Tube amp phase inverter buzz

Hi guys

I'm new in this forum and I'd like to resolve an issue with my guitar tube amp based on Bassman 64 tuxedo. It is build on pcbs and works fine but there is an issue with
buzz 50HZ comming from phase inverter obviouslly. When take out this tube buzz disappears from power tubes. Everything is ok up to this tube. I've checked all DC power supplays heaters, all components soldering and it seems OK, heaters are tap grounded. Preamp, power supply work ok. Scope measurement shows inequal amplitudes between both phases but this doesn't seem a problem because sound is great. I'm pretty inexperinced with tube amps so I'm not sure if this is a problem. Any suggestions?

Regards from Slovenia,

I figured out bias voltage was mainly the problem, there was a huge 5Vpp 50Hz component presented in negative bias voltage. It looks strange for me because there is very smal load at this point. Nevertheless additional filter in negative voltage was added and 50Hz was removed from output signal. Now there is another but much smaller problem, this when I turn up volume then between 7 and 10 of volume 50hz shows up again, of course 90% smaller then before but still it bothers me. This looks more like input problem, so I checked grounding and looks fine. I know there is a lot of data on web but if someone knows any practical solution I'd be happy. Preamp schematic is atached. Works great, just to solve this last issue.

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