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.
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?