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Re: Why would cathode cap burn out?

5/11/2005 11:13 AM
Tom Phillips
Re: Why would cathode cap burn out?
"But why is my screen voltage higher than the plat voltage? I thought the plate was supposed to be more positive than the screen. Also, the voltage for the screen should be less due to voltage drops across R13 and R12. I double-checked my wiring and it looks like it matches the schematic. "
If the wiring is per the schematic then my explanation would be that the voltage drop caused by the high plate current through the resistance of the OT primary is more than the voltage drop of the low current through R12 and R13. I have seen this before but the screen was only a couple of volts above the plate at idle and quickly fell below the plate voltage when signal was applied.  
You are on to something when you discovered that the noise stopped when the input grid was grounded. You could troubleshoot the pre-amp and the power section separatly with the grid grounded. Do you have a scope?  

Clint Tom said: "If the wiring is... -- 5/11/2005 1:08 PM