I have found that thermal cycling is usually only useful for bringing
about thermal intermittents so that you can properly troubleshoot.
Spraying around the chassis, looking for offenders is a last ditch
effort after your imagination has been depleted. I have had good
results with resoldering current carrying joints and vibration prone
joints with Silver bearing solder, as some can get hot enough to
oxidize the entire joint or vibrated enough to fracture, causing a
marginal joint to rise in temperature and resistance as it handles
current.
Have you considered this might be drive related? Is the power
really increasing, or is the impedance in the output changing?
Is the "power" you are seeing on the intended Fo or is there
another spur comming up to add with the intended signal and
fooling your wattmeter? Can you reduce the drive to maintain
the amp within limits as it begins to creep? If so, how is it that
the ALC action won't accomplish this?
What you have told us so far, is that the efficiency of the amp
is increasing dramatically, without any changes in the operating
parameters other than Ip. This is impossible unless your drive
is increasing or your tube is having a growth spurt and you
have more headroom than your power supply can handle. I
submit that maybe your malfunction is also leading to some
measurement error.
Jim, ac6tk
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