Measuring the life of a TUBE

[Amadeuz]

Hi, this is a call for all you tube wiz's.
Any one knows if there is a way to measure the remaining life of a tube?

How does one go when purchasing a mic that uses really hard to get tube like the ac701 found in popular choices like Schoeps 221b?

Also, I have heard that some designs use only half a tube, thus giving the possibility of "turning" the tube to use the newer half, as the component ages to near failure. If this is true, can visual examination determine this?


Cheers!
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Amadeuz

[guitarbill]

Hey A, It's been my experience that preamp tubes last a very long time, assuming that they have not been abused by wrong voltages or mechanical or electrical shock.

Really old tubes will show some discoloration around the bottom of the black cap (interior flash coating on the top of the tube ; sorry, I forget the technical name) if they have been on for a long time. However, even when discolored a little they may still perform well. I have a bunch of RCA 12AX7A's and 12AY7A's that are slightly discolored and performing well that are over 30 years old.

As far as rotating a dual element tube in a single element mic pre-amp, you may not gain anything from this because when a tube gets gassy or otherwise has been hammered, the whole tube is bad. You would have to change the wiring on the socket as well. That may not be a good idea in a vintage mic. I wouldn't do it myself unless I had good soldering skills and knew how to safely disassemble and reassemble the mic without damaging anything in the process.

gb

[Amadeuz]

Thanx GB!

Originally Posted by guitarbill

That may not be a good idea in a vintage mic. I wouldn't do it myself unless I had good soldering skills and knew how to safely disassemble and reassemble the mic without damaging anything in the process.

gb

It's true, I would never attempt this myself. I originally heard about tube rotation from Tony and his gang (Telefunken USA). They were commenting on how many legendary tubes have been tossed away, when all one has to do is turn them and use them for another 40 years.

I just wonder if anyone in here knows about such designs, and what mics use them. (u47?, 251?, etc.)

[BOWIE]

To measure the life of your tubes you can use a tube tester but it has little bearing on the sound quality. Some of my best sounding tubes have horrible terter readings.
One very innacurate way of taking a rough guess is to look at the glass bottom where the pins protrude. In newer tubes the bottom is often perfectly clear whereas it gets blacker as time goes on. This is not a good way of measuring but there is some validity to this though it mostly applies to mini tubes (12AX7, etc).

As far a "turning the tube" goes, you wouldn't actually turn the tube but rather re-wire the socket that holds the tube. If done properly, you could definitely use the other triode (provided that you're only using one to begin with).

EDIT: oh, I see GuitarBill allready stated most of this. Sorry for the repeat.

[Matt Syson]

Hi
Using the 'other half' of a dual tube probably won't achieve anything unless the original only uses one half of the heater (12AX7 and similar duals here). If the heater has been on and no anode current drawn then it may well suffer from 'cathode poisoning' so may be worse than the original half.
Tube testers must be used with caution as thre is a distinct possibility of testing to destruction or certainly beyond where they are happy so 'stuffing' a perfectly good tube. Stabilised heater power, applied gently and proper operating conditions should provide many thousands of hours operation. As with all things, if abused they will tend to fail 'early'. The proper test is to fit it in it's new home and leave it running for a week, then check the performance.
Matt S