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In Reply to: RE: ST70 EL34 plate glowing like Rudolph's nose posted by Joe Curcio on September 17, 2009 at 17:50:31
Hi Joe,
I did go through and read your troubleshooting guide, which is the white paper I think you are referring to(?)
I think I understand what you say about the tube socket. I will say that the EL34 sockets are high quality ceramic sockets I installed new 4 years ago. If I understand the socket theory, if the connection on pin 5 is bad, the bias will go to zero causing current increase on the plate. This morning I ran amp with tuner input on for >45 minutes and without the cage on. After initial stabilization, the bias on right sided showed 1.26 V and left at 1.28. (Which brings up another question. Should bias be measured with input source running?) After 30 minutes the right bias was 1.21 and left was 1.25. Both had gone down but both stayed at this level for the next 15-20 minutes. This is a little low since I like to keep it at 1.3V which I believe I had seen recommended here. But at no point did the plate runaway. Since the only time I have seen the plate runaway was when I had the cage installed, I'm wondering if this is a thermal issue. Maybe the added heat from cage on is causing say a bad solder connection (pin 5?)to open?
Bill C.
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