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! There are several variations of our Power Scaling Kits (SV1, SV2, SV84, SV-TT and SV3). See Power Scaling Kit Selection for more info.

Is the SV1 Power Scaling Kit for you?

To be suitable for SV1, your amp has:

<750V plate and screen voltage
fixed bias, or switchable fixed bias to cathode bias

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 the following amps – any amp you do not see listed is likely very similar to these:

Fenders: (note that most Fenders will also require our RBX Raw Bias Auxiliary Supply Kit) – all AA-series and 6-series of all vintage Blackface and Silverface models, Bandmaster, Bassman, Blues Jr, Concert, Deluxe, Deluxe Reverb, Dual Showman, Hot Rod Deluxe, Princeton Reverb, Pro (5E5A and later), Pro Reverb, Showman, Showman Reverb, Super (5E4 and later), Super 60, Super Champ, Super Reverb, Tonemaster, Tremolux (5G9 and later), Twin (5E8 and later), Twin Reverb, Vibrolux

Marshalls: (note that most Marshalls will also require our RBX Raw Bias Auxiliary Supply Kit except 1959, 1966, 1992) – all vintage models rated for 50-100W, all 800s, all 900s, DSL-2000s, Vintage-Modern 50W, vintage 20W amps like the 1974

Hiwatts: (note that Hiwatts will also require our RBX Raw Bias Auxiliary Supply Kit) – all 50W and 100W models

Ampeg, Dumble, Earth, Laney, Peavey, Soldano, Trainwreck, Traynor, and all other brands: (note that these brands will also require our RBX Raw Bias Auxiliary Supply Kit) – all 40W to 200W models; plus any other brand that is in this power range

Univox, ENGL, MusicMan 50-150W amps. These amps use voltage-doubler supplies with Va=700V or so, and Vs at half this value. SV1 accommodates these amps.

SV1 Tech Details

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.

For amplifiers such as Fender PS-series, and others, SV1 can be used to control the screen voltage and bias, with our SV3 Tracking High-voltage Regulator added to control plate voltages that are up to 450V higher than Vs. (Va is 650-850V.) The issue here is the heat dissipation of the Va regulator; SV3 has multiple devices for this task.

Multiple Power Scale (PS) pots are readily added and switched in multi-voice amplifiers. The PS pot is ground referenced but sustains up to +60Vdc depending on the supply voltage. Additional PS pots can be paralleled, requiring the change of one resistor on the SV1 board, with the wiper selection synchronized with channel switching. BJTs, mosfets or relays can be used for this function. Our PSX Power Scale Switch, compatible with ERK Electronic Relay Kit and Q-series Switching Kits both provide direct selection of the PS pots. The switching of Drive Compensation pots will generally require a relay, such as RLY.

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.

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 (<450V) or QS-HV (>450V) 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!

PCB dimensions: 3.2 x 1.5″, 82 x 38mm

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 <550V and cathode-biased ONLY, see SV2.

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

If your amp is Univox, use SV1 + SV3 + RBX. See SV3. Or, use SV84 + TBS + RBX for PS-TT (two-thirds Power Scaling).

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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