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In Reply to: Re: Non-match power tubes posted by Steve o on July 22, 2002 at 13:14:39:
Steve,So you've had supposedly "matched" tubes go ballistic---because only the Gm matched, but not the other parameters stated?
How do you compensate for this when the tubes "break-in" or start to wear out unevenly? Is simple re-biasing good enough? Especially, when all tubes are sharing one cathode resistor/bypass cap?
Follow Ups:
"So you've had supposedly "matched" tubes go ballistic---because only the Gm matched, but not the other parameters stated?" YES"How do you compensate for this when the tubes "break-in" or start to wear out unevenly? Is simple re-biasing good enough? Especially, when all tubes are sharing one cathode resistor/bypass cap? "
One of the reasons for buying "burned-in" tubes is to minimize the change in characteristics over time. Unfortunately, even tubes matched after a good burn-in may become unmatched over time. However, if the tubes were matched to begin with after burn-in they hopefully don't drift too far apart over time. If the amp employs individual fixed bias one can simply adjust each tube individually. If the amp uses the same fixed bias for both tubes there isn't much one can do but put up with it or replace the tubes. If the amp employs 100% cathode bias with a single unadjustable cathode resistor, there isn't much one can do here either although cathode bias is a bit more forgiving with regard to runaway. Some cathode bias amps have a balancing adjustment such as the original Williamson and Heath's versions of it as well as others.
Overall, the biasing scheme of the ST35 (and SCA35 and Heath AA151 etc.) are inexpensivve in the short term and problematic in the long term. This is why one popular mod for the ST35 is to separate the two channels so that only two 6BQ5s are on the same cathode resistor thus requiring two matched pairs instead of a matched quad.
Steve,Ever find a set of tubes, in which the grids age differently from one another? To the point of detriment and possible injury to the amp?
Not sure what you mean by "grids age differently". I have used pairs of TUBES that were well matched to start but after extensive use were somewhat unmatched although not to the point damaging an amp (unless you count sudden and catastrophic failure as aging). In all cases of aging or sudden failure I've beeen able to shut things down before serious damage occurs (nothing more than replacing screen stopper resistor and so on). Note that my tube stuff is mostly vintage and I DO NOT leave it on 24-7. If I can't get to it in 30 sec or less I turn it off.
Steve,I guess I meant if you've noticed certain tubes which the grids age differently---thus giving different internal resistance, etc, from one another as they age.
Not just the cathode and anode, but the suppressor, control, and screen grids. I never really thought much about variance, with age, among these grids and the effect on overall performance.
Guess I've never thought about tube aging in quite that manner i.e. the "grids" somehow changing over time. I've always looked at a tube as an overall system although my understanding of the wearout mechanism is: reduced cathode emission, gas/loss of vacuum or some combination of the two.
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