Also, since you've shorted half the output transformer, you can't play through the amp without unhooking the meter. It's too easy to slip and short the plate voltage to ground or through yourself. Don't trust your ability to hold the probe on the test points. If the tubes are balanced, you can divide this by 2 to get the bias current.Ī caution: the plate voltage of several hundred volts is present at the meter, so use alligator clips to wire the meter with the power off. Therefore the reading you get will be the sum of the current drawn by these 2 tubes. On amps with 4 power tubes, you're reading the current through 2 tubes. On amps with 2 power tubes, you're reading the current through one of the tubes. As long as the meter really is lower impedance, this works. The meter then measures the current flow. Since the meter should have lower impedance than the transformer, nearly all the current flowing through the tubes should flow through the meter instead. The plate on octal power tubes is pin 3 for the common ones. The center tap usually connects to a large filter capacitor and/or the standby switch, but you'll need a schematic to be sure where to find it. This involves putting the red lead of the meter on the center tap of the output transformer, and the black lead on the plate. With transformer shunt, you hook your meter so that it shorts half of the output transformer. Here's my experience with 2 methods, transformer shunt and cathode resistor. Measuring the current drawn by the tubes is the third way.

Variable bias vs fixed bias guitar amp