Tube going bad in preamp? How to tell?

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soundchaser59

Reluctant Commander
Being new to the tube mic preamp thing, I have encountered unfamiliar ground here.

I have a generic dual channel (low voltage) tube mic preamp. (not a "tube mic" but a tube preamp) When I first turned it on, I found that both channels could be set with identical knob settings and they would produce the same sound. I could switch mics around and still both channels would sound the same.

But now, one channel is becoming distorted, and the output level is very hot even if I turn the knobs down (ccw) really low. It sounds like the same distortion I would get if I ran an instrument too hot into the mixer trim, that kind of hi freq buzzy/fizzy/static-y distortion. I know it's not the mics because if I switch the mics the distortion stays in the same channel all the time. It's mildly annoying because the tubes only have about 20-30 hours of use on them. Shouldn't they last a lot longer than that?

Is this an accurate description of what happens when a (12AX7) tube starts to go bad?? I'll find out for sure (I Hope!) tomorrow night when I remove it from the rack and open it up to change the tube. But just thought I'd ask to see if anybody out there can say for sure, "Oooooh Yes! That's exactly what it sounds like!" or "No way, SC, you need to hire a real engineer!"

Thanks!
 
A 12AX7 should last something like 5,000 hours, longer if the filament is starved. However you problem could be a bad tube.

When you open it up, swap the tubes between the channels, and see what happens.
 
mshilarious said:
When you open it up, swap the tubes between the channels, and see what happens.
Click to expand...

5,000 hours......cooooL! I'd settle for 1 or 2 thousand!

Good idea, switch the tubes and see if the problem changes channels! Thanks!
 
Preamp tubes will literally last for years.... However, they are delicate. Even some bouncing around in a car can cause them to get messed up. Especially with all these solid state componant tube stuff floating around these days.

If you bought it used that's a good possibility that the tube could be bad. If you bought it new I seriously doubt it's the tube, but swap and check. You can buy a new one for like $10. Even if preamp tubes will glow a very light blue when audio is sent through them. I'm talking about the actual filiament,,, if it's not glowing It's not working.
 
If the output is always really hot on that channel, I would suspect that the gain knob took a dump.
 
I have a Sebatron that is about 2 years old, but which I use only sparingly whenever I have a project of my own to do.

So I would be nowhere near the 5000 hours of use. Does the tube deteriorate to any significant extent when not in use?
 
Actually most tube wear is from the thermal shock of warm-up and cool-down from turning the unit on and off...

Now I'm not saying that you'll get longer life from a tube in a unit that you use once a month by leaving it turned on for the whole year... but if it see's daily use... the tubes will last longer if you just keep it powered up...
 
If you can get access to a tube tester do an emissions test. Lightly tap on the tubes and listen for static type noise in the output or just tap on the chassis.. There used to be radars and other tube type gear that had a standby mode which put half voltage to the filaments to increase the life of the tubes. It also decreased the warm up time....

After reading your post again it sounds like a component failure and drastically changed the bias on that tube. Look for burned resistors or look at the end of the electrolytics for leakage. A tube swap would indicate which one is at fault.
 
soundchaser59 said:
I have a generic dual channel (low voltage) tube mic preamp.
Click to expand...

What's a "generic" tube preamp? I guess tube preamps are now generic, kind of amusing. Which unit are you using?

I like the suggestion about swapping the tubes between channels, that will tell you if it's the tube or something else in the electronics.
 
SonicAlbert said:
What's a "generic" tube preamp? I guess tube preamps are now generic, kind of amusing. Which unit are you using?
Click to expand...

he's embarrased to say he doesnt like his Avalon 737 that he bought to record awesome rapers. i MEAN RAPPERS!
 
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