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