Ken:

Yes, there was an existing 1K resistor in the cathode of the 6cw4--bypassed
for rf, I simply left it, it became the source resistance...

Regards,
John

"Ken Scharf" wrote in message
...
| William E. Sabin wrote:
| "Murray" wrote in message
| ...
|
| It should be remembered that FETs are a voltage controlled
| current device. A tube is a voltage controlled voltage device.
|
|
| A tube is also a voltage controlled current device because of its
| transconductance (gm) as well as a voltage controlled voltage device
because
| if its amplification factor (mu).
|
| A FET can be thought of in the same way if the gate is not forward
biased.
| MOSFETs are even more closely related. The difference involves the way
that
| the device is used. In some transistor circuits the "topologies" are
amost
| identical to tube circuits.
|
| FETs almost always operate at lower B+ voltage than tubes, so drain
voltage
| max-to-min swings are smaller and drain current max-to-min swings are
| greater, which suggests lower values of drain load resistance.
|
| Fifty years ago I designed pulse amplifiers at work (GE)for nuclear
event
| detectors using the 6CW4.
|
| At one time special low plate-voltage (12.6V) tubes were made for car
radios
| in order to eliminate the high voltage power supply that used either
| vibrators or the 0Z4 tube.
|
| Bill W0IYH
|
|
| The "plate" (well drain) curves of a jfet have nearly the same
| slope as a pentode tube. So replacing a 6cw4 with a jet would be
| like sticking in a 6ak5 (or similar) tube. The big difference
| between a fet and a pentode is that you can change the slope of
| the curve somewhat by varying the screen grid voltage, you
| can't do that with the fet (the screen voltage is built in and
| fixed).
|
| Reminds me of early regenerative receivers built with fets.
| It was quickly discovered that varying the drain supply voltage
| to control feedback didn't work very well (it won't work with
| a pentode either!). So most fet regen sets used a throttle cap
| to control feedback. You would be better off putting a pot
| in the source circuit to raise it above ground, thereby increasing
| the gate bias. This would move your operating point down to
| a lower curve, reducing gain. But I digress....
|
| My point is that fet's are more like pentodes than triodes, keep
| that in mind when you make a circuit change from tubes.