Posted by Mark Kelly [ 203.166.238.94 ] on March 31, 2005 at 19:17:02:
In Reply to: Cathode drive posted by Tnuctipun on March 08, 2005 at 11:42:37:
Yes it works. I have done it with an SE 801 and got 11 watts output. The sound was lovely - much "fuller" than a standard 801 in A1 and of course three times the power.
Cathode Z in in A1 is of course rp /(mu+1). Cathode Zin in A2 is rg in parallel with that which is rg x (mu +1) x rp / ( rg x (mu+1) + rp). The value of rg = Ig/E(g-k) and can be estimated from grid current curves or assumed to be roughly 1k.
Yes the cathode needs power but this power comes out in the plate circuit thereby raising efficiency - you can get around 55% without too much trouble. For a 211 IIRC you need about 4 watts to drive it - I have a transformer specially built to run PP2A3 into PP211s. If you are serious about this I will lend you the tranny although transport from Australia will likely be as much as getting a tranny built.
A much greater problem is RF oscillation - grounded grid transmitting triodes are wonderful RF amps. I found I had to put a snubber across half the primary of the output transformer for the 801 but then I was using a transformer designed as a PP 211 transformer (but my transformers are always air gapped anyway).
I wrote a short article about this for Glass Audio it appeared in issue 1 for 1999.
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