I "finished" working on the power amp section. I removed the Atoms, (including the stick-on plastic things that they were mounted to), re-stuffed both cans, replaced all the Astrons, and installed a 3-prong power cord. I also took some metal polish to the chassis and buffed out the pitting to the best of my ability. You can see the interior and exterior.
I tested the tubes. The phase inverter (6SL7) is branded "Ampeg", so it's probably original. It tested good with remarkably well-matched triodes. I left that. The power tubes (Mullard EL37s) both test good, but unfortunately, were very poorly matched in current draw. I tested them at +380v on the plate with -33v bias and got 50ma and 34ma of cathode current. I swapped out the 34ma tube with another from my cache that also pulls 50ma. The OC3 and the rectifier both seem fine.
I reconnected the power chassis (the power switch is in the preamp chassis), pulled the preamp tubes, and powered it on. I get 430v on the plates of the power tubes, and 320v on the screens (I'm used to amps that run the plates much closer to the plate voltage). 120Hz hum was quite low. Everything seemed to check out OK.
I tested the preamp tubes. The 6SL7 seems original, but tested good. The 6SN7 was dead. I replaced that with a random draw from my stash (a Ken Rad, I think) that tested good. The preamp tubes lit up, but I got no signal from the preamp. Time to go to work on that chassis. I've already identified an open resistor connected to the grid of the 6SN7, so I guess that is why there is no signal. But it doesn't look like anyone has ever touched the preamp, so I think I'll shotgun it. The parts count is not very high.
I tested the tubes. The phase inverter (6SL7) is branded "Ampeg", so it's probably original. It tested good with remarkably well-matched triodes. I left that. The power tubes (Mullard EL37s) both test good, but unfortunately, were very poorly matched in current draw. I tested them at +380v on the plate with -33v bias and got 50ma and 34ma of cathode current. I swapped out the 34ma tube with another from my cache that also pulls 50ma. The OC3 and the rectifier both seem fine.
I reconnected the power chassis (the power switch is in the preamp chassis), pulled the preamp tubes, and powered it on. I get 430v on the plates of the power tubes, and 320v on the screens (I'm used to amps that run the plates much closer to the plate voltage). 120Hz hum was quite low. Everything seemed to check out OK.
I tested the preamp tubes. The 6SL7 seems original, but tested good. The 6SN7 was dead. I replaced that with a random draw from my stash (a Ken Rad, I think) that tested good. The preamp tubes lit up, but I got no signal from the preamp. Time to go to work on that chassis. I've already identified an open resistor connected to the grid of the 6SN7, so I guess that is why there is no signal. But it doesn't look like anyone has ever touched the preamp, so I think I'll shotgun it. The parts count is not very high.