Power Scaling Kit Selection

London Power's Power Scaling Kits:
The Cutting Edge of Power Scaling Technology from the Inventors of Power Scaling

Selection GuidelinesSV1 Power Scaling Kit

The first selection parameter is how the amplifier is biased: cathode biased or fixed biased. 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-bias 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:

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

*Most bias supplies in fixed-bias amplifiers are too low in voltage and/or too high in impedance to fully support the Power Scale regulators. For these amps, 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'.

The problem with these stock bias supplies are 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 TUT2 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.

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

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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