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In Reply to: RE: New production 6L6/KT66? posted by classfolkphile on March 19, 2015 at 09:55:29
Just wondering, have you ever performed any actual frequency response tests when changing tubes? There are many reasons replacements sound different, including the need to retune the amplifier.
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Buy Chinese. Bury freedom.
Follow Ups:
No. How does one "re-tune" an amplifier?
What TK is talking about is,to really optimize an amplifier when using different tubes,you really should tweak the feedback and the bias for lowest distortion and the best and flattest frequency response..I'm glad he recognizes the importance of audio analyzers and test gear because it really does give you a leg up in performance when you do this.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
Ok, got it. I adjust the bias, of course, but I have no way manually to adjust the feedback on this amp and I'm not technically knowledgeable enough to test and change parts.
At some point I'd like to order a custom integrated amp: I'll have to include adjustable feedback as one of it's features.
Thanks!
In addition to feedback and bias, many amplifiers use frequency compensation networks that need to be tweaked when tubes are changed. This all means that the audible effects of tube rolling are really only appropriate for the specific amplifier used to test them. It's not fair to say that this tube or that has better bass, better mids, more air, etc. in a general sense unless each was tested in an optimized environment. Further, it's very possible to overlook the tubes that are actually a best fit if the amplifier isn't retuned for each type. That's the real shame when someone performs tube rolling without test gear. Of course, in the end, tube rolling is about finding the sound you like best. There's still a potential benefit, measurements or not. :)
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Buy Chinese. Bury freedom.
In addition to feedback and bias, many amplifiers use frequency compensation networks that need to be tweaked when tubes are changed. This all means that the audible effects of tube rolling are really only appropriate for the specific amplifier used to test them
BINGO!
You have to admit that the analyzer has showed you a lot of things we took for granted when we rolled tubes..Even hooking a pentode up in triode is going to change because the tube now has a more linear loadline, lower plate resistance, lower gain, and higher transconductance.The problem is,the amp wasn't designed to run this way so when you take measurements in the Pentode designed amp that is wired as a triode,you will see the distortion go up while open and closed loop gain changes significantly.Distortion almost triples in many cases so tube rolling does have its drawbacks even in the smaller tubes because as you say,the values have to be tweaked when you change tubes.
We often hear people say,I put these different number tubes in my amp or preamp and I was floored by results and to their ears that may be the case however,the changes they are hearing may be a rise or dip in frequency response at different parts of the spectrum or maybe a different distortion number..Once they see this behavior on an analyzer,they don't seem to like that tube(s) anymore..Funny how that works..
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
Edits: 03/20/15
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