organ Posted December 24, 2003 Share Posted December 24, 2003 OK guys, last night I was making cookies while the equipments were still on in the basement. I heard a loud hum coming out of one of the RF-35 and immediately ran downstairs to turn off the Dyna ST-70. The bias was set too high and one tube on the left was glowing red. I turned down the bias, turned the amp back on and everything still sounds good. So do you think the Svetlana EL-34 is still good? Thanks Maurice Quote Link to comment Share on other sites More sharing options...
3dzapper Posted December 25, 2003 Share Posted December 25, 2003 Maurice, I'd be more curious as to why the bias rose so high. You probably saved that tube and your OPT by catching it in time. But, I'd watch the bias on that tube for a while. You might have a coupling capacitor leaking DC into the grid circuit. I've had that happen to me with a Dynaco SCA-35 rebuild. The bias on one tube would set good at 350mA cold then, as the set was on longer would climb as high as 700mA. Not good. Tore my hair out trouble shooting mostly when cold. Then I left it on for a couple of hours while taking measurements and walla! There's the culprit! Bad NOS Sangamo oil capacitor. It's been a few months now since I swapped out the caps and the bias has been stable since. Rick Quote Link to comment Share on other sites More sharing options...
organ Posted December 26, 2003 Author Share Posted December 26, 2003 Thanks for the info, Rick. I don't know where the coupling cap is. The reason the bias drifted is because the bias on the amp is not steady. It usually drifts +/-0.15 volts. A lot of folks at audioasylum experience this as well. I also moved the amp arond quite a bit so the pots could have changed positions a bit. I've replaced the bias supply with new caps and a diode. The quad cap was also replaced. I changed the tubes to EH 6CA7 fat bottles and everything's been running smooth. Just not sure if I damaged the Svetlana. The still sound good though. Maurice Quote Link to comment Share on other sites More sharing options...
ZAKO Posted December 26, 2003 Share Posted December 26, 2003 Keep an eye on it you dont want it to burn the house down. Quote Link to comment Share on other sites More sharing options...
Tubinhard Posted December 26, 2003 Share Posted December 26, 2003 Maurice, It's time to send the amp in for a check up. If the bias is drifting, then it needs to be looked at. I'm sure Craig would give it a looksy at reasonable cost. You need to protect your investment. Quote Link to comment Share on other sites More sharing options...
3dzapper Posted December 26, 2003 Share Posted December 26, 2003 Dean, I just noticed the latest change in your everchanging sig: Van Alstine Transcendence 7. I am expecting a full report and review when you get your Khorns settled in. Rick Quote Link to comment Share on other sites More sharing options...
organ Posted December 27, 2003 Author Share Posted December 27, 2003 Thanks for the help guys. Who is Craig and how can I contact him? Is he in Canada or the US? I'm in Canada and shipping for the ST-70 will cost an arm and a leg. I'm still in college and working part time. Anybody from Toronto, Canada who knows a repair shop that will service Dynaco amps? Thanks Maurice Quote Link to comment Share on other sites More sharing options...
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