K&K Audio / Lundahl Transformers

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Question re 6BX7GT in 300B push pull designs

Posted by David Loader on July 21, 2016 at 15:07:28:
Greetings Kevin et al. I'm presently researching circuits that operate 300B's in Class A push-pull for a potential forthcoming build, and Kevin's designs from back in the day interest me greatly - the ones with a 6BX7 driver and 6SN7 in the first stage.

So my first question is this: The only concern that I have about the 6BX7 is that it's not a valve being manufactured currently, and ideally I'd like to use 'current manufacture' valves so that I don't have to run the gauntlet of potentially unscrupulous eBay sellers.

Last night I was poring over some data sheets and I noticed that the KT66, when triode connected, has near identical figures to the 6BX7 for plate resistance, amplification factor and transconductance at 250V plate voltage. Just requires a little more current (60mA vs 42mA). It's also about as linear a device as you could want.

I've found almost no references to the use of KT66's as 300B drivers so am I missing something obvious?