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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
tls49's avatar
tls49
1,351 posts
 
I agree with 'pweg', the KT120's may have many hours of use and getting weak. The only way to do a true comparison is to use all new tubes.

m-db's avatar
m-db
930 posts
 
After giving us, and assuming the manufacture, the type and brand of your vacuum tubes is there a reason you don't mention the brand and model of your amplifier?

You mentioned to the manufacture the edginess of the120 tubes and his response was not to worry about the low bias because it made little difference? That's it? He made no comments about using the 120's or any other suggestions?

atmasphere's avatar
atmasphere
11,574 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
3,059 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. 
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