Tube change...what do you do?

[Skorpio]

My Simple SE has been running very nicely for weeks now, using old second-hand tubes (unknown GZ34/EL34 and Mulland ECC81). I have installed a hour-counter in the power supply so I can monitor the runnng hours of the tubes.

The question is: When is the right time to change tubes (and at what pros and cons)?

I see some possibilities:

1. When something stop working
2. When you feel the sound is degraded
3. When you have exceeded xx hours of expected life..

What are your personal preference?

[rknize]

1 and 2, obviously. In the case of the SSE, the ECC81 was the first to go for me. The gain started to drop a bit and then I started to get some crackle. I think this is from the CCS running out of headroom trying to compensate for the weak tube. The GZ34 going soft will eventually lead to sagging B+, which manifests at louder volumes at first. Note that the GZ34 has a fairly easy time in most builds of the SSE, so even a weaker one will work just fine for a while.

[Skorpio]

Perhaps not clear, but being relative newbie with tubes, my considerations were if it would be normal just to wait for something to happen, or that if so something drastic could happen (damage to transformers or circuit)...

[Structo]

Usually the high frequencies are the first to suffer when a tube starts wearing out.
Generally it happens so slow that some times we don't perceive it.

Vintage tubes seem to be much more robust and stable than current production tubes.

So barring a catastrophic failure, such as a shorted rectifier or power tube, they just gradually wear out.
I don't think you can quantify that with a hours meter, maybe on an oscilloscope.

[rknize]

You don't need to worry about damage for normal tube wear. If you are unsure about a tube, it is easy to tell that it is weak by measuring its plate current in the circuit in question. You know what the operating point is supposed to be, so just take a few measurements and see.

For example, if the tube has a cathode resistor on it, you measure the voltage across it to calculate the current through it and then measure the plate voltage.

Russ

[Skorpio]

Thank you both!

Regarding the hours counter, that is just to get some experience...

[Jpeg]

Lets see, tube failures... So far I have had:
1) vintage 6BQ5 where the filiment sagged and came into contact or got to close to somthing causing a fine 60hz buzz. 1) Vintage 6sn7 that simply got very quiet. 1) Shuguang Treasure that also got very guiet, then lit up like a street light. I turned the amp off right away, I would have like to have gotten a picture... 3) tubes that rolled off the table and on to the floor. Ok that might not qualify as tube failure.

As far as tube life, I have heard stories about tubes lasting 30-40 years, or, like my Treasure, about 3 months/200 hours. There are a lot of factors that affect tube life, such as voltage, current, vibration, heat, ect. A tube in a class AB curcuit will outlast one in a class A curcuit. I have heard that 2000 hours is a petty good average lifespan for a current-production output tube, and that signal tubes generally last twice as long as output tubes. That may or may not be true I probably read it on one of these forums. I would venture a guess that most audio tubes get replaced because we think we found some that sound better. They don't usually harm the curcuit, but I would reccomend killing the power as soon as you see anything strange or get one quiet channel. Tube testers are nice to have (or a friend with a tube tester) to take some of the guesswork out of it.