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Tube Physics Question

At least I think it's Physics. 

Short version:  Is it logical that going to a less powerful power tube in an amp can yield better dynamics?  What's the Physics behind tube design/operation and their ability to reproduce the swing of musical dynamics?  What part does bias play?

Longer version:  A couple of years ago I acquired a used tube amp for a second system.  It came with three sets of power tubes - the original KT88s, a set of Gold Lion KT88s and a set of Tung Sol KT120s.  The amp had been biased for the 120s, and those are what I used and have been happy with.  More power = better, right?

Recently I swapped out the speakers in that system for a more efficient and transparent pair.  While the change was nice and an obvious speaker upgrade, I started hearing a little edginess in the treble.  Since I'd never checked the bias I put a meter on it and found it low (avg ~12mV vs. a recommended 45-50mV for KT120s).  Running the pots up to max only reached ~35mV, but it did improve the sound.  Called the mfr and he said not to worry about a lower bias setting, as it made little difference.

Not content, I swapped out the KT120s for the Gold Lion KT88s, checked/reset bias to 35mV (recommended for 88s) and wow, what an improvement they made.  Without getting into a bunch of hyperbole, what was immediately and most noticeable was the increase in dynamics.  So my question is, what about the design/production and implementation of a tube affects its ability to reproduce dynamics?  Or is it more likely that running a lower power tube at relatively higher bias causes the effect?
tmcclintock
atmasphere's avatar
atmasphere
11,793 posts
 
As far as I can tell, the KT88 is a better tube than the KT120, which appears to be on the way out in favor of the KT150.

KT120s get a lot of bad comments on account of excess distortion. Its not at all surprising that even though there is less power that the KT88 would otherwise be preferred.
tmcclintock OP
35 posts
 
Thanks for your responses.  True, I don't know how much the KT120s have been used (likewise the KT88s), but was told they didn't have many hours on them.  Second system, so I haven't put many on them.

I didn't mention the amp manufacturer because that was the constant.  What changed was the tubes.  And I only discussed with him the bias maxing out before reaching recommended voltage, not the sound.  The amp is a Music Reference RM200.

My question is more about what feature of a vacuum tube can cause a difference in the dynamics an amp produces.  (hope I'm using that term correctly - soft passages really soft, loud passages really loud - big swing between the two)  And does bias setting play any part in the resulting sound?  For instance, does increasing bias voltage move an amp more toward class A?  Did the fact that I couldn't adjust bias to the recommended value for KT120s put the amp in a better state with KY88s?

And FWIW, I'd side with noromance and atmasphere that KT88s sound better (at least in my amp).
rcprince
3,070 posts
 
Kevin Hayes of VAC has also preferred the KT88s to the KT120s.   I believe his thoughts on the tubes, which include a technical discussion, are in a blog on his website. 
m-db's avatar
m-db
971 posts
 
While my manufacture, Bob Carver, feels the KT 120s are a sonic improvement in his VTA180 design I've returned the KT 88s back into service while my 120s are collecting dust. 

There is definitely an audible difference between the tubes but nothing like what you've described. To me the much older 88s simply sound right to my taste. Possibly another amplifier design might showcase the 120s.

At some point I may consider the upgrade needed to use the KT 150. Good luck with it.
fundsgon
150 posts
 
The question from the op is really interesting; can anyone address it? Does the KT120 compress the signal, while the KT88 does not?
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