Marshall 6100LM Two different plate voltage readings?

[67Mopar]

I have a Marshall 6100LM that reads 488VDC on the plates, with no power tubes installed. When I install the tubes, I get a plate voltage reading of 470VDC on outer sockets V8 and V11. The plate voltage on the inner tubes V9 and V10 read 452VDC. The tubes are a matched quad... If I swap the tubes around, the readings are the same. Any idea what could be causing this?

[67Mopar]

Anyone?

[danman]

I'm not sure how your output tubes are paired up on that amp. If v8 and v11 are one half of the OT and v9 and v10 are on the other half, it could mean that your OT is severely out of balance on one side. You would need to make sure that everything is working properly on all four tubes first though. Are all tubes conducting and showing fairly matched current draw? Do all four screen resistors read pretty close to each other on your meter? You need to rule out all other possibilities before looking at the OT. I suppose the last thing would be to remove all tubes and check the resistance on each half of the OT's secondary for any drastic difference between the two.

[67Mopar]

I'm not sure how your output tubes are paired up on that amp. If v8 and v11 are one half of the OT and v9 and v10 are on the other half, it could mean that your OT is severely out of balance on one side. You would need to make sure that everything is working properly on all four tubes first though. Are all tubes conducting and showing fairly matched current draw? Do all four screen resistors read pretty close to each other on your meter? You need to rule out all other possibilities before looking at the OT. I suppose the last thing would be to remove all tubes and check the resistance on each half of the OT's secondary for any drastic difference between the two.

Thank you. I ended up soldering 1 x 5.6k grid-stoppers to pin 5 of each power tube socket, and changed the tubes to Tung-Sol 6L6-STR's. There are already 5.6k stoppers, but they are soldered to the pcb instead of the socket terminals. Odd design... All plates now read 455.2VDC!

I'm going to replace the "fizz" cap across the PI plates with a 100pf/120pf to help "smooth" it a bit. Maybe I'll try a 12AT7 at the PI first? Channel three has a lot of top-end, so maybe it would be better to center my efforts there.

[danman]

Glad you got it fixed! I was following your thread over at the Amp Garage where they determined it was oscillation causing the issue.