John Smith wrote:

Yes, that is exactly what is occuring here, a "new" circuit is being
designed... and, a turns ratio change may have to be contemplated in the
drain circuit (existing rf transformer)--not the gate circuit, that existing
circuit, decoupled through an isolating cap should be just fine (the fet is
a high input impedance device)--this is the front end of a receiver, the
actual signal levels here are low...

But, I am interested, what roadblocks do you see to "I don't think you have
any hope of making a plug-in compatible 6CW4 replacement..." ?????

Even though a FET and a tube bear superficial resemblances, the FET is a
much lower impedance device, with different restrictions on allowable
gate (grid) voltage, drain (plate) voltage, source (cathode) currents,
etc. The interelectrode capacitances are, in general, markedly
different and the whole FET circuit operates at a markedly different
impedance level than the tube circuit does. If that weren't enough the
mechanisms by which the tube gain drops off as frequency increases is
different than those for the FET, and the general circuit behavior of
the two changes differently as a function of frequency.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com