Avery Fineman wrote:
In article ,
(The Eternal Squire) writes:
-- snip --
4. Designing a circuit using battery powered, directly-heated
filaments as a differential pair is going to be difficult...unless you
have a separate "A" battery supply for that differential pair.
Since the cathodes ARE the filaments, not separate as in
indirectly-heated tubes, those cathode-filaments are going to
be elevated or, if run near common, will require a "B-" supply
for the long-tailed pair's large "cathode" resistor.
-- snip --
I agree with everything else, but at RF you can just make a
bifilar-wound filament choke -- you'll be at DC ground but you can
inject whatever signal you want. Since everything is so high impedance
and you want low power you can make it the final inductor in a matching
network, for that matter.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com