BEZ T9B Intermittent problem

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First let me say hello to all since this is my first post here. Hopefully some here may be able to help me.
I have a Pair of Bez T9B kt88 mono blocks. You use to see these on Ebay.
They use 4 ea Kt88 output tubes and 3 ea 6N1 tubes. The amp is a auto bias type. Now what happens to one of them is you can play it all day long with no problems with music going, but if you stop playing music and the amp idles to long you get a blasting 60hz hum out of the speaker. If you turn the power switch off for a second or two it goes away and you can go right on playing music again.
So my question is anyone have any ideas where i should start to look?
I have opened the amp up and all looks fine no over heating or anything outright bad.

 

[pwgtang]

If you make sure power supply section was good,then may be
one of your amp's tube had internal leak or short between
cathode and filament.

 

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All output tube are new and matched it did the same thing with the old set. I did not try to change any of the input tubes 6N1's (12Au7's) according to the labeling on the amps.

 

[pwgtang]

Switch all the tubes between the two units to see the problem
follow the tubes or not.

 

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Bez T9B

You where right on the money, It ended up being one of the 6N1's , Funny thing it would only act up after an hour or so and only when not playing music. Go figure ? Thanks for your help