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

Regards,
John

I have been toying with pen and paper...
It looks like like the grid circuit to the 6CW4 could have an isolation
capacitor inserted to remove and DC bias from the gate of the replacement
fet. And, the plate of the 6CW4 circuit could have the B+ side of the rf
xfrmr moved to a low volage supply (perhaps rectify and filter a 12v AC
heater supply to supply the new voltage.)
And, a proper bias supply constructed to supply a necessary bias to the
gate...
And, ideas?

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

John Smith wrote:

I have been toying with pen and paper...
It looks like like the grid circuit to the 6CW4 could have an isolation
capacitor inserted to remove and DC bias from the gate of the replacement
fet. And, the plate of the 6CW4 circuit could have the B+ side of the rf
xfrmr moved to a low volage supply (perhaps rectify and filter a 12v AC
heater supply to supply the new voltage.)
And, a proper bias supply constructed to supply a necessary bias to the
gate...
And, ideas?

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?

I've seen works on "transistorizing" tube radios, but the
characteristics of tubes and transistors are sufficiently different that
in general you have to approach this on a case by case basis. So while
I don't think you have any hope of making a plug-in compatible 6CW4
replacement, I _do_ think that you could often use most of a circuit
designed for a 6CW4 with some other device.

Having said all that, I would probably either buy another 6CW4, or
design & build a new circuit inspired by the old.

--

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

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..." ?????

Warmest regards,
John


"Tim Wescott" wrote in message
...
John Smith wrote:

I have been toying with pen and paper...
It looks like like the grid circuit to the 6CW4 could have an isolation
capacitor inserted to remove and DC bias from the gate of the replacement
fet. And, the plate of the 6CW4 circuit could have the B+ side of the rf
xfrmr moved to a low volage supply (perhaps rectify and filter a 12v AC
heater supply to supply the new voltage.)
And, a proper bias supply constructed to supply a necessary bias to the
gate...
And, ideas?

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?

I've seen works on "transistorizing" tube radios, but the characteristics
of tubes and transistors are sufficiently different that in general you
have to approach this on a case by case basis. So while I don't think you
have any hope of making a plug-in compatible 6CW4 replacement, I _do_
think that you could often use most of a circuit designed for a 6CW4 with
some other device.

Having said all that, I would probably either buy another 6CW4, or design
& build a new circuit inspired by the old.

--

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

I should have mentioned the freqs in question are under 30 Mhz... even a
junkbox fet should do it!

Regards,
John

"John Smith" wrote in message
...
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..." ?????

Warmest regards,
John


"Tim Wescott" wrote in message
...
John Smith wrote:

I have been toying with pen and paper...
It looks like like the grid circuit to the 6CW4 could have an isolation
capacitor inserted to remove and DC bias from the gate of the
replacement fet. And, the plate of the 6CW4 circuit could have the B+
side of the rf xfrmr moved to a low volage supply (perhaps rectify and
filter a 12v AC heater supply to supply the new voltage.)
And, a proper bias supply constructed to supply a necessary bias to the
gate...
And, ideas?

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?

I've seen works on "transistorizing" tube radios, but the characteristics
of tubes and transistors are sufficiently different that in general you
have to approach this on a case by case basis. So while I don't think
you have any hope of making a plug-in compatible 6CW4 replacement, I _do_
think that you could often use most of a circuit designed for a 6CW4 with
some other device.

Having said all that, I would probably either buy another 6CW4, or design
& build a new circuit inspired by the old.

--

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

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

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

"John Smith" wrote in message
...
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
Hi John,
Guess I'm just a boatanchor fan and would leave the circuits alone-
especially if there is no improvement in functionality.

Dale W4OP

well, that is fine, however if you don't have a 6cw4 this fix would be a God
send...

I don't know of anyone making them anymore, like the gold, the timber, the
water, and like oil, an end will come...

But these rigs can still live on...

Regards,
John

"Dale Parfitt" wrote in message
news:2nhbe.3058$Yc.2448@trnddc06...

"John Smith" wrote in message
...
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
Hi John,
Guess I'm just a boatanchor fan and would leave the circuits alone-
especially if there is no improvement in functionality.

Dale W4OP

"John Smith" ) writes:
Anyone ever replaced a 6CW4 Nuvistor with a fet or other transistor?
Got a schematic? Idea of one?

Regards,
John



I can't provide any references, but converters and preamps were some
of the first things that were transistorized. They were simple with
a single stage or at most a few, so once FETs came along they were
put to use. ON one level it was likely just to try it, but of
course the lower noise figure probably was a lure.

Given that that there might have only been one tube in the thing,
probably many were treated just as a foundation without much concern
for a direct plug in. I can picture an article in "73" from 1967
where someone talked about putting an FET in a nuvistor preamp, and
it wasn't much more than shorting out the cathode resistor (or something
like that) and a returning. The basic scheme would be the same
whether it was a triode or an FET, so it certainly is a relatively
easy thing. The same article had the author putting FETs into
other things, like a Command set transmitter, or rather the VFO
section, and that too was pretty simple.

Decades ago, I got ahold of some Collins PTOs that covered the
broadcast band, and I didn't have to do much to put an FET in
them beyond soldering in the FET and running the thing off a
12V or so supply.

It tended to get more complicated with multiple stage units, or
perhaps just that it ceased to be easy to put it back together
if it didn't work right. There'd be articles about conversions
that sometimes were about replacing function, some about modifying
the circuitry so the FET or whatever would fit in, and of course
the simplest (for the equipment, but not so much for the replacement),
a universal plug in.

Michael VE2BVW