[Amps] Alpha 76A - 120hz hum on 160m only

[Jim Barber]

After replacing the filter caps and equalizing resistors, I fired up
the 76A today on 40m and had a couple of nice contacts. The 120hz hum
problem *appeared* to be gone. The spectrum analyzer showed a nice sharp
clean carrier, with no "funnies" in the sidebands or for a few mHZ
either side.

And then I tuned up on 160m. 120hz hum like crazy on a carrier, hum 
distortion on SSB. Unusable.

Tuned up on 75m, clean as a whistle.

So - it hums like crazy on 160m, but not on the other bands.
Plate choke and/or bypass caps? It loads up fine on 160, just hums like
crazy.

Ideas? (other than exorcism)

Thanks,
Jim N7CXI