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When to replace preamp tubes?

When do you replace preamp tubes like 12AU7sand 12AX7? Do you wait till they blow or start making noise? It seems I’ve had them in for a long time and are in use everyday. I have replaced them in the past when they become microphonic but should I wait that long, should I just replace them after a certain amount of hours?
djf1
yogiboy's avatar
yogiboy
5,764 posts
 
Preamp tubes last many thousands of hours. I have antique radios that are over seventy years old and still have the original tubes. Even power amp tubes have a long life span! I've never had a tube that went microphonic. 
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elevick
2,638 posts
 
Preamp tubes rarely blow.  They just fade away...
If they lose dynamics or get microphonic it maybe time.  A tube tester is a must or find a local shop with a tester.
gs5556
1,239 posts
 
It depends on the preamp, some of them are tougher on tubes than others so you really can’t get a definitive answer. You can install new tubes from the same date batch for each channel and one will give it up after 6 months while the other can stay strong for five years. Happened to me a lot over 25 years with the tube-eating CAT SL-1

What I do is every two years is substitute a new set and if I don’t hear a difference I put the old ones back. This way I don’t have to stress over whether the tubes have lost something that I can’t hear because of the gradual deterioration nature of tubes. Other than that, when you hear hissing, pops or crackles, obviously then it’s time for a new tube.
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