Any cure for a noisy valve?

[Max N]

I have a 12sn7 that starts to hum after a few minutes.
Swapping it out cures the problem, so it’s definitely the valve. I thought there was some info in MJ about ways of trying to salvage an otherwise worthless valve - maybe by burning off dust between heater and cathode?
Anyway I can’t seem to find the info. Has anyone ever tried it?

[Paul Barker]

Sorry, no idea, but normal expectation of 6SN7 family off valves. You have to select the
less noisy ones out of those you have afaik.

[Max N]

Maybe I’ve been lucky, or more likely my circuit is quite high level so less sensitive to noise. Either way I have a lot of 6sn7, 12sn7, 6j5, 7n7 etc and this is the only one where it’s noticeable in this amp. It is very noisy, the hum just builds up over time. A bit of probing with a multimeter usually sorts it out temporarily.
Shame because it’s a Tung-sol with angled plates, not the very best sounding type but one of the good ones, and I only have two more.
If anyone can suggest something to try, or if I manage to find the info, I’ll give it a go.

[Nick]

Unlikely but worth checking it's not oscillating at rf.

[Max N]

Good point, I’ll do that, thanks

[Mike H]

Makes me wonder if there's insulation breakdown occurring between cathode and heater. Altho the Voltage offset is supposed to be quite high (200V, 100V DC + 100V peak signal AC [if cathode follower)).

What I usually got with them is mild but annoying crackling or rustling noises. No amount of cleaning the pins and sockets did any help. Move the valve around, the noise goes with it. Conclusion, it am boogered.

I've probably still got them but with 'DO NOT USE' written on the boxes in felt tip.

[Max N]

Yes, I am guessing it is low heater to cathode resistance. Finally found the MJ suggestion in BVA 2nd edition: 150% heater voltage for 10 seconds.
Will check for oscillation first as that is obviously non-destructive….

[Max N]

Measured resistance to cathodes, cold, both off the scale (>2000M ohms)
Measured with heaters on, one cathode still off the scale, one 20M Ohms.
Tried the 150% heater voltage suggestion, no improvement.
Oh well it was worth a try….

[IslandPink]

I would find a recording of 'The last post' on Youtube, then you can give it a respectful send-off.

[Max N]

Or maybe I’ll wind the heater right up and send it ‘flaming into the night’.