Output Tube Biasing Problems

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.

There are relatively few causes of output tube bias problems. All of them involve the grid not being held at a negative enough voltage with respect to the cathode.
• In any amplifier:
  • If the output tubes have just been replaced with new ones, "infant mortality"/early dying of the new tubes.
  • Leaky/shorted coupling capacitor from the driver tube plate to the power tube grid. Note that the chances are that only one of the multiple output tubes will have this problem.
  • Dirty, corroded or just old tube socket not making good contact to the tube grid pin Note that the chances are that only one of the multiple output tubes will have this problem. Eagle-eye the socket hole contacts with a magnifier and try to see if they are corroded or dirty looking or maybe the contacts have lost their springiness, and don't make good contact. Try removing the tube, squirting a little tuner cleaner into each socket hole, then wriggling the tubes back in. You can also very gently pry the contacts out so they have more pressure on the tube's pins.
  • The socket could be broken.
  • Rarely, a very poor or inexpert rebiasing attempt by an unskilled technician.
  • Rarely, an output tube itself that is shorted grid to cathode Note that the chances are that only one of the multiple output tubes will have this problem.
  • Rarely, the resistance from the tube grid to its bias supply will be open. This can lead to "runaway" in power grid tubes. Measure the resistance from grid socket pin to bias supply point in fixed bias amps or to ground in cathode biased amps and make sure that both (all) output tubes have the same nominal resistance - usually 100K to 220K, never more than 470K. If one grid has a lot higher resistance to ground or to the bias supply, there is a problem with the grid leak resistor on that tube.
  • Even more rarely, a wiring flaw in the wires getting the bias voltage to the grid. I have seen an amplifier that came from the factory with all of the wires inserted into the socket lugs and wrapped around the contacts, but not soldered. It worked that way for 20 years, and had intermittent troubles the whole time.
• In fixed bias amplifiers:
  • If the tubes have just been replaced with new ones, failure of the owner to get rebiasing done, or "infant mortality"/early dying of the new tubes.
  • Failure of the bias section of the power supply, especially the adjustment pot or the bias supply filter cap
• In cathode biased amplifiers:
  • If the output tubes have just been replaced with new ones, "infant mortality"/early dying of the new tubes.
  • Leaky/shorted cathode resistor bypass capacitor
  • Rarely, a noninductively wound cathode resistor with an internal short
  • Rarely, a broken or incorrect wire.