Terry2
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It is true tubes degrade over time, but not in the ways you might think. Sound wise they degrade very little, as long as the cathode can emit electrons. And that is the key really.
I have experimented quite a bit with tubes and their operation, I have run up a pair of SV83's well beyond their voltage and current limits. Usually what I have found to happen with them is once the tubes get "spent" they lose control over their ability to limit current via biasing and sort of go into run away within limits. It would appear that -1 volt was the new zero and so on. But before that the output wattage before clipping becomes weaker and weaker, and this is no doubt because the operating point is moving closer to -1v or whatever the new 0v bias on the grid for no current restriction.
I just wanted to find out what the real limits of a vacuum tube was so I tried several different things. Oddly enough brand new tubes seem to produce more distortion at first but once run in for a few hours they settle in to the same sound. No wonder some companies "burn in" their tubes before selling them.
In short the more "hot" you bias a tube, meaning its published max specification wattage rating for the plate, the less length of time they will last. But if you under rate them then they last for years perhaps seemingly beyond their datasheet hour limit or range. There is of course the published maximum current rating of the cathode, which I have never exceeded(that I know of), I should tried to with a lower B+ that would have allowed me to stay within the published max wattage of the plate.
So the question came up: If I biased a tube beyond its published maximums would it sound better. In short NO, it didn't sound any different to me. So as to keep it an ideal operating range to make use of the added current(to exceed the max wattage on the plate), at times I needed to increase the B+ voltage above its maximum as well. What happened was I gained more output wattage from the amp in general, but not so much that it would be worth ruinning tubes so quickly.
It turned out much better to just use a different tube, unless your just dead set on that tubes sound. Although I felt tubes with higher plate capacity really gave many other benefits that I don't know why you would want to over drive a tube for more wattage. I never did experience arching in the tubes when running the voltage up to 50% over their published maximum.
A much better alternative to me was to run two tubes in parallel, and if matched they didn't seem to really sound any different than a single tube. Well I didn't really hear a difference with unmatched tubes. Although I didn't like the idea of unevenly yoked tubes pulling the sound cart (so to speak.) I have tried up to five power tubes in parallel with good results, although the added needed circuitry and harware really increased cost over simply using a larger tube.
There is some charm for me in trying a tube that was not meant for power tube operation by using many of them. Although the results isn't always what one would expect, nice to dream about it though. I have dreams of monster single ended tube amps that can output hundreds of watts. But as of yet haven't tried one. One such case was using five GM70's per channel to attain 100w output to the speakers. For whatever reason when you start using higher power tubes the sound although amazing seems more scary, the sense of real power over a solid state or a push pull equivalent is different.
Oops sorry I got off on a tangent, that happens with me, sorry.
Terry
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