Biasing an Ampeg SVT 3 Pro

[dpm309]

Don't know if this belongs here or in the Solid State section. I am working on an Ampeg SVT 3 Pro with a tube preamp (3 12AX7s) and a hybrid power section (12ax7, 12AU7 and transistors). After the power is turned on, it sounds OK for a few seconds and then exhibits bad distortion and volume. Tried swapping out tubes with known good ones with no success. I have read that this could be caused by bad biasing. Have not found any information on how to bias one of these. I have attached 2 schematics (Preamp, and Poweramp). Would appreciate any help with this. Thanks

[tdvt]

Wading in as I have the non-PRO version of this (which has just the preamp tubes), not that I know much about SS.

But looking at the schematic, the tubes in both sections look permanently biased by the cathode resistors chosen, not something you would typically fool with unless the resistor values have drifted.

Looks like 300-ishV on power rail feeding the tubes & some familiar cathode resistor values, so I would look for cathode voltages similar to a regular tube amp.

[dpm309]

OK, I did find out how to bias the SVT. I am reading 28mV after letting the amp warm up with no load and the gain set at 0. The recommended value is 24mV so it seems it is in the ballpark. Don't know if a 4mV difference would cause all the distortion etc. Next I am looking at replacing all of the tubes since the ones in here are the original ones.

[tdvt]

Totally misread that I guess, as I thought you were talking tubes, not the bias of the power section.

What you've got sounds in the ballpark to me as well.

[dpm309]

Solved the problem. I neglected to check the 2 tubes in the power section and it turned out the 12AU7 was bad. Pulled one from one of my builds and popped it in the Ampeg. Fired it up and the amp is working with no ugly distortion, loss of volume, eyc.