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Do tubes performance degrade with use? (Read 5345 times)
flargosa
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Do tubes performance degrade with use?
11/19/16 at 16:58:45
 
Do tubes degrade with used or does it sound the same until the day it dies? Does it it fall out of spec the more hours you put in it?
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SonicSeeker
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Re: Do tubes performance degrade with use?
Reply #1 - 11/20/16 at 13:09:52
 
A tube can degrade with age.
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Dave1210
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Re: Do tubes performance degrade with use?
Reply #2 - 11/20/16 at 14:49:18
 
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flargosa
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Re: Do tubes performance degrade with use?
Reply #3 - 11/20/16 at 15:41:33
 
Seems like opinions vary depending on the forum.  Some say there is noticeable degradation while other’s say there is little to none until the tube expires.   Do you guys listen for audible degradation then switch out your tubes, or simply replace it when it dies?
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Lon
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Re: Do tubes performance degrade with use?
Reply #4 - 11/20/16 at 15:45:21
 
I definitely hear sonic degradation and don't wait for a tube to die to replace sets.
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flargosa
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Re: Do tubes performance degrade with use?
Reply #5 - 11/20/16 at 19:51:51
 
In general at what point in a 6,000 hr tube does the degradation become audible?
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Lonely Raven
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Re: Do tubes performance degrade with use?
Reply #6 - 11/20/16 at 20:12:51
 
IMHO, it depends on a lot of variables - yes, all tubes physically degrade, it's simple physics. But what and how this effects sound depends on so many variables. I've had gear that used preamp tubes for 20 years and swapping in a new tube does almost nothing to change the sound. Then I've had amps where I'd swear barometric pressure changed the sound (slightly exaggerating of course).

But yes, tubes degrade with age, it's physics.

Being the way things are, and how slowly they degrade, it's always a good idea to either swap them out at specific intervals (like a tune-up for your car), have them tested on a proper tube tester, or have a fresh set that you can compare to. Because the change is usually so gradual, it'd be difficult to say "gee, I need to swap these out, they sound soft".  Though some people, if they really know their gear can spot it. Stevie Ray Vaughan's guitar tech used to tell a story about how SRV (being the Texas Hick he is) would say "My transformers are getting soft...can you change them out?".  But obviously, he was noticing his tubes change.
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Terry2
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Re: Do tubes performance degrade with use?
Reply #7 - 11/20/16 at 22:20:35
 
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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