Hi everyone. This is my first post ever and it is because I need help. Before this past December all I knew about tube amps was that I liked them a lot. I’ve read a lot and learned a lot since then, but not enough to turn this beautiful paperweight into a functioning jtm45!
Let me also just say thanks to everyone who generously shares their knowledge freely on this site. I would t have gotten nearly this far without all of your help.
On first power up I followed the metro amp instructions. Heater voltages were fine (except getting 4.5vac to v6 instead of 5vac). Measured 300vdc at standby switch as well as -40vdc on control grid (pin 5) of each power tube socket.
Inserted preamp tubes and started it back up. Voltages were pretty much ok EXCEPT plate #1 voltage on v2 was 325vdc and plate #1 voltage on PI was a whopping 345 vdc! WHY! I checked both the 8.2k and 82k resistors that seem to separate the b+ from the PI plate and they both read fine. Anyway being the noob I am, I ignored this and inserted kt66 tubes.
I turned the amp on and it screeched at what must have been over 100db. Intel on this site indicated that this was related to the NFB circuit, so I disconnected the 27k resistor and the board and that allowed me to switch the amp on without any screech. Assuming I had found the issue, I reversed OT wires and went to bias, but now I have a second problem. Only one of the OT plate wires is showing my 400vdc. The other shows no signal at all.
I’m at a loss. Are these two issues related? Could the high plate voltage on the PI have damaged the OT? Maybe the positive feedback loop damaged the OT? With my multimeter on resistance I can’t even get a signal out of one leg of the OT primary, so I’m assuming it’s...damaged.
Please help, I’m very lost. Thanks
Nate