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In Reply to: Re: What would make one output tube go bright cherry red!! too much drama posted by el_37 on December 2, 2006 at 07:04:11:
"The X-101-C is a cathode biased amp..."Go here and look at this schematic:
http://oldtech.net/Fisher/X-101-C/page5.gif
The amp is partially cathode biased unless the schema is incorrect... There is a bias adjustment pot present that changes the grid voltage.
Follow Ups:
Hi Jim,Yeah, I think you're right, I think it's biasing. I think everyone talking about a defective tube are probably barking up the wrong tree.
I have made a bit of a specialty around modding AA-151's, I have a small herd of them now, and I have witnessed one el84 going cherry in two of them that were otherwise OK - that is, the iron was OK.
I changed el84 makes, and it didn't make a bit of difference, went right to cherry on the same socket.
My fix was I changed the circuit from a single cathode bias resistor for all four to a separate resistor for each pair, and raised the value a little, and got rid of the bypass cap (with a separate cathode resistor for each push-pull pair, I could tell absolutely NO difference with it in or out, so out it went), and that fixed it.
HOWEVER, I think your suggestion about lowering the grid resistors is a great one, and I shall do that mod forthwith!!
best, charles
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
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