Is there a way to tell if a tube needs to be changed?

[PepeJara]

I would like to know if there is a scientific way of telling if a tube is too old to be used, testing it in the amp...I mean, the same when you test the bias, plate current and so on, is there a test to tell if a tube is too old and needs a replacement? Often I read that when you hear the amp "lifeless" and the tubes have more than "x" hours they need to be replaced, but I would like to test them
At this moment, my only tube tester is the "good/bad" type I had a Hickcok that died

[CurtissRobin]

I can't imagine that a test machine (at any price) could tell you that with any measure of confidence. I'd just swap the suspect tube with a known good one to see if there's a significant difference in the tone. Big difference: replace the tube; small difference: decide if it's a big enough difference to replace the tube; no discernible difference: put the known good tube back into your spare stock. If it's the rectifier tube, with the Standby switch in the play position I'd measure the B+ with the suspect tube then with the known good tube then decide if one of them is out of line. Swap or don't accordingly.

KennyO

[stevesuk]

A cautionary tale

I work on 18 watters so much every day I usually just quickly replace one valve at a time when looking for a faulty valve. Replacing the valve I have just pulled out, if a new one doesn't make a difference.

I recently had a hard gigging 36W in with old Mullards and Brimars I built probably 8 years ago ?
It was in for a terrible screech and howl at any volume. This lead me straight to the valves. I did my usual replacing one valve at a time.

No joy after replacing every valve including the GZ34.

I eventually took it out of it's cab and replaced all valves with new.
Problem cured.

Putting the old valves back in, one at a time I discovered not one, but two EL84's (one from each pair) were faulty.

I reluctantly hauled the valve tester out from beneath a pile of amps and tested the EL84's, yep, internal shorts, hence the deafening screeching with the volume controls down.

It had me going for a while until I walked away and came back fresh to the problem.

Steve UK
www.valvepower.co.uk

[shasen]

I can't imagine that a test machine (at any price) could tell you that with any measure of confidence.

Actually my Hickok 539B and most other Mutual Conductance Tube testers will be able to tell you how much gas is left in the tank of each tube. Having tested thousands and thousands of tubes, a good tube tester really does work. The Hickok 750, 539A, 539B, 539C models also allow you to test for microphonics.

The Triplett 3444 and 3444a are another two awesome testers that have more voltgae options when testing big power tubes.

The king of the hill IMHO is the Amplitrex AT1000 tester. This baby even gives you curves and can test big power tubes to the most ideal conditions. http://www.amplitrex.com/at1000.html

I never put a tube in my amp unless it has been tested but the one area that can't be tested by any of these testers is how the tube will sound to your ears.

[CurtissRobin]

is there a test to tell if a tube is too old and needs a replacement?

I can't imagine that a test machine (at any price) could tell you that with any measure of confidence.

I never put a tube in my amp unless it has been tested but the one area that can't be tested by any of these testers is how the tube will sound to your ears.

Which is the point I was trying to make (not too well, I guess).

KennyO