About Our Kits

London Power’s kits allow the hobbyist or amp builder to add features to an existing project or to build a new project from scratch. Each kit contains the circuitry relevant to itself. For example, a preamplifier kit contains the circuitry that makes up the audio signal path ONLY; the kit will require a power supply  to support it.

All of our kits use high-quality components, including: metal-film 1% tolerance resistors; flame-proof metal-oxide and wire-wound resistors where required; long-life electrolytic capacitors; plastic and ceramic capacitors; select semiconductors; ceramic tube sockets; industry-standard jacks and pots. The pots and jacks have PC-pins and we include small PCBs to mount them on and for the wiring to be easily attached to.

Our printed circuit boards use twice the industry standard for copper thickness to assure reliable and reworkable connections. For some kits, you can order just the PCB and notes, or also the “hard to get parts” unique to the board. For either of these variations, the pot and jack boards are not included.

Kit notes include the schematic of the kit, the board layout, installation and mounting instructions. Some kits include mounting hardware if there is a single way to mount the board. Kit notes are only available with the kit. For tutorials on how a given kit or circuit functions, refer to our books.

The product description for each kit contains dimensions for the PCB, dimensions of the assembled kit, and some information about the kit’s function. Tube-based kits DO NOT include the tubes. Tube-based kits can usually be mounted two ways and the PCBs are silk-screened on both sides of the board. A special mounting instruction note is included with these kits.

The assembler must provide wire, solder and a chassis to support the kit safely, as required or as relevant to the kit.

General Component Selection Guide
Resistors are the most common component in most linear circuits. In a tube circuit, plate resistors and supply decoupling resistors should be 5-600mW and ideally rated for the supply voltage present – yes! resistors have a voltage rating. Cathode- and grid-circuit resistors can be 0.125W  or higher, most often 250mW for cost and availability. The PCB footprint for these low-wattage positions will range from 0.2″ long to 0.4″, although there may be occasion where the leads are left longer to span traces on the board.

Power resistors are metal-oxide or wire-wound and flame-proof. These will be 1W and up, rated for 500V and up.

Filter capacitors and plate coupling caps must be rated for the supply voltage present. Note that in  the case of a direct-coupled cathode-follower and preceding gain-stage anode, the caps here are rated for the supply voltage present. All other capacitors are rated for the signal size they must face: EQ caps should be 100V or higher; grid-circuit caps can be 50V and up; cathode caps only see the cathode voltage, so typically 6V or more for preamp stages but 16V or higher for EL-84 and 63V or higher for 6V6 and other power tubes. Grid-circuit caps in fixed-biased amplifiers should be rated for the sum of the bias voltage plus the driver plate supply.

Note that electrolytic caps do not require derating as they are listed with WORKING VOLTAGE. Similarly, plastic film and mica caps do not require derating. Ceramic caps can be used close to their rating unless the circuit is a switch-mode type, where the cap must be rated for two to three times the voltage present to exhibit its rated capacitance at the switching frequency and its harmonics.

Diodes should be rated for the voltage of the circuit they are in. Note that a centre-tapped power transformer plate winding has an end-to-end voltage that the half-bridge diodes must fully block. For example, a 300-0-300Vac winding is 600Vac total, or about 850V peak. A 1kV diode will be marginal and is likely to fail given that the unloaded transformer voltage can be 10-20% higher. In this application, higher-rated diodes are required, or diodes wired in series for each position.

Similarly, transistors and other semiconductors can be used close to their voltage limits provided their safe-operating limits are respected, i.e. keep current and heat low. Mosfets are particularly sensitive to being over-heated despite the claims regarding their robustness.

Fuses need to be derated by 25% typically to account for self-heating and ambient effects, and to avoid nuisance fuse failures.

Wire should have a voltage rating appropriate to its position in the circuit, and its current capacity should be similarly appropriate. Hobbyists and some builders often waste money on very heavy wire where the current is exceedingly low.

TUT3 (The Ultimate Tone volume 3) expands on all of the above and details proper layout and wiring methods for lowest-noise and best note articulation. The Galactic Ground Method shown in TUT3 is universally applicable to all technologies and to all construction formats.

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

Selection 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 grid-modulated tremolo:

2 – Fixed biased with bias-modulated tremolo:

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

4 – Cathode-biased with single-node supply and grid-modulated tremolo:

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

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

7 – 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 to be 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 using RBX-LT. Although many Fender amps have separate bias windings, these are all too low in voltage to support a Power Scale circuit, as are Hiwatt’s bias windings. These need to be fully lifted from ground and wired using a voltage doubler. 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  Power Scale kit. 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.

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 highest-power amps tend not to be Power Scaled as the users of such amps have no interest in playing quietly.

To accommodate this range of amp voltages, particularly the screen, SV boards provide an easy means for matching the stock ratio of Va and Vs by a simple resistor change. The kit notes explain what is required. let us know the amp model you are installing the kit into, and we can provide a custom resistor value.

SV1 is the Power Scale solution for fixed-biased amps. In most cases, the amp will also require RBX Raw Bias Auxiliary Supply as the stock bias supply may not support SV1.

SV2 is the Power Scale solution for cathode-biased amps. These will also require VCK Voltage Clamp Kit to maintain preamp voltages and tone.

The voltage limits for SV1 and SV2 are:
Va <700V
Vs 4V to (Va-4V)

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. For the rare single-node fixed-biased output stage, add TBS Tracking Bias Supply and RBX Raw Bias Auxiliary Supply to SV84.

The voltage limit for SV84 is 450V.

SV1 and SV2 have recently been made about one-third smaller than their original form, allowing them to be fitted into any amp that needs them.

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.

SV1 Power Scaling Kit
All kits require proper heat-sinking, usually accomplished by mounting the pass elements to the chassis. 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 the links below and visit our forum theultimatetone.com

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
[email protected]

Vancouver, British Columbia, Canada
email: [email protected]
phone: (778) 808-587

Lake Elsinore, California
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
[email protected]

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.

Bruce Egnater
Maywood, CA

Ben Fargen
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
Geoffray Herbrand [email protected]

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
[email protected]

Brian Ohman (Tube Amp Technologies)
Central Ontario
Tel: 705-833-2727
[email protected]

Rick Hayward (Hayward Amps)
St. John’s, Newfoundland
[email protected]

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
[email protected]

John Suhr (Suhr Guitars)

Albert Rivera (ACR Amplification)
Tel: 808-292-8331
[email protected]

Frank Lamarra (Amp Lounge)

Kevin Silva (Uncle Albert’s amp repair)
Tel: 800-416-2444 [email protected]

Europe – UK
Shane Dolman
West Midlands, Great Britain
[email protected]

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

London Power's QRK-LATCH Quad Voice Controller for Guitar Amps

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 [email protected] 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
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


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