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RE: 6V6 Amp Feedback

Poinz switched to hand matched 6GK5s in the LTP. The triodes in the 6CG7/6FQ7 are the same as those in the venerable 6SN7. Superb linearity, but (most unfortunately) way too little gain for these 2 stage setups. Remember, in these 1 grid driven LTPs 1/2 μ is the theoretical max. gain and 30% of μ is doing well, in the real world.

I too like "fixed" bias in a PPP setup, as exact setting of each tube's "idle" current is important. The downside is that grid to ground resistance limit is lower, when "fixed" bias is employed. Therefore, the NET impedance the driving circuitry is working into drops to a point where both gain and linearity are degraded. That trouble spot is dealt with by buffering the LTP plates. The high gm/low R P types we've been discussing will have no problem in driving DC coupled IRFBC20 source followers. Put a protective 15 V. Zener diode between the plate side of the gate stopper resistor and the FET's source electrode.


Eli D.

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