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
SV1 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 SV84Power 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.
SV-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 SVPower 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:
London Power was founded in 1990 by Kevin O’Connor, after two decades of research into innovative audio amplification techniques. Power Press Publishing is its book publishing arm.
Kevin is an audio designer, author and speaker, best known for his proprietary methodology called Power Scaling. He has written 11 books on audio subjects. Whether you seek amps, kits or information, you’ll find useful answers here: not just the “how” but the “why” of audio amplifier design.
London Power is located near Thunder Bay, Canada, in the beautiful boreal forest. Our website offers a blend of information for industry/builders/hobbyists, and an online store.
Please enjoy the information on this site, and don’t hesitate to contact us with questions.
Mailing Address: London Power/Power Press PO Box 10012 Thunder Bay, ON P7B 6T6 CANADA
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.”
CANADA STEPHENSON AMPLIFICATION POB 176 Parksville, BC V9P 2G4 250-954-2292 10am-5pm M-F Pacific Time www.stephensonamps.com 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.
CARL’S CUSTOM AMPS New Mexico carlscustomamps.com Carl Carrasco builds custom amps with Power Scaling
CIRCUITRIX Rhode Island www.circuitrix.com 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.
FARGEN AMPLIFICATION – EXCLUSIVE AMPS Ben Fargen North Highlands, CA exclusiveamps.com
EUROPE – UK MJW AMPLIFICATION www.mjwamps.com 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.
RAT ELECTRONICS Andy Dokken modifies Valve Jr amps, adding Power Scaling and other mods. He sells only through eBay.
AMP MAKER ampmaker.com Offers a kit/prebuilt model with stepped Power Scaling – a modest 10dB loudness reduction at the lowest setting.
EUROPE – Belgium RUTHERBERG AMPLIFICATION Geoffray Herbrand email@example.com
EUROPE – Spain MPF-SOUND mpf-sound.es 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 LEHNERT AMPS Stockholm, Sweden www.lehnert-amps.com 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.
SOUTH AMERICA – Brazil LIFESOUND AMPS Alvorada Rio Grande do Sul lifesound.com.br 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.
AUTHORIZED POWER SCALING INSTALLERS
Looking for someone to install Power Scaling in your amp? Here is a list of recommended Power Scaling installers.
SOUTH AMERICA – Brazil LIFESOUND AMPS Alvorada Rio Grande do Sul www.lifesound.com.br 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.
Power 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 PowerSTUDIO 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.