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In Reply to: RE: As I always advise in situations such as this,... posted by ChasLieb on September 25, 2015 at 14:01:09
The best way to do it would be to invert the amplifier on a safe and sturdy workbench. Then remove the bottom plate and then measure the voltages with the tubes all in place. Each 6AS7 has two discrete sections. However, before following this line of investigation, I would suggest first of all to follow Ralph's plan of attack.
This weekend I have been dealing with almost the same problem on my humble diy M60. I measured the voltages on the sockets of all tubes (tubes taken out). Everything was fine except the grid voltage on V4 (-285). I suspected there is a fault in 5K trim pot, replaced it, and the problem disappeared.
I fixed my neighbor's Novacron, which had a problem with "low power output" (much like that of the OP in this thread), and it also turned out to be due to what appeared to be an intermittent problem with a bias pot, which resulted in operating one bank of 6C33Cs in near shut-off conditions. (Bias voltage was around -80V, which will pretty much turn off a 6C33C that sees the typical plate voltage in a Novacron.) I gave the pot a few turns in both directions, then re-set the bias, and that alone seemed to cure the problem, which has not recurred. Either that or he had just erred in setting bias. He seemed to know how to do it, so I tend to think the pot was faulty, but I really cannot be sure.
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