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Old 19th April 2008, 06:50 AM   #1
DougL is offline DougL  United States
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Default Low cost 6AV5 design questions (long)

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
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Old 19th April 2008, 02:00 PM   #2
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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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