Modified ST-70: Two Rectifiers down, X to go?

[hueyrescue]

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!

[Ty_Bower]

...you'll probably cringe at some of the things I did...

In the mean time, I used the amp on L channel only; I removed the tubes from the R.

That's probably the part where I cringe the most. You can't run the amp with half of the output tubes pulled out. When they don't pull their expected share of the current, it throws the whole power supply out of whack. Voltages end up much higher than they should be, cooking the remaining tubes.

Are you sure the hum is mechanical? Old Dynacos tend to get dried up can caps. When they start leaking DC, you get hum in the circuit.

[hueyrescue]

You can't run the amp with half of the output tubes pulled out

Well, that is good to know! Ignorance is bliss until the tubes start dying off. :) Would this have likely killed the rectifier as well? So next time a tube fails, I need to turn off the amp until I replace the tube or can I simply disconnect the appropriate channel input?

Are you sure the hum is mechanical? Old Dynacos tend to get dried up can caps. When they start leaking DC, you get hum in the circuit.

The amp has an upgraded circuit board with modern orange caps and the silver can (that's the can cap, right?) was replaced with a modern equiv. I am certain the noise was mechanical...maybe one of the transformers is loose?

[TomMcNally]

Often when a capacitor is bad, or the bias isn't set correctly, or a few other problems, the choke in the power supply in an ST-70 will buzz. Sometimes tightening it down will calm it down - but often it's caused by other problems. Not knowing the history of your amp, it's hard to diagnose. If you replace an EL-34, be sure to check and set the bias. It is best to replace all 4 tubes at one time. Was the amplifier output clean with no hum when you ran it on one channel ? Hum would indicate filter/power supply problems. No hum could just be bad tubes.

[erichayes]

Your third paragraph probably reveals the problem: you had an EL34 that went south. They have a nasty tendency to play games . . . intermittent operation, meltdown, etc.

Get some fresh output tubes (I know; yecchh) and fire it up again. If you want to be totally anal, replace the GZ34 as well.

Tubes fail . . . that's an unfortunate axiom that we have to accept.

[hueyrescue]

Thanks for the advice! I'll give it a try with new EL34s and another rectifier.