I've got an amp that happily had MOSFET followers in front of its power tube grids for quite a while. One day, I got a bug in me to swap out the tubes from EL34s to those JJ 6V6s that can take such a beating and all. Something blew and the tubes started redplating.
A postmortem revealed that my crude old bias circuit had died due to overvoltage on the gate, in the process, had caused a grid to cathode short in the 6V6s
I took the opportunity to gut the bias circuit, redesign it, and install a new one. I've attached a rough schematic - I don't have all the specific parts I used in LTSpice, but these are close enough.
The bias circuit appears to be working fine now, even the old faulty tubes didn't faze it, but when I crank it all the way it still pops the 500mA fuse between the power tube cathodes and ground. I need to stop it from doing this, because whenever it does the amplifier stops working.
I have two ideas as to what might be going wrong, but I wanted to get some input here because I know that some people here have messed around with class A2/AB2 before and know much more than I do about it. I think it is possible that I'm pulling too much grid current and causing the power tube to fail; in that case, my guess is that the appropriate fix would be to limit positive signal to no more than some number of volts above ground with a zener somewhere or another. My other idea is that there is something non obvious about throwing a JJ 6V6S into a circuit meant for an EL34 that I am missing completely.
Thanks for anyone who can help me out here - maybe this should go into troubleshooting, but that forum seemed more concerned with production amplifiers than tinkering, so I posted here instead.
A postmortem revealed that my crude old bias circuit had died due to overvoltage on the gate, in the process, had caused a grid to cathode short in the 6V6s
I took the opportunity to gut the bias circuit, redesign it, and install a new one. I've attached a rough schematic - I don't have all the specific parts I used in LTSpice, but these are close enough.
The bias circuit appears to be working fine now, even the old faulty tubes didn't faze it, but when I crank it all the way it still pops the 500mA fuse between the power tube cathodes and ground. I need to stop it from doing this, because whenever it does the amplifier stops working.
I have two ideas as to what might be going wrong, but I wanted to get some input here because I know that some people here have messed around with class A2/AB2 before and know much more than I do about it. I think it is possible that I'm pulling too much grid current and causing the power tube to fail; in that case, my guess is that the appropriate fix would be to limit positive signal to no more than some number of volts above ground with a zener somewhere or another. My other idea is that there is something non obvious about throwing a JJ 6V6S into a circuit meant for an EL34 that I am missing completely.
Thanks for anyone who can help me out here - maybe this should go into troubleshooting, but that forum seemed more concerned with production amplifiers than tinkering, so I posted here instead.
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