Yes, and most of that would be taken care of just by choosing an "rf fet" as
compared to an "audio fet."
At least in the circuit I am looking at, the 6cw4 is operating at in input
impedance of ~500K-1 meg--the fet circuit will of course, expect 1 meg--no
sweat for a even a cheap rf fet...
As mentioned, the blocking cap removes all question of danger of HV bias (I
will supply the bias--most likely though a resistor bias arangement (perhaps
clamping it with a diode if there is any danger of damage), and the power
supply mentioned removes all danger of high B+ voltage...
This 6cw4 is the preamp in a frontend, signal levels are very low, there is
a buffer stage following, with an amp behind that... certainly a signal
level compatible with a small signal fet...
The 6cw4 is a "hybrid" device... I am looking for a datasheet on it now, I
think replacement will go quickly and be simple, probably not even requiring
revamping of the rf xfrmr on the drain (will revamp if noticible degrade in
preformance....)

I was just hoping someone had walked this path before and could speed my way
and hold my hand....

At this point, I am simply wondering, "Why hasn't someone done this before!"

Is there a big call for these devices? Perhaps I can build them and market
them on Ebay... grin

Warmest regards,
John

"Tim Wescott" wrote in message
...
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