[Collins] Self balancing audio phase inverter circuit

[LHNDMW@aol.com]

I'm working on a homebrew AM TX, 4-125's modulating a 4-400. Check out my website for more info. (http://home.san.rr.com/km5tz/km5tz.htm)

I have just about finished building the modulator and I started checking it out today. 

Hum was the first problem, solved by grounding the metal cased mic amp tube.

The 4-125's are driven by a pair of 6SJ7's in a standard ARRL handbook 'self balancing phase inverter' circuit. The output of the 6SJ7 in the main signal path looks fine but the phase inverted output from the other 6SJ7 is not so linear. The lower half of it's waveform appears compressed and the upper half expanded. I tried swapping tubes but that made no difference. The 4-125's are out of the circuit. All bias voltages look correct. Anybody have experience with this circuit? suggestions?

Thanks.

David
KM5TZ