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?
maybe - how many megawatts does your radio station put out?
Since the amplifier was originally designed for KT88s, why couldn't the improvement simply be due to the fact that they are a better match for the circuit than another tube type, though compatible from the standpoint that it won't damage anything, that has different specs?

"If this much is good, then more must be better" can be tempting whether tube wattage, car horsepower or a zillion other items, but often the best performance is obtained when things match. 
or maybe the the KT120's were at the end of their life?
Not exactly. Nelson Pass is famous however for noting that too many amplifier makers focus on the extremes of power delivery, and not enough reviewers, specs or designers worry about the linearity, quietness, etc. of the very first watt.
The KT120 can sound a little edgy and sterile while a good KT88 is quite good. I have some original Genelex Gold Lion which are sublime in the midband.
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.

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?

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.
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).
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. 
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.
The question from the op is really interesting; can anyone address it? Does the KT120 compress the signal, while the KT88 does not?

Here's the link to VAC's take on the KT 120s:

I don't see anything there about compression, though.

Thanks for the link, rcprince.  A lot of what Kevin Hayes says went over my head, but a couple of his points maybe speak to what I heard.

"3) You should expect more distortion with the KT120, possibly particularly at moderately low listening levels and with highly reactive loudspeakers"

"6) You’re likely to give up a few kHz of high frequency response"

And one of his main points is as mlsstl suggested earlier, an amp designed for KT88s will not sound as good with tubes having a different spec.

I'd still like to hear from anyone who knows what differs mechanically in tubes.  Like what is physically different between KT88s and KT120s - bigger plates? - thicker metal? - different metalurgy?

dunno - but 1st figure out if they are both triodes, pentodes or what

then you might find an old Radio Amateurs handbook to look at tho it may not have any hifi info it will give a starting place

you may wind up in some obscure corner of the IEEE journal stacks in a library...
I've been using a factory upgraded Jolida 502p for a few years now and KT120s sounded great and really lasted in that thing. Don't get the distortion issue at all as the 120s were really clean and powerful sounding until I wore them out (took a long time by the way)..went to 150s and they also sounded fine (although perhaps taxing the heater current draw on the 502) but before long I simply became curious to try the Gold Lions partly because they look cool (so sue me). Consequently, earlier this year I ordered an appropriately matched quad of GL KT88s from thetubestore and  they're GREAT sounding in the Jolida…sweet, snappy and accurate clear sounding tubes, and I might have to admit they're maybe a better match for this amp as it was shipped with 6550s. The Lions are currently ahead in points…"I like it, it's good."
Does the KT120 compress the signal, while the KT88 does not?

If a BMW has a Ford Cobra 500 hp engine installed in it, will it drive better, faster, or what?

Switching tube is more than matching the voltages.  Just because it fits in the socket doesn't mean it will work.

A KT120 or KT150 has a lot more 'horsepower' than a KT88.  If the amp you have is designed for X watt output and you now have X+2 or X+3 power tubes in it, you may well be over driving the output transformer and or demanding more 'juice' from the power supply that it can supply.  Both of these situations will not be ear pleasant.

Some mention that they can not get the bias pots to the desired mv reading when switching from KT88 to KT120 or KT150.  If you can not, then your tube is burning COLD and while that may make the tube last for 10 or 20 years it sounds ....cold, may even sound buzzy, and is running towards class B, or more simply to 'cut-off' of the power.  Not being able to bias to the recommended point (unless something is damaged) means that amp is not designed for that high an output.

If a BMW has a Ford Cobra engine installed in it, that BMW owner has too much free time.
The old AMC Gremlin with the 350 small block in it was a hoot, came that way from the factory. But I digress.
In high school oh so many years ago, my buddy had a Sunbeam Tiger w the 289 Shelby Cobra motor. The car was faster but unfortunately, the driver was not better.  He nearly killed us, and unfortunately, he did kill the car!
The Tiger was overpowered. A friend of mine had one with a 350 in it. Yikes!
I think it was Nelson Pass who said or made famous the phrase "If the first watt is not good  why would you want many more of them?" Or 100s more of them etc. 
The answer to your question can be ascertained by looking up the parameters available from the current manufacturer. Try your question on tube forums, like audio asylum tube asylum, for additional discussion.

The real answer however  is simple. Magic! 
The Tiger was kind of fun (a friend had one) and seriously cool but didn't handle so well.  From that era I preferred a Lotus Elan I was lucky to drive for a bit as those were REALLY fun, and felt very fast. 

Nelson Pass is too cool. Somebody wrote about hanging out with him listening to a First Watt amp with a meter showing how many watts were being used while listening to some fairly loud music…around 3 or 4 watts or so max, with not particularly efficient speakers. The "first watt" idea is completely correct and can be illustrated by any electric guitar player…try a 50's era Fender Tweed amp (Julian Lage uses 2 of these live…a beat up looking Champ and a Deluxe) and see what a first watt can sound like.

Thanks, glassandlight.  The part about not being able to adjust bias to the point spec'd for the 120s seems legitimate in my case and part of my original question.  The 88s operate in their range, the 120s don't.

As to what's physically different about the tubes, I'm OK with the magic answer.

Re: Sunbeam Tigers, revving up and dumping the clutch would make the leaf springs buckle like crazy.  No wheelie bars needed.  (currently checking out a hi-po 289 swap for my M3)