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|19th April 2008, 06:50 AM||#1|
Join Date: Feb 2004
Location: Wheaton IL.
Blog Entries: 30
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.
Scienta sine ars nihil est - Science without Art is nothing. (Implies the converse as well)
Mater tua criceta fuit, et pater tuo redoluit bacarum sambucus
|19th April 2008, 02:00 PM||#2|
Join Date: Apr 2004
Location: Monroe Township, NJ
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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