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In Reply to: RE: Question re 6BX7GT in 300B push pull designs posted by David Loader on July 21, 2016 at 15:07:28
I have used EL34s for this purpose in two similar amp designs over the past 5 years with somewhat better results than I was able to get with 6BX7GTs. The voltage and current requirements are greater, so that is a disadvantage, but as you point out, the tubes are still available and since they are single element tubes, matching can be accomplished in a straightforward way, and they are arguably better drivers for high swing requiring tubes like 300Bs. When using EL34s I used the LL1671/PP as the interstage, as it's a better match and that eliminates the need to deal with high frequency out-of-band resonances.
I would say that we were missing the mark by not using a little more current to create better driver stages for challenging output tubes!
Kevin Carter
K&K Audio
www.kandkaudio.com
Follow Ups:
Thank you Kevin. I remember coming across some posts mentioning this resonance issue with the LL1660 and 6BX7 when I did a search before.
Forgive my further question but I presume this occurred as a result of the output capacitance/impedance of the valve and the inductance of the interstage? I'd like to confirm what I need to look at to calculate the projected behaviour with a triode connected KT66 and predict if the same issue will occur.
The resonance(s) are a result of the interaction of the tube's parameters and the leakage inductance/stray capacitance of the transformer. If you had all of the necessary parameters for both parties, you could probably simulate the behavior to some unknown degree of accuracy, but the simpler way, in my view, is to choose the interstage transformer that has just enough primary inductance to provide the desired low frequency cut-off. Shooting for low frequency cut-offs at very low frequencies, just because you can, puts you in a position of choosing interstage transformers with higher leakage inductance and stray capacitance to get bass response that is likely irrelevant to real in-room performance.
In any case, the resonances can usually be minimized with a carefully chosen RC network across the entire secondary.
Kevin Carter
K&K Audio
www.kandkaudio.com
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