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RE: Phase inverter tube choice question

V1 is a differential gain block phase splitter, AKA long tailed pair (LTP). The theoretical max. gain for such a setup is 1/2 the amplification factor (μ). In practice, roughly 1/4 μ is obtained. Kudos to the designer for using a constant current sink (CCS) load in the LTP's "tail".

That 1/2 V2 is a ubiquitous common cathode voltage amplifier. The bottom line is that all the small signal tubes in this setup are amplifying and, therefore, the non-linear 12AU7 should be eschewed.

As long as sufficient heater current is available from the power transformer, only 12BH7s should go into that unit.

Eli D.

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