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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
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?


randy-11
1,913 posts
 
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...
wolf_garcia
6,695 posts
 
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."
atmasphere's avatar
atmasphere
11,793 posts
 
Does the KT120 compress the signal, while the KT88 does not?
No.
glassandlight's avatar
glassandlight
19 posts
 

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.

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