Learn about Power Scaling from Its Inventors
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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 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.
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 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:
Who can install Power Scaling for you?
Licensed Power Scaling Builders and Installers
