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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? |
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