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.