SV1 – Power Scaling Kit for Fixed Bias

CAD $92.00CAD $180.00

London Power’s SV1 Power Scaling Kit, along with our RBX Raw Bias Auxiliary Supply Kit, make up the Power Scaling solution for 99% of guitar and bass amps. These are the kits you need to Power Scale your 50W or 100W amp, and many lower-powered amps as listed below.


Our SV1 Power Scaling Kit for Fixed Bias offers cranked sound, as quiet as you desire!
If your amp is cathode-biased only, see SV2 Power Scaling Kit for Cathode Bias

  • For fixed -bias, or fixed- to cathode-bias
  • up to 750V plate (Va)
  • up to 750V screen (Vs)
  • Power range: 100% down to zero
  • Loudness range: unusably loud to unusably quiet
  • PCB size: 2.3″ x 1.3″  58.4 x 33 mm
  • Built height: 1.3″ 33mm
  • Controls included: Power Scale, Drive Compensation, bias

Is the SV1 Power Scaling Kit for you?

SV1 is the ideal Power Scale solution if your amplifier is fixed-biased, or has is switchable from cathode-bias to fixed-bias.

SV1 works with supply voltages up to 750Vdc. Most amplifiers have a screen voltage that is just a few volts lower than the plate voltage, which this is accommodated using the stock values in the kit. If Vs is much lower than Va, one resistor can be changed to accommodate the difference, as the kit notes detail.

Most amplifiers have poorly designed bias supplies that will not support SV1 by themselves; therefore, our RBX kit is required for nearly all fixed-bias amps. See our Product Information article “When Do You Need RBX Raw Bias Auxiliary Supply?” to determine if your amp requires RBX – most do, but there are a few exceptions.

If you determine that you need SV1 + RBX, order the combo as a Bundle and save! SV1 + RBX Bundle.

Compatible Amplifiers

SV1 is the best Power Scale solution for ALL fixed-biased amps.
Univox, ENGL, MusicMan 50-150W amps use voltage-doubler supplies with Va=700V or so, and Vs=350V. SV1 accommodates these amps.

SV1 Tech Details

SV1 is now  one-third smaller so it can fit into tighter chassis spaces. The construction method requires care in assembly and soldering, but the resulting module is compact and easy to position inside the amp chassis.

The low-level resolution of the Power Scale control is also improved, allowing you to dial in the exact quiet level you desire.

Screen voltage is typically just a few volts less than plate voltage, accommodated by the stock values in SV1. Changing one resistor allows Vs to be far less than Va, in amplifiers with voltage-doubler supplies, or where a large resistance ties Vs to Va with a corresponding large voltage difference.

A Drive Compensation control is provided, allowing four operating modes for the amp, as well as the ability to maintain tones when dialing down – dial PS and DriveComp together to maintain a sound. Single-knob Power Scale solutions compromise loudness reduction or tone, so we strongly encouragee two-knob solutions (Power Scale + DriveComp). If your amp has a master volume that regulates ALL preamp voices, then that control will perform the Drive Compensation function.

A Bias pot is provided. Multiple bias pots can be paralleled to the one provided, with each controlling an individual tube or circuit half. Bias is mounted on the tube plane of the chassis so it is protected from being knocked, yet is accessible just like the tubes. Bias is set at full power. Add one of our BMK Bias Mod Kits to have correct and safe bias monitoring and adjustment.

SV1 uses small, non-critical pots, allowing easy placement of these components on a crowded front panel. Note that only Power Scale and Drive Compensation are mounted on the panel and used for performances. Pots supplied are 16mm. These mount on individual PCBs to make wire connections easy.

In amps with “half power” or “triode” switches, those features are still functional with Power Scaling added. Note that adding our QS-LV (<500V) or QS-HV (<900V) Quiet Supply kit provides super-quiet operation in all operating modes.

For amps where the idle current is set very high, adding our VCK Voltage Clamp Kit to maintain a constant voltage to the non-scaled circuits helps to maintain tone consistency from loud to quiet, and also provides clean power to those low-level audio circuits. VCK is not required in most fixed-biased amps.

SV1 requires a low- to medium-impedance bias supply of high-output voltage for proper performance. If the stock amp does not have such a bias supply, add our RBX Raw Bias Auxiliary Supply to assure a problem-free installation. Most amps need this!

The installer must be able to locate the relevant circuit points within the subject amp.

Available configurations:

Standard DIY Kit: Requires assembly. Includes PCB, components, pots, and comprehensive notes with installation instructions + layout and schematic diagrams of the kit.
Built Kit: PCB comes pre-assembled, with comprehensive notes with installation instructions + layout and schematic diagrams of the kit.

If your amp is <750V and cathode-biased ONLY, see SV2.

If your amp is <450V and uses a single supply node for plate and screen, see SV84.

For more information about Power Scaling Kits, see also:

Selecting a Power Scaling Kit

Power Scaling Q&A

About Power Scaling

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

Questions bout the SV1 and RBX? Join the discussion of these products on The Ultimate Tone Forum!

The Ultimate Tone Forum

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