Output Tube Biasing Problems

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Output tubes handle probably 85% of all the power used by your guitar amp. If they are biased incorrectly or if there is a fault in one of the biasing components, it can cause a number of power supply and output section problems. A failed biasing component that lets the grid assume the same voltage as the cathode will cause an output tube to act almost shorted. Tubes which are conducting too much bias current (older tube-techs would say these are "underbiased" or "biased too hot") can cause blowing fuses, excessive power supply ripple and 120Hz hum, burned out rectifier tubes and could in the long run kill an output transformer or power transformer. They overtax in general everything in the electrical path from the AC power plug to the output transformer.

You will need to know whether your amplifier is fixed bias or cathode biased. If you don't have a schematic make sure the amplifier is unplugged. Remove the output tubes and measure the resistance from the cathodes of the output tubes to chassis ground. If this is under 10 ohms, you have a fixed bias amp. If it is 50 ohms or more, you have an amplifier that is cathode biased. Between these two could be a flaw in the amplifier or could be one of the very rare amplifiers that use a combination of cathode and fixed bias.