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Substitute for 1/2 a 12AX7?

Posted by David Anderson on March 8, 2002 at 14:11:38

Hi,

I have a pair of Dynaco MkIVs that I'm thinking of converting to Kevin Kennedy's input circuit (very close to the front end of an EICO HF-87). But for a mono amp, I'd only need 1/2 of the 12AX7 used in Kevin's ST-70 design as a first stage voltage amplifier.

I know I could just use half the 12AX7, but I was wondering if there is any way to get approximately the same in-circuit performance from, for example, a pentode like the EF86 strapped in triode mode. I'm not sufficiently experienced in tube circuitry to figure out if this is practical or if it would cause too much of a change in the circuit's gain structure.

Of course, I could just make the circuit like the front end of the EICO HF-35 with the EF86 in pentode mode. Would that be a better idea? Popular sentiment seems to be against pentodes these days......

Thanks,
David

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