First schuffle tubes to identify the bad one...
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Hmmm...this does not sound like a tube problem. Sounds more
like one of two problems:
1) a cold solder joint going intermittent; or
2) The volume/gain pot or the MM/MC switch is dirty.
First swap the tubes to see if one of the tubes is faulty.
I would first try to eliminate possibility #2 as it's easy.
Next time this happens turn the gain knob up/down and flip
the MM/MC switch (make sure system volume is LOW!!!). If you
hear any sort of crackle or channel loss/regain get yourself
a can of Caig Deoxit and clean the pot/switch.
If it's a cold solder I would send the unit in for service.
My old MAC-4100 showed the exact same symptoms. Cleaned all
the switches and pots but no luck. Sent it in for service
and sure enough they found a cold solder joint...it took
them 5 hours on the bench to find it!
The unit hasn't done it again. However I did notice a small crackel noise in the gain knob. Very slight but it is noticeable. Maybe I need to clean the pot switch as directed by Meisterkleef. I swaped the tubes prior and well maybe that's why it hasn't happened again. Or the switch just has some dirt on it. Thanks for all of your help in the matter too...........