strange preamp tube issue?

[ALBATROS1234]

i got a nos 6113 which is a ruggedized 6SL7 preamp dual triode. it was made by national union 39th week of 1951. it worked fine when i got it and used it periodically in a couple of amp build and/or tube rolling to listen to different 6SL7s in certain situations with different power tube so as to optimize desired tone. i am sure i have put less than 4 or 5 hours playing time on it. was it truly nos who knows?

anyway to the problem. now when i have it in an amp and turn it on it sounds fine from the time it starts to produce sound for 20 to 30 seconds. it then goes a bit crackly and looses volume and gain by approx 40%. if i touch the tube it sounds good agaain. when i say touch i mean even very light touch of the finger without moving the tube at all. the tube does it in different amps and other 6SL7 types work fine back to back in the same amps. it seems to me like it must be a internal grounding issue due to the slightest touch to the glass fixes it.

i am figuring there is no fix but i found it to be a bit curious so i thought i would start a thread to see if others have experienced similar things. i know of the microphonic issues these tubes can potentially have but this seems different than what i perceive microphonic to be. anyone have any ideas?

[sluckey]

Break the glass and throw it away.

[ALBATROS1234]

i got that its busted and of no use to me anymore but, i was just wondering if this is something that happens sometimes? i have read quite a bit by now about microphonics and such but have not encountered this. it seems odd that after warmup it sound good for a bit then weak and crackling unless a finger lightly touches it. i assume some spot has a micro crack that opens after heat is applied for a time but why would it stop with the slightest touch of a finger . wondered if anyone had seen this symptom before?

[sluckey]

I've seen lots of tubes that would work properly if you put a finger on them. Will you use your pickin' hand or your frettin' hand?   

Seriously though, you may be able to put a couple silicone O-rings on the tube. Or put a ring of copper tape around the tube as a shield. Solder one lead of a 47-100pF cap to the copper shield and connect the other end to chassis. You might get lucky. You already know what the sure fire remedy is.

[ALBATROS1234]

thats what roadies are for, lol. but seriously, i hold no hope it will work i just found it odd and wondered if others had seen this happen. i paid like $20 bucks for it nos(supposedly) guess it had a flaw. i just shook it and heard some jingleing. i have a handful of 6SL7s but i liked that one, it sounded as good as my 49 rca. btw the national union 6v6 i have sounds almost as good as my rac 6v6 too, i guess n.u. is a good brand.