch tube preamp kit, and similar circuits. ERK has options for one or two LEDs at the amp end but no footswitch LED. The optional foot switch available for ERK is a standard push-on/push-off alternate-action type with a large actuator. Players or builders can buy ERK without this switch and save half the cost of the kit providing their own.

…to Intermediate…

Many players prefer to have independent control over the functions of their system, even if this might mean that sometimes they have to hit two buttons to go from one sound to another. This depends on the playing style of the individual. For these intermediate-complexity systems, we have a sophisticated solution: DFC Dual Function Controller.

DFC uses non-latching normally-closed push buttons just like the Q-system below, so DFC can aesthetically match the Q-system. DFC uses electronic latching circuitry to allow any style of switch to be used from very small to very large – for finger tips or feet. Each half of DFC can control relays, shunt-jfets and series-jfets. The series-jfet output can also control grounded-source power mosfets as high-voltage switches for selecting tubes, for example. Each half of DFC is independent and not effected by the other, so the player has independent control over each feature of his sytem.

Each half of DFC also has double-indication of its status. This means that the nominal-off state and nominal-on state both have a LED that lights to show that condition. This is great to have when one section is used to switch between two preamp channels. One LED shows ch-1 is active while the other shows ch-2 is active. The LEDs can be individual as for this application, or they can be combined as a 2-colour 3-lead type. Of course, you can use a single LED or no LED per half of DFC, so anything up to four LEDs can be driven by the circuit – two on at a time. Current-steering allows the circuit to draw only the current that would be used by a single LED.

For players who like the large ERK-style foot switch (FSW), these can be wired for momentary or ‘pulse’ duty, as shown in our book The Ultimate Tone (TUT) fig. 9-77. These can be applied to the Qsystem as well.

For about the same cost as two ERKs, DFC provides three-times the output switching cability and more options for the actual switching devices.

…to Unlimited Sophistication

The Q-system family of switching products provides direct random access for any number of voices, as well as potential for two-way communication between two or more switch stations.

The first building block is Q-LATCH , which provides an interface for up to four switches – either panel or foot – with auto-canceling of previous selections and latching of the new selection. Multiple Q-LATCHes can be ganged “side-ways” for more than four voices to be selected, or “daisy-chained” for 2-way communication between as many stations as you need. Of course, sideways expansion and daisey-chaining can be done simultaneously just by adding more Q-LATCHes.

At the amp end, a Q-LATCH (or a few) can be used for the local voice selection. To control relays, Q-RLY is added to control up to four relay groupings. These typically control features or channels. The relay control groups can be linked to be activated by any or all of the four voice lines from the Q-LATCH. If there are more than four voice lines, the voice expansion card Q-VX is added to allow the voice control lines of the added remote Q-LATCH to access the blocks of the Q-RLY. Note that Q-VX is not needed at the amp end if there are Q-LATCHs at the amp end.

Similarly, if jfet control of features or signals is required, Q-FET has four blocks to do just this. Each block can provide the correct control voltage range for shunt or series switching. Each block can be accessed by any or all of the four voice control lines, and with Q-VX can be controlled by more than four voices. Q-FET provides a shunt control line and a series control line for each of its four blocks.

Many amps will have few things to switch, so we also offer a single relay controller as Q-SR, and a single jfet controller as Q-SF. Q-SF has a shunt control line output and a series control line.

Lighting the Way

ERK has the ability to drive one or two LEDs, or to drive a 3-lead bicolour LED. ERK is an exclusive switching system, and thus has two states, both of which have positive indication if the builder wishes to incorporate it.

RLY has no inherent or in-built LED drive or indication of its states. The builder must add his own LED driver with current limiting for the LED.

DFC has positive indication of both states for each of its two independent control blocks.

Q-LATCH has drive circuitry for local LEDs to indicate which voice is active. LEDs can be wired directly to the board, or the Q-LED board can be used, most often in combination with Q-SW as a panel selector.

Q-RLY and Q-FET also have voice line LED driver circuits. Where one might decide to have the “smart footswitch” with Q-LATCH and a “dumb amp” with no panel switches, it is still desired to have LEDs on the amp to indicate which channel has been selected even though there are LEDs on the footswitch.

For simpler systems, Q-LDV LED driver can be used to control the amp’s panel LEDs to display the voice selected. If one chooses, the LEDs driven by Q-SR and Q-SF show that the feature is active. When the voice indication “overlaps”, as in the case of a 2-channel preamp with one extra feature, two of the four voices engage ch-1 while the other two voices engage ch-2. It is typical here to place a LED beside each channel’s controls to show which is active. An additional LED can be placed by the FX loop controls to show when it is active. There are two ways to deal with this situation as the downloadable PDF describes.

Q-RLY, Q-FET, Q-SR and Q-SF also have “function” LED drivers, as the control blocks do not necessarily relate to voices. For example, one control block might be for channel-1, another for channel-2, another for reverb and the fourth for an effects loop. We might wish to have a LED light when the reverb is active and one for when the loop is active, or for whatever the function is.

The Q-system accommodates whatever one might need, and can be easily reconfigured if you change the amp or other parts of the system.

For more information, download our informative PDF: London Power Switching Kits Selection & Application (135 kB, 22 pages).

TUT Book Selection

Which “The Ultimate Tone” Volume to Get First?

TUT-DealTUT – The Ultimate Tone, is an ongoing series of distinct books about guitar amp technology with a heavy focus on tube circuitry. Currently there are six volumes. You can buy the whole series as a package (TUT-DEAL) or buy just one or two at a time. But… which one should you get first? This depends on your skill level and your particular interests regarding the format of the amp and its features. Note that the first volume of the series does not have a number and is simply TUT1; subsequent volumes have a volume number, as TUT2, TUT3, etc.

TUT - The Ultimate Toneschematic – a drawing using symbols to represent different components which shows how those components connect together; akin to reading music
layout – a drawing showing how the physical parts should look when connected together correctly; akin to reading tab (tablature)


If your skill level is “beginner” (you cannot fully read or understand schematics, but you can follow a layout drawing to build something):

Evolving-skill recommended reading order: TUT3, TUT5, TUT2, TUT, TUT4, TUT6

If your skill level is higher, reading order depends more on your interests. See below.

If you are eager to build an amp that has the lowest noise and best note articulation, regardless of your schematic reading skill, before you build an amp you should definitely read TUT3 – it is the best reference for correct wiring methods, proper layout, lead dress, wire and fuse sizing, and mechanical and thermal design. TUT3 and TUT5 both have layout drawings as well as schematics, so these make a great package for hobbyists who are eager to build. Which volume you buy next depends on your skill and interests:
“Eager-to-build” recommended reading order: TUT3, TUT5, TUT2, TUT, TUT4, TUT6

If you can read a schematic and understand how electronic components function, and want to learn how guitar amps work:

Reading order for:
How amps work:
Preamp design: TUT, TUT5, TUT6, TUT2, TUT3, TUT4
Effects loop interest: TUT, TUT2, TUT5, TUT3, TUT4, TUT6
Channel switching: TUT, TUT2, TUT5, TUT3, TUT6, TUT4
Power amp design: TUT2, TUT5, TUT6, TUT4, TUT, TUT3
Power Scaling interest: TUT4, TUT6, TUT5, TUT2, TUT3, TUT
Power supply design: TUT2, TUT4, TUT, TUT3, TUT6, TUT5
Super Scaling interest: TUT4, TUT5, others any order
GMX interest: TUT4, TUT3, others any order
Design philosophy: TUT4, TUT3, others any order
Biasing methods: TUT2, TUT4, TUT3, TUT, TUT5, TUT6
Low-noise wiring and grounding: TUT3, TUT5, TUT, others any order
AC-30 amp issues: TUT3, TUT, others any order
Ampeg amps interest: TUT3, TUT, others any order
Fender amps interest: TUT, TUT3, TUT2, others any order
Hiwatt amps interest: TUT3, TUT2, TUT6, others any order
Marshall amps interest: TUT, TUT3, TUT2, TUT6, TUT4
Laney amps interest: TUT3, TUT6, TUT,  others any order
Traynor amps interest: TUT, TUT3, others any order
Bass amp interest: TUT3, TUT2, TUT6, TUT5, others any order
Dumble amp interest: TUT6, others any order

The recommended reading orders above reflect a “density of information” about a given topic within each book. Some topics are discussed in many places within a book volume, or in many volumes, whereas other information may be packed into a chapter or section of its own.

We hope you enjoy the series!

Amp & Gear Ordering

Made in Canada

London Power‘s line of pre-built equipment for studio use is produced to order on a short turn-around cycle. All units are hand assembled and hand soldered, and then rigorously tested and burned in. Because this takes time, we prefer to discuss the needs of each client prior to accepting the order.

Contact us at amps@londonpower.com or 807-473-0952 with your purchase inquiry, and we can discuss your needs and determine the shipping cost to your locale prior to processing the order.

Prices are in Canadian dollars.
GOOD NEWS – Our prices are in Canadian dollars (CAD), and are economical compared with US dollars (USD).
At present (Spring 2020), $1 CAD is worth approximately 75 cents US.

How to Pay:

Contact us via phone or email to find out the ship rate to your locale and the order total.


We accept payment by Bank wire transfer. There is a $14 bank fee for the wire transfer (our banking fee, passed on to you).
The received amount in Canadian funds must be the amount for product plus shipping, plus $14.
A shortage in your payment will delay processing and require a further transfer. Your bank will make the currency exchange between your local currency and CAD$.
Contact London Power for bank transfer info.


For Canadians only: you may use Interac e-Transfer once the product cost, shipping and applicable taxes are determined.

What’s Included with Your Amp:

Preamp tubes are included for all units that use them, including our STUDIO, KC25, 3-D T, 3-D TL, All-Tube Bass Preamp.
Power tubes are NOT included, as the user is encouraged to experiment with various types and combinations.


Price List SPRING 2020

Product$ CND(Equiv. in USD)
ATBP All-Tube Bass Preamp2,640Approx. 2,110 USD
STUDIO All-Tube Power Amp2,820Approx. 2,260 USD
KC25 All-Tube EL-84 Power Amp2,820Approx. 2,260 USD
3-D S Solid-state Reverb with LEDs2,010Approx. 1,610 USD
3-D SL Solid-state Reverb with LEDs2,010Approx. 1,610 USD
3-D T Tube Reverb, no LEDs2,430Approx. 1,950 USD
3-D TL Tube Reverb with LEDs2,430Approx. 1,950 USD
ICON Six-Channel All-Tube Guitar Preamplifier2,840Approx. 2,280 USD
SUPER-STANDARD Vacuum-Tube Guitar Preamplifier2,130Approx. 1,710 USD
MINI-MARSHMALLOW Vacuum-Tube Guitar Preamplifier2,130Approx. 1,710 USD
FS4-2W Footswitch for Pro Gear360Approx. 290 USD
1U6-BRKT Rack-Mount Bracket for 1U Units39Approx. 30 USD
2U6-BRKT Rack-Mount Bracket for 2U Units45Approx. 35 USD


Back to Amps & Gear.

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