Ian White GM3SEK wrote:
Grumpy The Mule wrote:

I've only read about them. G2DAF wrote extensively on pasive grid
linears.

I think the ultimate value of grid resistor will depend on the grid
capacitance and layout parasitics. The articles I've read use 300 Ohms
as the grid swamping resistor but with a 4CX250 450 Ohms might be
possible.

Here is an article you may find useful.
http://www.radioamator.ro/articole/files/291_1.pdf

I have a bucket full of 4CX250B's and would like to try passive grid
someday. Please Let us know how yours shapes up if you decide to
build one.


The defining feature of a G2DAF amplifier is not the untuned passive
grid input, but the method of grid biasing. It has no control grid bias
and the screen voltage is developed by rectifying the RF drive.

Most passive grid tetrode amplifiers use conventional fixed bias
voltages on g1 and g2. Commercial examples include the Alpha 91b/99 and
the Australian Emtron range.


G2DAF's published articles on amplifiers are on my web site, and the
RSGB historical collection still has his original receiver, transmitter
and amplifier, along with many hand-written notes. The 2 x 4-125A
amplifier that featured in his published articles still exists, but at
some later time he had removed the screen rectifier tubes and converted
the amplifier into a very basic grounded-grid configuration.



I really don't see any advantage in his circuit over a conventional zero
bias grounded grid amp, at least not with the 4-xxx series of tubes.
With the 4CX250 series it makes some sense since these tubes CAN'T be
run in zero bias grounded grid due to the fragile grid structure in
these tubes.