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Biasing an amp requires some knowledge of tube amp circuits and experience with high voltage. If you are uncertain that you are experienced enough to work on a live high voltage amp, please take your amp to a professional to get it biased.

This calculator uses the power equation

~P~ is the power rating of the power tube, ~I~ is the bias current, and ~V~ is the plate voltage of the power tube. Solving for ~I~ we get

When biasing to 70% or whatever percent you’re looking for you simply multiply the answer by that number divided by 100. We’ll call this variable ~x~.

Let’s say we have a Deluxe Reverb that needs to be biased. We know the 6V6GT tubes have a power rating of 14 Watts (~P = 14\text{W}~). Next we measure the plate voltage. Let’s say we found it measured 420VDC (~V = 420\text{V}~). This is a Class A/B amp so we’re going to use the standard 70% (~x = 70~). Plugging everything into the equation we get the following:

If this number seems small that is because it is in amps rather than the typical milliamps we’re used to seeing. Simply multiply the outcome by 1,000 to get