The NU 6N7GT/T is a great sounding tube - the gray glass are from the 40's.
I have been able to fix some noisy tubes (pops and crackles - not hum) by heating each tube pin with a 30W soldering iron for around 30 seconds. Just hold the tube upright so that any solder that might flow doesn't end up inside the tube.
For hum and microphonics I have had luck by applying and stretching a tight band of self fusing silicone tape around the glass.
Question: When you measure a quad temperature, are you referring to the tubes themselves (hottest point) or the amp chassis? I would think that what counts is the amp temperature.
IMHO I have also noticed that tubes that have been burnt in and used for a longer time tend to run less hot compared to when new.
PS: It seems to me that that NU is gray glass (haven't seen any black glass 6N7GT) but who knows.....
@mordy you gave me your 30 second soldering iron tip a while back, finally got around to giving it a try. I have a beautiful Fivre 6N7G that tests perfectly in my tester, but is plagued by noise. Using your method took the noise from a 10 down to a 1, and I suspect a second go-around might take it to a 0, I am in your debt! This is a special tube for me and I was very distraught at how unusable it was. I will try this on the EL32 next, they have given me a lot of trouble.
Here it is with a pair of Western Electric 421A. Listening to Aphex Twin - Selected Ambient Works Volume II
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