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