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#1
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Passive grid linear
Anybody here ever build a passive grid linear amp?
That's where you simply terminate the grid of a tetrode with a 50 ohm resistor and just drive it directly. Svetlana recommended that configuration with their 4cx400, 4xc800 and 4cx1600 tubes, and it should work well with more commone surplus 4cx250's. I was thinking of using a 4:1 or 9:1 balum and increasing the grid resistor value to 200 or 450 ohms to reduce the amount of driving power required. With the 4cx250 a peak grid drive of about 50 volts is required in class AB1. With a 50 ohm termination the driving power would be 50 watts, with a 200 ohm termination 12.5 watts, and with the 450 ohm termination 5.6 watts. That's actually just the power sucked up in the resistor, but the tube requires less than 1 watt of drive itself in class AB1. Some power might be lost in the balum, so maybe the actual driving power might increase by a watt or two. Since I wanted this to be a final for a QRP rig the larger terminating resistance looked like a better way to go. The only issue is how high can you go with the termination resistance and keep the tube stable without neutralization being required? |
#2
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Passive grid linear
Howdy,
I've only read about them. G2DAF wrote extensively on pasive grid linears. I think the ultimate value of grid resistor will depend on the grid capacitance and layout parasitics. The articles I've read use 300 Ohms as the grid swamping resistor but with a 4CX250 450 Ohms might be possible. Here is an article you may find useful. http://www.radioamator.ro/articole/files/291_1.pdf I have a bucket full of 4CX250B's and would like to try passive grid someday. Please Let us know how yours shapes up if you decide to build one. 73, Grumpy ken scharf wrote in news Anybody here ever build a passive grid linear amp? That's where you simply terminate the grid of a tetrode with a 50 ohm resistor and just drive it directly. Svetlana recommended that configuration with their 4cx400, 4xc800 and 4cx1600 tubes, and it should work well with more commone surplus 4cx250's. I was thinking of using a 4:1 or 9:1 balum and increasing the grid resistor value to 200 or 450 ohms to reduce the amount of driving power required. With the 4cx250 a peak grid drive of about 50 volts is required in class AB1. With a 50 ohm termination the driving power would be 50 watts, with a 200 ohm termination 12.5 watts, and with the 450 ohm termination 5.6 watts. That's actually just the power sucked up in the resistor, but the tube requires less than 1 watt of drive itself in class AB1. Some power might be lost in the balum, so maybe the actual driving power might increase by a watt or two. Since I wanted this to be a final for a QRP rig the larger terminating resistance looked like a better way to go. The only issue is how high can you go with the termination resistance and keep the tube stable without neutralization being required? |
#3
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Passive grid linear
Grumpy The Mule wrote:
Howdy, I've only read about them. G2DAF wrote extensively on pasive grid linears. I think the ultimate value of grid resistor will depend on the grid capacitance and layout parasitics. The articles I've read use 300 Ohms as the grid swamping resistor but with a 4CX250 450 Ohms might be possible. Here is an article you may find useful. http://www.radioamator.ro/articole/files/291_1.pdf I have a bucket full of 4CX250B's and would like to try passive grid someday. Please Let us know how yours shapes up if you decide to build one. 73, Grumpy ken scharf wrote in news Anybody here ever build a passive grid linear amp? That's where you simply terminate the grid of a tetrode with a 50 ohm resistor and just drive it directly. Svetlana recommended that configuration with their 4cx400, 4xc800 and 4cx1600 tubes, and it should work well with more commone surplus 4cx250's. I was thinking of using a 4:1 or 9:1 balum and increasing the grid resistor value to 200 or 450 ohms to reduce the amount of driving power required. With the 4cx250 a peak grid drive of about 50 volts is required in class AB1. With a 50 ohm termination the driving power would be 50 watts, with a 200 ohm termination 12.5 watts, and with the 450 ohm termination 5.6 watts. That's actually just the power sucked up in the resistor, but the tube requires less than 1 watt of drive itself in class AB1. Some power might be lost in the balum, so maybe the actual driving power might increase by a watt or two. Since I wanted this to be a final for a QRP rig the larger terminating resistance looked like a better way to go. The only issue is how high can you go with the termination resistance and keep the tube stable without neutralization being required? That's an interesting article. He shows no matching network between his exciter and the linear so there would be a 6:1 SWR with the 300 ohm termination, though a tube type transmitter with a Pi network might still be able to load into it. Deriving the screen voltage from the rf drive seems a hybrid method of doing grounded grid. I was thinking of an amplifier with normal bias and screen supplies which would reduce the drive required. I have a pair of used 7034's (4cx250's with glass seals) but I have no idea how much life (if any) are left in them. My idea was to build an amplifier chassis that could be used to experiment with several tube configurations. I've found a source for a 400VA toroidal power transformer with two 550v and two 6.3v (5A) secondaries and a 120/240 v primary for about $65 postpaid. In a bridge rectifier configuration with a capacitor input filter using large capacitors it should yield about 1400 volts under load. This would be a suitable plate voltage for a single 811A/572B or a 4CX250A. With the bridge rectifier configuration the center tap of the HV secondary will provide 700 volts that can be regulated down to the 300-350 volts required for the screen of a 4CX250. One of the 6v filament windings would power the tube (unless I find some 4CX250F/G's which would require an extra 25v transformer) and the other winding could be used with a voltage multiplier for the bias supply (or drive another filament transformer bassackwards for that purpose). So I could build up a power supply and tank circuit that could serve either configuration, and swap out one of two chassis; one for a single 811A/572B and one for a 4CX250.350/400A. The target amp would be between 150 to 250 watts output (a nice afterburner for a 10-20 watt QRP rig I'm thinking about). |
#4
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Passive grid linear
Grumpy The Mule wrote:
I've only read about them. G2DAF wrote extensively on pasive grid linears. I think the ultimate value of grid resistor will depend on the grid capacitance and layout parasitics. The articles I've read use 300 Ohms as the grid swamping resistor but with a 4CX250 450 Ohms might be possible. Here is an article you may find useful. http://www.radioamator.ro/articole/files/291_1.pdf I have a bucket full of 4CX250B's and would like to try passive grid someday. Please Let us know how yours shapes up if you decide to build one. The defining feature of a G2DAF amplifier is not the untuned passive grid input, but the method of grid biasing. It has no control grid bias and the screen voltage is developed by rectifying the RF drive. Most passive grid tetrode amplifiers use conventional fixed bias voltages on g1 and g2. Commercial examples include the Alpha 91b/99 and the Australian Emtron range. G2DAF's published articles on amplifiers are on my web site, and the RSGB historical collection still has his original receiver, transmitter and amplifier, along with many hand-written notes. The 2 x 4-125A amplifier that featured in his published articles still exists, but at some later time he had removed the screen rectifier tubes and converted the amplifier into a very basic grounded-grid configuration. -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek |
#5
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Passive grid linear
Ian White GM3SEK wrote:
Grumpy The Mule wrote: I've only read about them. G2DAF wrote extensively on pasive grid linears. I think the ultimate value of grid resistor will depend on the grid capacitance and layout parasitics. The articles I've read use 300 Ohms as the grid swamping resistor but with a 4CX250 450 Ohms might be possible. Here is an article you may find useful. http://www.radioamator.ro/articole/files/291_1.pdf I have a bucket full of 4CX250B's and would like to try passive grid someday. Please Let us know how yours shapes up if you decide to build one. The defining feature of a G2DAF amplifier is not the untuned passive grid input, but the method of grid biasing. It has no control grid bias and the screen voltage is developed by rectifying the RF drive. Most passive grid tetrode amplifiers use conventional fixed bias voltages on g1 and g2. Commercial examples include the Alpha 91b/99 and the Australian Emtron range. G2DAF's published articles on amplifiers are on my web site, and the RSGB historical collection still has his original receiver, transmitter and amplifier, along with many hand-written notes. The 2 x 4-125A amplifier that featured in his published articles still exists, but at some later time he had removed the screen rectifier tubes and converted the amplifier into a very basic grounded-grid configuration. I really don't see any advantage in his circuit over a conventional zero bias grounded grid amp, at least not with the 4-xxx series of tubes. With the 4CX250 series it makes some sense since these tubes CAN'T be run in zero bias grounded grid due to the fragile grid structure in these tubes. |
#6
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Passive grid linear
On Thu, 27 Nov 2008 13:45:33 -0500, ken scharf wrote:
Anybody here ever build a passive grid linear amp? That's where you simply terminate the grid of a tetrode with a 50 ohm resistor and just drive it directly. Svetlana recommended that configuration with their 4cx400, 4xc800 and 4cx1600 tubes, and it should work well with more commone surplus 4cx250's. I was thinking of using a 4:1 or 9:1 balum and increasing the grid resistor value to 200 or 450 ohms to reduce the amount of driving power required. With the 4cx250 a peak grid drive of about 50 volts is required in class AB1. With a 50 ohm termination the driving power would be 50 watts, with a 200 ohm termination 12.5 watts, and with the 450 ohm termination 5.6 watts. That's actually just the power sucked up in the resistor, but the tube requires less than 1 watt of drive itself in class AB1. Some power might be lost in the balum, so maybe the actual driving power might increase by a watt or two. Since I wanted this to be a final for a QRP rig the larger terminating resistance looked like a better way to go. The only issue is how high can you go with the termination resistance and keep the tube stable without neutralization being required? This should jump out at you with the proper circuit analysis. Just analyze the circuit with a parallel tank load on the plate, and look for negative resistance showing up at the grid. Your grid swamping resistor would need to take care of the worst-case negative resistance. Then spend a month tearing your hair out over parasitics that increase the plate-grid coupling over and above what the data sheet says they are. -- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html |
#7
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Passive grid linear
On Nov 28, 12:00 am, Tim Wescott wrote:
On Thu, 27 Nov 2008 13:45:33 -0500, ken scharf wrote: Anybody here ever build a passive grid linear amp? That's where you simply terminate the grid of a tetrode with a 50 ohm resistor and just drive it directly. Svetlana recommended that configuration with their 4cx400, 4xc800 and 4cx1600 tubes, and it should work well with more commone surplus 4cx250's. I was thinking of using a 4:1 or 9:1 balum and increasing the grid resistor value to 200 or 450 ohms to reduce the amount of driving power required. With the 4cx250 a peak grid drive of about 50 volts is required in class AB1. With a 50 ohm termination the driving power would be 50 watts, with a 200 ohm termination 12.5 watts, and with the 450 ohm termination 5.6 watts. That's actually just the power sucked up in the resistor, but the tube requires less than 1 watt of drive itself in class AB1. Some power might be lost in the balum, so maybe the actual driving power might increase by a watt or two. Since I wanted this to be a final for a QRP rig the larger terminating resistance looked like a better way to go. The only issue is how high can you go with the termination resistance and keep the tube stable without neutralization being required? This should jump out at you with the proper circuit analysis. Just analyze the circuit with a parallel tank load on the plate, and look for negative resistance showing up at the grid. Your grid swamping resistor would need to take care of the worst-case negative resistance. Then spend a month tearing your hair out over parasitics that increase the plate-grid coupling over and above what the data sheet says they are. -- Tim Wescott Control systems and communications consultinghttp://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes,http://www.wescottdesign.com/actfes/actfes.html Hey OM: I don't see plate grid coupling with a grounded screen and the cathode at negative screen voltage? eh? 73 OM n8zu |
#8
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Passive grid linear
ken scharf wrote:
I really don't see any advantage in his circuit over a conventional zero bias grounded grid amp, at least not with the 4-xxx series of tubes. With the 4CX250 series it makes some sense since these tubes CAN'T be run in zero bias grounded grid due to the fragile grid structure in these tubes. You only needed 20W of drive power and a single HT supply. I have one using a pair of 813s, 10W from my TS-120 drives it to about 200W output. It looks to be exactly the same layout as the one on Ian's site but has had the valve rectifiers replaced with solid state at some time. Steve H |
#9
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Passive grid linear
Stev eH wrote:
ken scharf wrote: I really don't see any advantage in his circuit over a conventional zero bias grounded grid amp, at least not with the 4-xxx series of tubes. With the 4CX250 series it makes some sense since these tubes CAN'T be run in zero bias grounded grid due to the fragile grid structure in these tubes. You only needed 20W of drive power and a single HT supply. I have one using a pair of 813s, 10W from my TS-120 drives it to about 200W output. It looks to be exactly the same layout as the one on Ian's site but has had the valve rectifiers replaced with solid state at some time. Steve H 200W from a PAIR of 813's? You must be running them with less than 1.5KV on the plates. That's the problem with 813's, they don't live up to their potential (pardon the pun) unless you run them with lots of HV. The point though is that in conventional grounded grid (all three 'grids' grounded) you still don't need a SG supply and the drive power should be about the same. |
#10
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Passive grid linear
raypsi wrote:
On Nov 28, 12:00 am, Tim Wescott wrote: On Thu, 27 Nov 2008 13:45:33 -0500, ken scharf wrote: Anybody here ever build a passive grid linear amp? That's where you simply terminate the grid of a tetrode with a 50 ohm resistor and just drive it directly. Svetlana recommended that configuration with their 4cx400, 4xc800 and 4cx1600 tubes, and it should work well with more commone surplus 4cx250's. I was thinking of using a 4:1 or 9:1 balum and increasing the grid resistor value to 200 or 450 ohms to reduce the amount of driving power required. With the 4cx250 a peak grid drive of about 50 volts is required in class AB1. With a 50 ohm termination the driving power would be 50 watts, with a 200 ohm termination 12.5 watts, and with the 450 ohm termination 5.6 watts. That's actually just the power sucked up in the resistor, but the tube requires less than 1 watt of drive itself in class AB1. Some power might be lost in the balum, so maybe the actual driving power might increase by a watt or two. Since I wanted this to be a final for a QRP rig the larger terminating resistance looked like a better way to go. The only issue is how high can you go with the termination resistance and keep the tube stable without neutralization being required? This should jump out at you with the proper circuit analysis. Just analyze the circuit with a parallel tank load on the plate, and look for negative resistance showing up at the grid. Your grid swamping resistor would need to take care of the worst-case negative resistance. Then spend a month tearing your hair out over parasitics that increase the plate-grid coupling over and above what the data sheet says they are. -- Tim Wescott Control systems and communications consultinghttp://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes,http://www.wescottdesign.com/actfes/actfes.html Hey OM: I don't see plate grid coupling with a grounded screen and the cathode at negative screen voltage? eh? 73 OM n8zu I think he means that any feedback which would induce instability can be translated as a negative resistance seen at the grid (all an oscillator is after all, is a negative resistance generator). And ANY tube exhibits some degree of plate to grid coupling in the form of stray capacitance between the elements. Even Pentodes with two grounded (for rf) elements between the plate and grid show SOME coupling (though in the case of screen grid tubes the value is reduced to values of a few pf or less). The grounded grid circuit reduces the feedback even more by introducing a phase shift as well, but some grounded grid amps still need some neutralization (especially if you mess things up by putting 4 bottles in parallel.) |
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