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McIntosh MC60 Plate Voltage

T

tubes&vinyl

New Member
Hello, I recently had my Mac 60's gone through. When I got them back they sounded great however very soon (on my first use) one plate on a 6550 began glowing red. They still sounded great however I shut them down and sent the one back to the Tech, He more or less sent it right back and said the 12AX7 that controlled the bias was bad, and since I have some extras, he did not replace it.
I have yet to try the Amps but did test that 12AX7 on 2 decent testers, one is a recently calibrated Sencore. the other a 539B. The tube in question showed no issues on any of the test parameters, nothing showed the slightest problem ?
The Tech told me that it did not function properly as the bias for that 6550 ? So this likely being correct as the Tech is very savvy and experienced, how does one recognize this issue before sending the Amp to a repair facility thinking there is a serious problem ?
Thanks in advance for the help......
Mike
 
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a schematic so we're looking at the same thing here

http://www.tubebooks.org/file_downloads/McIntosh/MC60_sch.pdf


Usually with this kind of thing I stick a voltmeter in there and compare to what the schematic or service manual says it should read. A bad 12ax7 could upset the bias on the output tubes, but bad coupling caps at C9 or C10 can upset the bias on the 12ax7, which will mess with the output tubes. A problem in the bias supply will do it too.

I have had tubes test very strong on testers, but thats not always a good thing. Ones with too much transconductance will usually bias different than a "bogie" tube, and in certain applications that can upset the circuit. If the tester you're using is an emission tester and just gives output in % it may not tell the whole story. Try another tube, or just switch it with the other 12ax7 and see what happens.
 
Seems like a good tech would have asked for your extra 12ax7s to find one that worked, or would have installed a good tube so it wasn't up to you to troubleshoot tubes to fix your own unit.

(I know that's not real helpful, haha)
 
I'd like to assume the tech swapped 12ax7 tubes, verified the problem went away, and sent the amp back with the understanding that OP would install his own tube to sort it out.
 
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