2. amps-preamps
  3. 127348-3465

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

Showing 3 responses by tmcclintock

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).
tmcclintock OP
35 posts
 
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?


tmcclintock OP
35 posts
 
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)
More to discover