Edistojim wrote:Just found this informative forum, Very nice gentlemen ! Thank you for the opportunity.
Although I'm new to tube amplification, I'm a long term audio
listener and enthusiast.
I own two ST70s, one completely redone with new everything, she is running just fine, sounds wonderful
and I couldn't ask for more. The second unit………..not so much. She was running fairly well till this morning,
low hum that started as nothing then louder and louder, I looked up and the front left tube was glowing
cherry red, no other symptoms whatsoever. I shut her down quickly and completely and unplugged her.
Here is the problem: I know squat about whats happened, I know it ain't good and frankly I'm a little
scared to plug it back in and look. No other outward sounds, rectifier looks ok, no light show at all, just that one left front EL34 and the hum.
Let me add they are older tubes and mismatched for the most part.
Could this be a simple tube failure or something more sinister ?
I did a search and didn't find the exact same problem, If I missed it, sorry.
Thanks in advance for any help.
I'd start with removing the left el34's. Make shure speakers are connected during the whole
Then power on and measure on pin 5 on one of the el34 sockets on the left, if more then -30V
and varies with the bias pot the i'll say that bias circuit is ok. Turn the bias pot to get the
most negative voltage and leave it there.
power off, check the cathode resistors ( check that it is 15.6 ohm from pin 8 in any el34 socket to ground.
If all above is ok, install a new matched pair of el34 in the left sockets, power on and measure the
voltage on pin8 of the external socket ( it has a small arrow ), it should be less then 1.5V when
the tubes are warmed up. If it rises above 1.5 V turn off immediatly as this indicates somthing else then a tube failure. ( see page 10 of the manual for detailed instructions). Turn the
biad pot for left channel and make shure that the voltage changes, stop when 1.4V is reached.
Do the same adjustment on the other side.
If voltages are stable and no hum is heard you have done the repair. If not, problems are
If you feel uncertain of these procedures, don't hesitate to turn it in to a qualified repair shop.