Wanted to follow up on this repair - customer needed the amp back so I stopped working on it. I did replace the transistor with no change. I also pulled both of the larger transistors and they seemed to test ok. Baffled would be my state. Hopefully I can understand the dynamics going on here one day when the amp returns. Thanks for the help!

In review of my comment I realized that the connector shown on the right is actually the preamp output to the power amp (the tank return is out of the photo). There are only two wires from the reverb circuit send to the tank. Also there are only two wires from the tank out to the preamp reverb recovery. They all share common ground. What range of voltage should I expect to see on the signal send to the reverb tank? Guessing... my reading is low and the tank is absorbing all the signal. This might point to a faulty drive transistor?

Here are some voltages and a layout shot of the reverb output section to the tank input. The dual molex connector on the left edge of the photo is the send to the tank. The one on the right is the return from the tank output.

Thanks Phil - I appreciate the explanation very much. I need/want to comprehend the theory of operation of everything I work on. I have much to learn.

The Reverb circuit (preamp) schematic is posted earlier in this thread. I will post voltages again tonight for that part of the circuit (at the correct magnitude to ground) if you or anyone else wants to take a look. I have successfully tested the wires/jacks/tank and all are ok. Here is a close up - the +/- 50 Vdc input voltage is good.

I was measuring to the voltage rail as well as looking at mV as V, so you will see a much different situation. (I am assuming that the power amp is working correctly maybe not at peak performance, but output is strong and without hum). All voltages were taken between chassis ground and leg on my chart. The Reverb circuit is still not functioning. Is the voltage  variance on the ballast resistors something of concern?

Voltages attached

I will double check this evening and post voltage readings. Thank you

Schematic

Biggest smoothest clean I have ever heard was  Peavey Nashville 400 with a 15" Black Widow. Really impressive.

Greetings kind people. I am in the middle of a repair on a Peavey Session 400 from the 70's. So far I have replaced a resistor in the Reverb send/rec section and re-attached two components where one leg had lifted off the PCB. I am getting output now but I have an issue in the power supply rails to the output transistors. I am getting a solid +50 vdc on one rail and -50 vdc on the other. I can signal trace an input signal and 50vdc  off of 5 of the 8 sand box resistors in connection with the output transistors. The other 3 have 104vdc and a buzz. I pulled one of the output transistors (5 seem to be working fine) and tested with diode checking and it appears to be ok.I also have no signal at the send of the reverb circuit section, but the return and tank are ok. I'm not that experienced with SS power section diagnosis, so I'm hoping someone here might give me some advice on where and how to start best. Thanks for reading/thinking etc - (I have a schematic from Peavey BTW)