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In Reply to: RE: Will direct coupling shorting tube life?? posted by bouncy ball on May 20, 2012 at 19:04:57
In direct coupling, it is very easy to put a high voltage on the output tube grid during the warmup transient. Some tubes are fairly tolerant of this abuse, and others are very touchy about it. I have seen tubes that withstand it for years of daily use, and tubes that die in a day. It's still abuse.
Worrying about these transients is one of the prices you pay for direct coupling. Done carefully enough, there is no reason such circuits should not have a very long tube life. I am not claiming that I have always done it carefully enough - how else would I have learned what's in the paragarph above?!
Incidentally, once the transient is eliminated, sometimes the tube's emission will come back after a few hours of running.
Follow Ups:
Hallo Paul... can the Bottlehead 2A3 kit be ordered with a 240v primary?
Seems like I remember seeing a schematic with a tube damper diode following SS rectifier for surge limiting or am I wrong?
I suspect that the damper diode is used as a time delay element (to delay B+ tirn on) and not for surge protection. Damper diodes such as the 6DE4 take a long long time to warm up - something greater than 20 seconds, if I recall correctly. Which brings up a point. If you need a damper diode for delay, why not get rid of the SS diodes and replace them all with damper diodes?
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