Another thing to try is a small value cap from the grid of V2a to ground. I get this from the Rainer zur Linde Audio Valve Amplifiers book in one of the schematics where there have an EF86 directly connected to the phase splitter. They are using 3.3pF and mention it is there to stop any tendency for the amp to oscillate. It should be rated at your B + voltage.
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Hold the phone here. If you mean going by the schematic Edcor game me yes. Which is Blue-plate white/blue-screen, brown-plate white/brown-screen. So naturally I put white/blue with the blue on one tube and white/brown and brown on the other tube.
But this is what I get when I resistance checked it:
center tap to brown 168.4
center tap to white/brown 42
center tap to blue 202
center tap to white/blue 43
brown to white/brown 210
blue to white/blue 250
brown to white/blue 125
blue to white/brown 160
So basically this has been a wild goose chase because Edcor messed up? Looks to me I need to switch the screen leads going to the output tubes. Or am i going crazy?
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Eureka! NO MORE SQUEAL ON SHUTDOWN!!!!!!!!!!!!!!!
That was it, no really that was it. I can't believe I checked and matched every resistor/cap in this amp before the build but I never checked the OPT. Who would have thought that a company would screw up this bad. Do they even have a quality control before these things go out? I guess not.
I also went back to the stock 47pf value cap on the EF86 plate stabilizing network. I got my highs back now and it sounds good. And the best part is no more squealing!
Going back to kevinkr's question about R13, the value I used was 5.6k with the 8 ohm tap. Are you saying I am better off removing the cap in parallel with it?
That was it, no really that was it. I can't believe I checked and matched every resistor/cap in this amp before the build but I never checked the OPT. Who would have thought that a company would screw up this bad. Do they even have a quality control before these things go out? I guess not.
I also went back to the stock 47pf value cap on the EF86 plate stabilizing network. I got my highs back now and it sounds good. And the best part is no more squealing!
Going back to kevinkr's question about R13, the value I used was 5.6k with the 8 ohm tap. Are you saying I am better off removing the cap in parallel with it?
Great news that the amp is now working correctly, odd about the screen tap swap..
If its working and there is no evidence of oscillation you can leave C9 as is..
When you do finally get a scope it would be a good idea to revisit the value of both caps and optimize for good square wave response and unconditional stability into real speaker loads.
If its working and there is no evidence of oscillation you can leave C9 as is..
When you do finally get a scope it would be a good idea to revisit the value of both caps and optimize for good square wave response and unconditional stability into real speaker loads.
Ok thanks Kevin.
And thanks to everyone who helped me, I feel terrible that I wasted everyone's time on something so stupid. And I am a little bent out of shape from having to waste so much time to track down something I shouldn't have had to, but you live you learn I guess.
Don't feel bad..I had the same problem on my Edcor transformers. But my amp squealed all the time not just on shutdown, and that was with nothing but the output stage connected, so it was a little more obvious to debug. It's a good idea to check resistances on the secondary before you wire up the transformers in the circuit. Transposing plate leads seems to be a quality control problem Edcor has on their PP models for some reason.
Don't feel bad..I had the same problem on my Edcor transformers. But my amp squealed all the time not just on shutdown, and that was with nothing but the output stage connected, so it was a little more obvious to debug. It's a good idea to check resistances on the secondary before you wire up the transformers in the circuit. Transposing plate leads seems to be a quality control problem Edcor has on their PP models for some reason.
Really? Wow this is the first time I went with Edcor and probably the last. I figured my case was a rare but I guess not. I like to buy USA made stuff but this just makes me sad.
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