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