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Biasing Tube Amplifier Calculator

Use this calculator to determine the bias current for an amplifier’s power tubes. Enter the plate voltage and type of power tubes to get the bias current at various levels of plate dissipation.
Amplifier Tube Values
Enter the values for your amplifier below.
V
The output tube you are using in your amplifier

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

$$P = I \times V$$

$P$ 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

$$I = \frac{P}{V}$$

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 $x$.

$$I = \frac{x}{100} \times \frac{P}{V}$$

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:

$$I = \frac{x}{100} \times \frac{P}{V}$$$$I = \frac{70}{100} \times \frac{14}{420}$$$$I = 0.0233A$$

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

$$I = 23.3\text{mA}$$

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