How do you spot bad tubes?

What I have done is depended upon sound to tell me when a tube starts going bad.  If your output sounds like the rustling of leaves, then one of your tubes is starting to go bad.  I've had a lot of tubed amps and pre-amps over the years and that is what I have always done.  However there may be other or even better ways to know.  

 

Generally my tubes have lasted many years (5) or above before I have started to hear the rustling of leaves sound.

 

Hope this helps.

Tube tester.

They still make those?

 

That part is called the "getter" It is there to suck up funky air that leaks into the tube. If it turns dull white or yellow (it should be shiny silver or black-chrome if its good) your tube is toast.

 

There are other ways a tube can kick it though. Several parts of a tube age under normal operation. To check for these you can run by ear, or get a tube tester. Or both. The downside to a tube tester is that they are expensive :(

 

The good news is that most tubes will last for several years of day-to-day use. 

 

Zvex makes a couple. Hagerman recently made a few VERY nice ones (with a curve tracer so you can actually match tubes under load... ooooh, aaaah)

They are not terribly difficult to DIY if you only want to test a few similar tubes.

 

Edited:

it may not be Zvex who makes the tube tester :( wavoman has the one Im thinking of. 

Edited by nikongod - 11/17/10 at 9:46am

You can also buy vintage testers that have been calibrated for varying degrees of expense, depending on how sophisticated they are. I got my B&K 606 for $150; the true mutual transconductance testing B&K 650 cost me $400 including the calibration.

IMHO it's important to check a tube for leaks, shorts and gas (not a big issue with audio tubes) before you use it.

A low reading tube may not last you long, but a shorted one can cause problems.

Different testers, even calibrated ones, will not give exactly the same results but you can find out if a tube is safe to use and if it has some life in it.

With double triodes like the 6SN7, the sound quality starts to deteriorate, one side goes first and then the other.

A tube which is testing good can have noise problems, which you can hear if the tester has a noise test with suitable headphones.

The final and best test will be your amp.

I took skylab's advise and started with a nice B&K 606, after that the tube tester addiction started.........

Transcendent Audio also makes sort of a tube tester.

You can pick up vintage testers for a reasonable amount. I think I paid $25 for mine a number of years ago. It's nothing special and needed a little surgery (I gave it new resistors and caps) but it works well enough.

My favorite indication of a dying tube is the purple haze. Beautiful, but you'll want to replace it.