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#11
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Passive grid linear
On Nov 27, 1:45*pm, 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? Ive seen one amp that I know worked using a 4cx250B using a 16:1 transformer. He used tha amp on 6M. I think he still added a little capacitance to tune out some of the inductance in the transformers. The best I remember he really didnt have to do this. He just did it so he would see a perfect 1:1 SWR beteween the exciter and amp. I modified an old 2X 6146 amp with tunned grid by ripping out the tuned circuit and replacing it with a 200 ohm resistor. 5 watts was more than enough drive. Jimmie |
#12
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Passive grid linear
On Fri, 28 Nov 2008 10:06:43 -0500, ken scharf wrote:
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). Yes, that's what I meant. And if you're looking to figure out the largest parallel grid resistance that will still effectively swamp out the instability, I think that's the best approach to take. 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.) In theory, any tube is going to have _some_ coupling straight through the grids. In practice its probably vanishingly small. But for many tubes you'll have to spend a long time optimizing the physical layout of the circuit before you have more coupling through the tube than you have around it. -- 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 |
#13
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Passive grid linear
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#14
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Passive grid linear
On Nov 27, 1:45*pm, 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? I was recently playing around with a 6DQ6 PA and it did not require netralization with 2K ohms on the gridI didnt try anything higher than that. Using broadband transformers will introduce some reactance that may have to be tuned out. I suspect this may not become a problem until you get up to 10M or so. Jimmie |
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