Problems with old valves

[John_A]

Sorry for the obvious technical ineptitude, but I hope someone can help.

While I'm reasonably happy with the sound of my amp using JJ pre-amp valves, I found some old Mullards in a cupboard from years ago and thought I would give them a try.

Popped one in the V1 position and immediately noticed an improvement in tone, but after 2 minutes the amp started making strange noises. Pt the JJ back in and everything was fine.

Repeated the procedure with another Mullard and exactly the same happened, it appeared fine at first but failed after a couple of minutes.

I have a few more but don't want to damage them, am I doing anything wrong or is this normal for old (not NOS but used) valved that haven't been carefully stored to fail after a few minutes use?

[MrMike]

What does "straneg noise" mean? Can you describe?

[PeddlerPete]

Strange enough to make one think there's something in the circuit which would cause numerous tube candidates of a type/brand to not perform well.
Easy answer is stick with the JJ's, but taking some voltage measurements of tube acting right and acting up may prove useful.
The expectations of the 18w on preamp tubes isn't especially demanding, any good 12ax7 *should work fine in it.

[MrMike]

PP tough to assume, thats why asking for relevent facts before providing a solution is important.

Is it microphonics? Popping and Hissing? A growl? Causing lack of bass? No treble? No signal?

Lots of different kinds of noises, many different solutions.

[John_A]

A loud oscillating 'white noise'

There is quite a possibility that the valves were duff to start with, but they definetely worked OK for a minute or two.

[DaveW]

Do they work as the phase splitter?

[MrMike]

John, this is very interesting. Most likely, as you say, the tubes are stuffed. Out of curiosity what frequency was the white noise? Perhaps 60Hz or lower?

[zaphod_phil]

Most likely, as you say, the tubes are stuffed.

Which is probably why they were lying around in a cupboard...

[clobo]

Hi,

You don't mention what kind of amp, but don't mess with this tube in a position that does not have a high value resistor in either plate or cathode circuit (ex 100K).

For example, in a typical Fender reverb driver circuit, a shorting 12AT7 can easily melt the reverb transformer (connected directly to plates), before fuse has time to blow.

Not an expensive replacement, but it affects the vintage mojo (and resale value).

Claude

[John_A]

Thanks for the replies everyone, I think as you have said they were probably stuffed to start with! I've got some known good ones coming so I'll give them a try.

I didnt say what amp it is as this is an 18 Watt forum, (definetely my next amp) and the amp is a Ceriatone JTM45.