![]() ![]() The plates will glow red-hot from many electrons pounding the metal plates. If you’re biasing the amp too hot, the power tubes could “red plate”. For cathode-biased amps, you should change the cathode resistor value for changing the cathode voltage and adjusting the bias. When working with fixed bias amps, you’ll have to adjust the grid voltage for setting the bias. It’s important to adjust the number of electrons that are flowing from the cathode to the plate. You’ll need to adjust the voltage difference between the control grid and the cathode. The higher the plate voltage is, the lower the plate current needs to stay under the plate dissipation limit. Plate dissipation is the plate-to-cathode voltage x plate current. Tube manufacturers normally list the tube’s max bias in the tube datasheet as maximum plate dissipation in watts. This results in fewer electrons on the grid for repelling the flow, such that the flow through the tube increases. If the signal is a positive voltage (scarcity of electrons), the grid will go less negative. ![]() This repels the negatively charged electrons that flow through the tube, slowing the flow. If the guitar signal is a negative voltage, it’ll make the control grid negative. This will give enough room for the positive and negative voltage of the guitar signal. Normally, you’ll want to set the idle flow in the middle of cutoff and saturation.
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