VariWattage Kit

For fixed bias amps from 20-150 watts.

VariWattage is our solution to varying the output wattage in many vacuum tube guitar amplifiers that have fixed* bias supplies. It uses voltage manipulation (similar to a Variac, but doesn’t affect the tube filament voltage) to control output power and power tube overdrive without affecting high-end frequencies like an attenuator can. Our VariWattage operates via one control knob that allows the user to dial down (brown) the B+ voltage, from full 100%, to very low (effectively 5%), for late night bedroom/apartment volume. That way, the same amp can be used anywhere – from the smallest recording studio, practice environment, or coffee shop gig, to the biggest stage the amp can cover at full volume.

Our kit includes all the necessary components, mounting hardware, and step-by-step instructions and wiring diagrams for installation in your amp.

We offer a several options – 1. A choice of a of Plexi-style or Pointer (chicken head) control knob. 2. Internally mounted, or chassis mounted bias adjustment pot. 3. Bias circuit modification components suitable to modify bias supplies derived directly from a HV secondary tap (as are many Marshalls and Hiwatts). 4. Additional components for 100-150 watt amplifiers (select in dropdown fields).

Bias circuit modification components include the following:
– 100uF@150V electrolytic axial-lead capacitor
– 39K1W carbon film resistor
– 10K1W carbon film resistor
– .047uF@630V film axial-lead capacitor
– 100V 5W zener diode
– 1ohm 5W wirewound resistor

100-150 Watt amplifier components include the following:
– 2 IGBT
– IGBT mounting kit
– (2) 1 ohm 5 watt resistors
– extra heat shrink tubing
– additional wire

Please note: this is an advanced kit – Knowledge of vacuum tube amplifiers, high voltage power supplies, semiconductors, handling static-sensitive devices, and bias supplies is required. Email support is limited to VariWattage installations in working amplifiers only. Please ensure your amplifier is working properly before installing VariWattage.

* VariWattage is for amplifiers that have “fixed,” bias supplies, referring to a negative supply voltage applied to the power tube control grids to regulate current flow, which is often adjustable. This kit should not be used with cathode-biased amplifiers. Please refer to our Variable Power Control – VPC1 Kit.


Beginning in 2018, instructions for modification kits are now online for your convenience. This allows us keep our prices competitive, and helps all of us reduce paper waste. Links to PDF files will be included in the order status email when the order ships, and always available in your order history. Please add to your email whitelist to ensure you receive our emails.

DISCLAIMER: User assumes all risks. Performing these modifications will most likely void the warranty on your amplifier. Working on amps can be deadly if you do not take proper precautions. Read and follow the diagram or instructions very carefully. Basic soldering skills and basic user-supplied tools are required. If you are at all unsure of your ability to perform these modifications STOP and take your amp to a professional to be modified. You can also send your amp to us for modification. Contact us for pricing.

DIFFICULTY LEVEL: ADVANCED [Follow safety guidelines for working with electricity. Advanced soldering skills and knowledge of working with AC/DC circuits. Identify all terminals of electronic components using data sheets. Read resistor color codes and capacitor values, as well as identify and handle PNP, NPN, JFET, MOSFET, IGBT semiconductor devices. Measure and connect high and low voltage AC and DC. Understand advanced working of vacuum tubes. Use all functions of a multimeter. Understand Ohms law and apply to basic and advanced circuits. Work with advanced hand and power tools.]

Pricing and Options


29 in stock (can be backordered)

Control Knob(s) *

Bias Adjust Pot *

Bias Circuit Mod Components *

Amplifier Wattage *


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