Hi,
There are two balance adjustments you can make to a push pull power
amp circuit. One is the more common DC balance. If you have separate
cathode resistors for each tube, just set the balance pots so the
voltage drop across each resistor is the same. If there are no
resistors, what you do is connect an AC volt meter across the speaker
terminals and adjust the balance pot till you get the lowest AC volt
reading. This will be where the tubes balance and where you get the
least hum.
The other balance commonly found is called AC balance. Here your
goal is to have each half of the phase inverter producing the same
voltage. So, you run a sine or square wave into the amplifier's input,
then turn up the level and just measure each half of the phase
splitter's output. Actually the best place to measure this is at the
control grids of the output tubes. Adjust the phase inverter balance pot
till both voltages are the same. The last adjustment is bias. Again, if
the amp has a cathode resistor, you can measure the voltage and
calculate the current using ohm's law. I find this more accurate then
shunting a miliamp meter across a low value resistor. If you don't have
cathode resistors, you can install a set of 10 ohm resistors between the
cathode and ground. This is assuming the amp is fixed bias.
Bill Bittle


Archangele sound: Offering service for all makes of tube hifi and
guitar amplifiers.