OTL Asylum

RE: 6C19 PP schematic?

89.253.73.146

Peter H-son

In Reply to: RE: 6C19 PP schematic? posted by Lew on April 21, 2008 at 09:33:32

I realize I need a cathode follower. I also realize a pair is not ideal. (The 6C19 has an Rp of around 200 and mu of 3 if I remember.) And you should be able to draw some conclusions from the apparent lack of such a circuit on the net.
Maybe there's no hope for such an amp. But SE isn't an alternative, even though it would be easier. Nor is the 6C33. But if it is possible, I want to give the 6C19 a try.
I have a bunch of 6AS7 and 6C33 pp amp designs on my hdd. I don't know how hard it is to tweak them for the 6C19. But it does take more work than changing output tubes in a traditional amp. Even an untested 6C19 PP schematic would be of great help. I just don't feel very confident with this.

Edits: 04/21/08 04/21/08

Follow Ups:

RE: 6C19 PP schematic? - Lew 10:49:51 04/21/08 (2)
The big problem for designing a low power OTL, is that you want a low output impedance. The major determinants of output impedance are the net plate resistance of all the tubes paralleled and mu. Kevin Covi has posted a formula for the output Z of a circlotron, for example: Zo = Rp/2+mu. So if you wanted an OTL using one pair of 6C19, the predicted output Z would be 200/2+3 = 40 ohms. Obviously this is too high to drive any real world speaker, even a very efficient one. The only cure is to use NFB. With a modified Futterman-type circuit, you get a slightly lower Zo, but there you definitely also need NFB. It's actually therefore easier to make a high power OTL that could get by without NFB. Check out the Atma S30 and the Transcendent website.
- RE: 6C19 PP schematic? - acres verde 14:46:32 04/22/08 (1)
  Not trying to be a perfectionist nit picker here but since the discussion is technical, the formulas should be technically correct and I was momentarily thrown off (but just momentarily, thank you).
  
  200/2+3=103
  
  200/(2+3)=40
  
  See ya,
  - Thanks for the parentheses, - Lew 08:57:12 04/23/08 (0)
    but it should have been obvious based on the right side of the equation when values were added. However, I stand corrected.

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