My modified ST-70 started doing some strange things a few weeks ago. I have no experience in troubleshooting tube systems. As you read the order of events, you'll probably cringe at some of the things I did in ignorance. I've owned this amp for 5 years.
The strangeness began with a mechanical hum from the transformers about 8 weeks ago. This would happen only occasionally and would often go away if the system was turned off and allowed to cool and then restarted.
Then, about 4 weeks ago, my rectifier tube (it was used when I bought the amp, probably near the end of its life) flashed over. My wife was quick to react and shut the thing down before any damage was done. I, however thought it would be a good idea to restart and see what was going on since I didn't witness the event...you can guess what happened. Fortunately, it only blew one of the four el34s; the front R tube.
Well, I replaced the rectifier with a JJ. I wanted to replace the whole quad of el34s and was researching best value options. In the mean time, I used the amp on L channel only; I removed the tubes from the R. Last week, the L rear tube failed to start up. I replaced it with the other good el34 from the R channel. I put the failed tube in the front R position to verify that it was the tube itself that failed. I turned the system on and the rectifier flashed a little and the fuse blew. I replaced the fuse and tried again. This time the rectifier flashed over completely. The system is now awaiting mx. Any input is appreciated as to how to proceed. Unfortunately, there are no electronics repair specialists in my area. My hope is that the second rectifier was simply bad but that doesn't seem likely. Thanks!