1961 Zenith Stereo Amp Problem

[davet753]

Last weekend, I drove 4 hours to buy a console stereo. (Don't laugh, I know that sounds crazy). I've been looking for an early 60's Zenith tube-type for a while now, and finally found a nice top-of-the-line model with the extended stereo set-up and the optional reverberation.

I got it home and immediately replaced all the wax paper capacitors and the electrolytics (other than the filter caps, which seem to be fine). I also replaced all the old carbon resistors in the audio output circuits.

The amp uses two 6BQ5 output tubes in a push-pull circuit per channel and a 12AX7 as a pre-amp / phase inverter tube. This should really sound great, but it leaves a lot to be desired at the moment. At lower volume levels, it's pretty good but when I turn the volume control up to half or over, it's bringing in a lot of distortion.

Is anyone here familiar with the "current balance" adjustment pot on these amps? There is a potentiometer between pin 3 of the two 6BQ5 output tubes in each channel, with the center arms connected to each other and going through a 100 uf electrolytic and a 63 ohm, 4 watt resistor to ground. There is a 330k resistor coming off the plate of each output tube to a set of small terminals poking up through a hole in the chassis. The factory service manual says to hook up a meter between these terminals and adjust the "current balance" pot to get a zero-volt reading.

On mine (with both pots cranked all the way), one channel is down to .3 volts and the other channel is right at 2 volts.

Any ideas?