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In Reply to: RE: Phase inverter tube choice question posted by Eli Duttman on May 08, 2020 at 12:01:51
I'm told that the VTA board can handle the draw of three 12BH7's. I'm OBVIOUSLY not an expert, but I believe the center tube splits but adds very little actual gain. I believe a fair part of the gain does occur in those tubes.Again, thanks
charles
Edits: 05/08/20Follow Ups:
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.
Thanks Eli, I put in an order with Jim this afternoon.
charles
Edits: 05/08/20
I took a quick look at the plate curves for the voltages and currents on V2 and it looks like it won't matter much which tube type, just pick a good brand.
*Freudian slumber, plenty of sound
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