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I've posted here before about this amp. Still confused by what I see. Q10 and Q11 running too hot to touch. Trying to understand their bias currents. Schematic shows Q7 setting up bias of Q9 through D27. I ohm D27 Cathode TOWARD Q7 not towards Q9. See the same thing @ Q8 where I find the Anode of D28 towards Q8 emitter (not cathode). Bad schematic? This unit has been a real challenge as I can't run it for more than a few minutes before I feel Q10 and Q11 are too hot to continue. They have both been changed once. Circuit board is awfully fragile. Hate to pull them again. Thought Id try to ohms check through the circuit but now I'm a little lost. Any help/comment greatly appreciated. My friend is patient but I'd love to get these amp off my bench and back on his stage.
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The schematic is OK. Bias in this amp is set by Q9 and voltage divider made of R16, R17 and R18 resistors (only). Other components you mentioned are not related to bias setting. It is possible that some other components are bad.
Mark -
Overheating of Q10 and Q11 in a Bassman 200 is not uncommon and attributed to the transistors being so close to each other that a runaway heating condition can occur. The solution, offered by Fender, is to replace Q10,11 and Q9 (often damaged and part of the bias circuit). Leave the full lead length on Q10,Q11 and separate them as far as possible. Check all bias components.
If this does not get it, the bias divider that Mark outlined may need to adjusted.Comment
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I would double R18 and R20 to 100 ohms each and call it a day.
This will cut dissipation exactly in half and I'm sure the power amp will still have ample drive current.
*or* I would replace wimpy MPSA42/92 by larger case, much higher dissipation MJE340/350.
Watch pinout, of course.Juan Manuel FaheyComment
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Thanks for some teriffic feedback guys. What I really needed was some validation that the heat was a known condition. What I've done is fashioned a heatsink out of 1/4 of a pepsi can and secured it to both Q10-Q11 with Artic Silver and Heatshrink Tubing to hold it in place. From what I can see the transistors are still pretty warm but I'm feeling a lot better about it. Cranking it now. We will let `er run hard for a couple of hours and then start the task of putting it all back into the box. Good to have Knowledgeable opinions. That's the sort of thing that makes the internet great. Thanks again!!Comment
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Although when on a 100VAC variac it seemed fine. moving it to a 120VAC wall plug caused an additional rise and a lot more buzz after about three hours on-time. Ordered Q9 today. (It was never replaced) Think I'll move Q10 & Q11 to the heatsink rail and wire them back to the board. Thought I'd put a dab of HSink compound on the face and secure them with epoxy.Comment
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