It should be remembered that FETs are a voltage controlled
current device. A tube is a voltage controlled voltage device.
A bipolar transistor is a current controlled current device.

A FET is therefore somewhere in between a tube and a transistor.
I did have some luck with a FRG7000. the IF (which does all
the work in a radio) was connected in the old triode configuration.
Signal to the 1Meg gate and the drain working into a parallel
IF can. I had a good look and the designers had used regular
transistor IF cans but ignored the tapping on the output side.

By cutting the tracks and making little jumpers i was able to
boost the rx signal by using the tap on the coil to the drain. I had to
put caps in to connect to the next stage - the IF cans were now working
as voltage transformers and improvement in reception was dramatic.
What was a 'deaf' reciever was now pretty useful.

Anyone wanting to use FETs for tube replacements should remember
they aren't voltage amplifiers - only current amplifiers.

Hope i have made myself clear? :-)

Murray vk4aok

John Smith wrote:
well, it works, there was a 1k resistor bypassed by a .01 cap on the cathode
of the 6cw4, I just hooked the source of the fet to it, adding a 500 ohm
resistor to the drain which passes though the primary of a rf xfmr feeding
the next stage, it is fed with ~16V B+. Works great, since the 6cw4 was bad
there is no way to compare how this affected preformance, but receive seems
hot... noise level good...
Even if a dozen 6cw4's dropped from the sky, I'd just look at 'em....

Regards,
John

"John Smith" wrote in message
...

Anyone ever replaced a 6CW4 Nuvistor with a fet or other transistor?
Got a schematic? Idea of one?

Regards,
John

"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

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.

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.

Yes, quite, and ohms law makes it plain that current through a resistance
causes a voltage drop across it...
And, logically following, a voltage placed across a resistance causes a
current to flow through the resistance--
hey, this is excellent idea for a voltage-to-current/current-to-voltage
conversion device--ya suppose someone else has thought of that??? grin

Regards,
John

"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 bipolar transistor is a current controlled current device.

A FET is therefore somewhere in between a tube and a transistor.
I did have some luck with a FRG7000. the IF (which does all
the work in a radio) was connected in the old triode configuration.
Signal to the 1Meg gate and the drain working into a parallel
IF can. I had a good look and the designers had used regular
transistor IF cans but ignored the tapping on the output side.

By cutting the tracks and making little jumpers i was able to
boost the rx signal by using the tap on the coil to the drain. I had to
put caps in to connect to the next stage - the IF cans were now working
as voltage transformers and improvement in reception was dramatic.
What was a 'deaf' reciever was now pretty useful.

Anyone wanting to use FETs for tube replacements should remember
they aren't voltage amplifiers - only current amplifiers.

Hope i have made myself clear? :-)

Murray vk4aok

John Smith wrote:
well, it works, there was a 1k resistor bypassed by a .01 cap on the
cathode of the 6cw4, I just hooked the source of the fet to it, adding a
500 ohm resistor to the drain which passes though the primary of a rf
xfmr feeding the next stage, it is fed with ~16V B+. Works great, since
the 6cw4 was bad there is no way to compare how this affected
preformance, but receive seems hot... noise level good...
Even if a dozen 6cw4's dropped from the sky, I'd just look at 'em....

Regards,
John

"John Smith" wrote in message
...

Anyone ever replaced a 6CW4 Nuvistor with a fet or other transistor?
Got a schematic? Idea of one?

Regards,
John

"Murray" bravely wrote to "All" (26 Apr 05 19:29:08)
--- on the heady topic of " Replace 6CW4?"

Mu From: Murray
Mu Xref: aeinews rec.radio.amateur.homebrew:9477

Mu It should be remembered that FETs are a voltage controlled
Mu current device. A tube is a voltage controlled voltage device.
Mu A bipolar transistor is a current controlled current device.

Mu A FET is therefore somewhere in between a tube and a transistor.

If we look at the characteristic I/V plate curves for a pentode it is
easy to see a similarity with the collector saturation plots of a
transistor. The FET in my opinion is different from the pentode
analogy but it is a little more like the triode.

The combination of bipolar transistor and FET is called a BiFET and
this is perhaps most like a pentode than anything. There is also power
HEXFETs which are something amazing with extremely low ON resistances
in the milli-ohm range. These might be more like a triode on steroids.
These come in pretty high voltage versions which seem to me more
suitable to replace a 6CW4 with some bias voltage adjusting of course.

A*s*i*m*o*v

.... "If it ain't broke, you're not tryin!" - Red Green

Asimov wrote:
. . .
There is also power
HEXFETs which are something amazing with extremely low ON resistances
in the milli-ohm range. These might be more like a triode on steroids.
These come in pretty high voltage versions which seem to me more
suitable to replace a 6CW4 with some bias voltage adjusting of course.

Have you compared the capacitances of the 6CW4 and Hexfet? Note also
that the FET capacitance is highly nonlinear, varying greatly with
reverse junction voltage.

Roy Lewallen, W7EL