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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Low cost 6AV5 design questions (long)

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I am trying to design my first low power / low cost amplifier.
I have been inspired by Lynn Olson's Amity

However, its not exactly low cost.
I found the Vivi-Voice which looked like a cool place to start.

Anyway, with TubeLab talking about 6AV5 and Edcor $20 transformers, I broke out TubeCAD PP calculator.
Goal is to be class A2, get about 8 watts a channel.

I ran the sim, and get an answer I need 60 peak volts.
I assume that's 120Vpp. That's what Mu of 4 gets you. :)

I was going to use an Edcor inter-stage and a 6H6pi, but Av is only about 20, and I calculate I need an Av of about 40 for a 6 Vpp input, running into a 2:1+1 input phase splitter. The Edcor is also disqualified because its designed for a max of 25V (rms), and I need about 40 V(rms).

I see my options as:
Use 6DJ8 transformer loaded with the Edcor, and get 5 Watts out.
Use 6N1pi CSS loaded with a Fet source follower. (powerdrive)
Use a 6SL7 with PowerDrive.
Go back to EL84 with its much lower drive requirements.

I also considered using the Edcor inter-stage as a choke load with the secondaries open, but decided that trick would work for tunes with Rp of 5k or less.

Would a CSS loaded 6sl7 be able to drive the gate capacitance of a powerdrive FET?

It seems silly to resort to a Cascode for Av of 40, and or Cascading multiple stages.

Any thoughts or comments on these ramblings would be welcome.

Doug
 
Doug,

The 6SL7, like its offspring (the 12AX7), is poor in the drive capability dept. IMO, caution indicates that nothing bigger than a ZVN0545A be used in close association with the high mu, high RP, low gm types. That little FET is not suitable as an A2 driver.

However, if you choke load a 6SL7 in SRPP, you get both the requisite voltage gain and the drive capability an A2 suitable FET (like the IRFBC20) needs.
 
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