About Our Power Scaling Kits

Learn about Power Scaling from Its Inventors

Our SV Power Scaling Kit Line

The new term in Power Scaling is "Super Versatile" (SV).

Our SV-range of Power Scaling kits represent 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, and most operate at <550Vdc - with the bulk of these below 500V. These amps can be described as having "dual-node" supplies, and 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 limts for the SV-kits are:

  • SV1 <700Va and Vs
  • SV2 <550V Va and Vs
  • SV84 < 450V Va

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.

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 KitPower 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 designed as a universal solution for both fixed-biased and cathode-biased amps, with supplies up to 900V. Note that in a single-node amp, SV-TT can be wired for Full-PS or PS-TT operation.

SV-TT Power Scaling KitAll 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.

Detailed Selection

1A: Screen voltage (Vs) and plate voltage (Va) are the same supply (single node) and less than 450V
These amps can use SV84, for Full-PS or PS-TT. SV2 or SV1 may also be used, but cost more in both money and space. Go to "1B".

1B: Single-node <450V; bias method?
If the amp is cathode biased, then SV84 is all that is required.
If the amp is fixed biased, then SV84 + TBS Tracking Bias Supply are required for Full-PS or PS-TT. In most of these cases, the stock bias supply will be inadequate to support the Power Scale regulator, so RBX Raw Bias Auxiliary Supply will also be needed.

SV2 Power Scaling KitSV-TT can be used here at greater cost of space and funds. RBX will still be required for the fixed-bias situation, as above. However, SV-TT is optimised for high-voltage operation.

2A: Screen (Vs) and plate (Va) are both less than 550V and have separate filter caps
SV1 or SV2 will be required depending on the bias method. These can be wired for Full-PS or PS-TT. SV-TT can be used for PS-TT wiring only. Go to "2B".

2B: Dual-node <550V; bias method?
If the amp is fixed-biased, use SV1 for Full-PS or PS-TT. In most of these cases, the stock bias supply will be inadequate to support the Power Scale regulator, so RBX Raw Bias Auxiliary Supply will also be needed.
If the amp is cathode-biased, use SV2 for Full-PS or PS-TT.

2C: Dual-node <700V; bias method?
If the amp is fixed-biased, use SV1 for Full-PS or PS-TT. In most of these cases, the stock bias supply will be inadequate to support the Power Scale regulator, so RBX Raw Bias Auxiliary Supply will also be needed.

SV-TT can be substituted for either SV1 or SV2 above, for PS-TT wiring only, with RBX as required.

3A: Dual-node; Va and/or Vs >550V and up to 900V
These amps will most often be fixed biased, with Va up to 800V and Vs anywhere from 500V to 800V. For Full-PS use SV-TT and SV3 Tracking High-voltage Regulator. For PS-TT use SV-TT.

4A: Screen (Vs) and plate (Va) are from voltage doubler supply
Voltage-doubler supplies produce one voltage that is twice the other. The higher will be Va and the lower will be Vs. Provided Va<900V, use SV3 to control Va for Full-PS, while SV1 or SV2 will be required to control Vs and bias provided Vs<550V.

For PS-TT wiring, leave out SV3 and use SV1, SV2 or SV-TT depending on conditions below. See "4B".

4B: Disparate-node; cascode BJT drive circuit ?
MusicMan RD-series and Peavey TX-series use BJT cascode drive circuits. In these amps, use SV3SV84 for Full-PS. For PS-TT wiring, leave out SV3 and use SV84 . If no BJT cascode, go to "4C".

4C: Disparate-node; conventional fixed-bias?
Univox amps use conventional bias methods, with most of their amps being fixed biased. In these amps, use SV3 + SV1 for Full-PS. For PS-TT wiring, leave out SV3 and use SV1. In most of these cases, the stock bias supply will be inadequate to support the Power Scale regulator, so RBX Raw Bias Auxiliary Supply will also be needed.

Fender PS-series amps are 150-450W output. For Full-PS use SV3 + SV1. For PS-TT use SV1 or SV-TT.

5: Switchable bias method?
If the amp can be switched from fixed-bias to cathode-bias, then the Power Scale kit required is based on the fixed-bias condition. By their nature, the fixed-bias-capable kits inherently accommodate cathode-biasing. This includes SV1 and SV-TT.

6: Tube type?
Tube type(s) used in the amp have no bearing on Power Scale kit selection.

7: Multiple preamp channels?
Power Scaling can be used "globally," like the "presence" control on vintage amps, or be synchronized with preamp channel selection. If the latter is the ultimate goal, then the switching methods required for the Power Scale kit should be considered.

All SV-kits require the use of either BJT, mosfet or relay switching to select between multiple PS pots. The switch element is tied to ground and must be able to sustain up to +60Vdc for Vs<550V, or up to +90V for Vs<800V..

Drive compensation can be attained using final volume controls for each preamp channel, if present, or by using the controls provided in the SV kit plus extras as required.

Switching between multiple Drive Compensation controls typically requires series switching best accomplished using relays. Automated drive compensation for fixed-biased amplifiers is achieved using the AD1 Auto Drive-1. For cathode biased amplifiers, use AD2 Auto Drive-2.

8: Multiple power amp channels?
Power Scaling multiple power amplifiers simultaneously can be achieved with all Power Scale kits.

The only potential complication here is with the Drive Compensation control. In single-PA applications, DriveComp controls two audio paths simultaneously. In a stereo PA, there may be four audio lines to control, depending on the topology of the circuit; then six paths in a three-PA amp, and so on. Multi-section pots become expensive at four sections, so alternate techniques must be used for drive compensation, specifically automated drive tracking of the PS setting. The AD1 Auto Drive-1 is best suited to this task.

SV84 Power Scaling KitDetailed Differences

All kits use 16mm pots
SV1: <550V; fixed/cathode bias; PS+DrvCmp+Bias
SV2: <550V; cathode bias; PS+DrvCmp
SV3: <900V, no controls related to this kit
SV84: <450V; cathode bias; PS+DrvCmp

When Do You Need the RBX Raw Bias Auxiliary Supply?

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

The problem with these stock bias supplies is two-fold. First, they are usually "high impedance", which means they are derived from the plate winding through very high value resistances (100-220k) or through capacitors. A high-impedance bias supply cannot support proper bias-set networks, nor will it support a bias regulator.

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

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

So, unfortunately most fixed-biased amps will also require RBX Raw Bias Auxiliary Supply if proper performance is to be attained 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 just the parts they needed. This also made the basic Power Scaling kit much less expensive.

About Our Sustain & Sag Kits

This section has moved - please see our page: Selecting a Sag or Sustain Kit.

Old "Classic" PSKs Sold until the end of 2007, our "classic" PSK-1, PSK-2, PSKPC and PS-BOX used switching regulators to achieve Power Scaling. Although this technique helps to keep waste heat in the regulator itself to a low level, installing the kits requires a very experienced tech. Any ground problems existing in the amp are brought to the forefront and must be corrected. Not all techs have the knowledge to correct such problems.

As switching regulators, the PSKs are inserted into the power supply ahead of the filter capacitors. This order requires current-limiting protection be built into the Power Scaling circuit. Circuity of this kind is very simple, but is relatively uncommon in guitar amps, so most techs are not familiar with how to adjust it for individual applications. This order also means that dialing power up is instantaneous, but dialing down depends on the audio circuit to discharge the caps, so it is difficult to have multiple PS pots.

Overall, the classic-PSKs are good for single channel amplifiers, but a tech experienced with Power Scaling and ground-flaw correction is required. PSK-1, PSKPC and PS-BOX were for fixed-bias amps; PSK-2 was for cathode-bias amps.

"Super Budget" & "Super Flexible" PSKs

The DC Power Scale kits of the mid-era encompass the DC-PSK1, DC-PSK-2, SF-1, SF-2, SB-1, SB-2 and SB-84. These kits represented developments and combinations of features and performance that have ultimately brought us to the present. The SBs were very simple, built around an expensive mil-spec pot, and were nonproblematic for even novice installers. The SFs required more skill but afforded certain flexibilities the SBs did not.

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


For more information about Power Scaling Kits, see also:
Selecting a Power Scaling Kit

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

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

About London Power

London Power was founded in 1990 by Kevin O'Connor, after two decades of research into innovative audio amplification techniques. Kevin is an audio designer, author and speaker, known for his proprietary methodology called Power Scaling, and for his eleven books on audio subjects. Whether you seek amps, kits or information, you'll find useful answers here on not just the "how" but the "why" of audio amplifier design. Please enjoy the information on this site, and don't hesitate to contact us with questions.

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