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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
lewm
6,770 posts
 
What Viridian and some others said.  Tubes usually just fade away.  You may notice a loss of gain, first of all, with tubes in a phono stage. Sometimes tubes also become noisy, but if no catastrophic event occurs, they just lose their mojo very slowly and gradually.  I think listening test is better than testing in a tube tester for determining when to change tubes.  Trust your ears first of all.  Some tubes can look marginal in a tester and actually sound fine.  This is because 99% of tube testers are incapable of testing tubes at the actual voltages and currents to which they are subjected in the context of  a working circuit.
harold-not-the-barrel
799 posts
 
^ Spot on. Tubes just lose their mojo very slowly. In my preamp new Golden Dragons last for about ten thousand in great working order but after that they just start to fade away. They simply start lose their dynamics and detail so the sound becomes weak and eventually just dull. Once I used for twelve thousand hours and went surprised how anemic the sound actually had been when comparing to a new set of tubes. I have never experienced microphonic nor blows in my preamp.
viridian's avatar
viridian
4,184 posts
 
Tubes do not age like light bulbs at all. Light bulbs are usually on, until they fail. Tubes age more like tires.
jab's avatar
jab
450 posts
 

Wow Virdm, I have read some tube POWER amps blow like you think a tire does, my amp would not go that way but many here purchase those poorly designed amps. I had a kt88 stop working, but I do not know when it stopped I just noticed the amps sound was unbalanced. Now that is a tube amp. If you were to look at the Continuous Automatic Bias System in the new VAC amps all of this talk is a waist of time.

But the original post talked about a PREAMP tube, so you are saying a 12ax7 will blow like a tire?

To the original posters question: 12-type tubes are like a light bulb, I turn off the light bulb when I leave a room, then turn on the light bulb when I enter a room, just like tubes, I turn on the tube when I want and turn them off when I am done.

Keep an extra set and don’t become paranoid like so many others

folkfreak's avatar
folkfreak
1,149 posts
 
In my experience the answer is - much sooner than you think. In the case of my old ARC Ref 2SE phono stage I noticed appreciable noise in one channel at 400 hours and replacing the set of 6H30s was a transformation in sound. I think factors such as on/off cycles probably play a part too, my listening sessions tend to be short so I may be hard on tubes. The only real test is to keep a broken in replacement set on hand and swap them out every several hundred hours or so once you get to say 500 or 1000
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