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Replace 6CW4?
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 |
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 |
Michael:
As usual, I am behind the times--would have been a great article to write up back in 1967, huh? grin Thanks for the encouragement. The fet I used only has house numbers but it came from the parts box where I keep hf rf parts--obiviously a N channel fet--it works great.... Regards, John "Michael Black" wrote in message ... "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 |
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 |
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 |
Wow!!! I saved about $14.30 figuring the cost of parts--approx. .50 cents!
grin But, then figuring my design costs and labor... frown Warmest regards, John "Tim Wescott" wrote in message ... John Smith wrote: well, that is fine, however if you don't have a 6cw4 this fix would be a God send... I don't know of anyone making them anymore, like the gold, the timber, the water, and like oil, an end will come... But these rigs can still live on... Regards, John "Dale Parfitt" wrote in message news:2nhbe.3058$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 $14.80 at Antique Electronics Supply, www.tubesandmore.com. At that price there's still plenty out there -- look at the price for the 2A3, or a '45 for comparison. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
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 |
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. |
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