2. misc-audio
  3. 1170020061

how do you know a bad tube?

I'm thinking of winding up all or at least mostly with tube gear. that said, how does a person know for sure which tube device needs a tube?

By that i mean, if you have multiple preamps, and amps, then I'd see it as not a lot of trouble determineing which one has perhaps an issue. but if you only have one pre and one amp both with tubes and things begin to sound funky, how then do you tell which unit is the culprit?

Past that then, how do you know which tube? (given there aren't any indicators on the chasis, and the tube itself isn't dead blown). ?? ...and apart from having on hand dupes of all the tubes in the system.

I am overlooking the obvious here for a reason. I'd as soon not have to get a tube tester. Unless there is a mighty simple one to use which has an oscillator in it as well as meters.

Sorry if it is a dumb question, but sure seems like a simple answer here will come in handy later on... as I'm looking for an "in house" solution that ain't way expensive and is simple enough tactilly for me to use.

thanks much
blindjim

Showing 3 responses by larryi

larryi
3,716 posts
 
I don't know of any kind of simple and cheap tube tester that will reliably tell you when a tube is going bad. Most testers are not cheap and many require calibration and maintenance.

Also, a tube can begin to go bad in ways that certain testers will not be able to detect. Even a good tester that measures transconductance, may give you an idea of the strength of the tube, but not other conditions, such as noise. I had a small signal tube, an EH 6sn7, that became gassy. It tests spectacularly strong for transconductance, but my tester (not cheap) also tests for gassy condition and it showed that this tube was horrible in that respect.
larryi
3,716 posts
 
You should have at least one extra set of tubes. That extra set will provide the tubes to swap and test for the bad unit. It also means your equipment is never out of commission just because a tube is going bad.
larryi
3,716 posts
 
Newbee,

The act of turning off a component, often results in noise, without any component necessarily being defective. That is why one should not turn off sources or the linestage/preamp first, because any such transient "pop' will be amplified by the power amp, perhaps to destructive levels.

Sometimes noise in tubes is eliminated by the very act of pulling and reseating the tube. Often noise comes from poor contact at the pins, other times, something deposited on a surface inside the tube can be shaken loose and the source of noise eliminated.

Perhaps the best reason to have a tube tester is not to find out if a tube is beginning to go bad, but rather, to determine if a new tube is safe to use at the outset. I own a tester and lent it to a friend. The tester determined that one of the rectifier tubes he was keeping as a replacement showed no voltage drop in either direction (shorted); it is frightening to think what that tube would have done had he inserted into an amp.
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