The question from the op is really interesting; can anyone address it? Does the KT120 compress the signal, while the KT88 does not?
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?
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?
- ...
- 25 posts total
Here's the link to VAC's take on the KT 120s: http://vac-amps.com/vacBlog/category/technical-musings/ 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? |
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." |
- 25 posts total