K&K Audio / Lundahl Transformers

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RE: Question re 6BX7GT in 300B push pull designs

Posted by KevinC on July 22, 2016 at 04:27:14:
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.