Any Ampeg experts out there?

[punkykatt]

I have a late 60`s Ampeg GS-12 Rocket 2 on the bench. Did a clean and service, installed a new matched pair of 7591A`s, after about 10 minutes the tube being driven by the plate of the PI (by the way this amp has a 12AX7 cathodyne for the PI and not the 7199) rises about 10mA while the other 7591A drops about 8 mA. I swapped the tubes and the same thing happened. So its not the tubes. I replaced both 0.1 coupling caps and the 270k grid to ground resistors, re flowed all the solder joints on the tube pins. No change. I disconnected the OPT center tap and removed the power tubes and checked the resistance between the CT and each plate lead. The  plate lead that drew the most mA read 101R to CT and the other read 90.9R to CT. Could  this be the cause of the problem? Any help is much appreciated. Thank you in advance. PS I could not find the proper schematic for this amp.
https://ampeg.com/support/files/Schematics/G%20Series/GS-12/GS12%20Rocket%202%20Schematic.pdf

[shooter]

after about 10 minutes the tube being driven by the plate

idling?  (no signal?)


the R difference in an OT is typical from what I've measured


the schematic shows a common R for both PA tubes, how are you measuring the different currents?

[punkykatt]

I have bias probes on each tube. I usually play amps that I test a little above bedroom volume as not to disturb neighbors. Did not try just letting it idle.

[punkykatt]

I had the amp on with no signal, all pots at 0 for 20 minutes. one tube drawing .049.9mA the other .059.9 mA. Still have that one side drifting up.

[shooter]

Still have that one side drifting up.

double check that by monitoring the cathode resistor voltage, should drift also

[PRR]

...one tube drawing .049.9mA the other .059.9 mA. Still have that one side drifting up.

Probably some extra decimal points or bogus units in there.

I would not call 50:60 a serious unbalance, but the drifting is worrying.

Replace both grid coupling caps on the suspicion of leakage.

[punkykatt]

I swapped the tubes again and for the last 25 minutes they are getting much more stable. Each running around 100% MPD now. I will check again in another 10 minutes.

[sluckey]

There is usually a schematic either on a side wall or the back panel of those old Ampegs. Does your amp have such?

[sluckey]

Maybe this is your schematic...

     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_gs12r_.pdf

[punkykatt]

This is the PI that is in this amp. see attachment.   The tubes in this position are holding nicely within 3 mA, even after  playing. Tomorrow I will swap sockets and see what happens.

[PRR]

Yeah, different, but not in how they set power tube bias.

[punkykatt]

Mystery found. Eyelet board conductive letting small +dcv on grid. Lucky there was an empty eyelet just to the left so I could easily move the 0.1 cap, 270k grid leak and 1k control grid resistor connection away from the B+ source a bit. The bias has been stable within 2mA.  I`m sure in time the conduction will eventually spread.    Thanks to all that responded.

[tdvt]

Mystery found. Eyelet board conductive letting small +dcv on grid. Lucky there was an empty eyelet just to the left so I could easily move the 0.1 cap, 270k grid leak and 1k control grid resistor connection away from the B+ source a bit. The bias has been stable within 2mA.  I`m sure in time the conduction will eventually spread.    Thanks to all that responded.

Thanks for reporting back on the issue.

I'll file that away, if I can remember to...