W8JI did a good analysis of this idea:
Scroll down to 'A Common "Bad Grid Idea" Super Cathode Drive'.
One take-away is that at some frequency in the HF spectrum that grid circuit is parallel resonant. That just seems like a bad idea. Of course this is not the only point in his analysis.
73
-Jim
NU0C
On Mon, 25 Mar 2024 20:21:46 +0000
"Clyde K2UE" <clydewashburn1@...> wrote:
The mechanism of feedback is the capacitive divider formed by the plate
to grid capacitance and the grid to ground capacitance. The average of
the various 3-500Z data sheets suggest plate to grid is 4.4 pF and each
tube has 600 pF of grid to ground capacitance, so it is a a roughly
136:1 voltage divider. Taking the 2500V case, the RMS plate voltage at
600W per tube is 1440V, which causes 10.6V to be applied to the grid, in
opposition to the cathode. The driving power without feedback is 46W
into 100 ohms, or 67.8V, so the feedback has increased that to 78.4 V,
or 61.5 W (ignoring the grid current loading). So so in the simple case
33% more power is required for the same output. Note that max output is
not reduced -- it just takes more power to get there. But if you have a
100W driver you are not able to supply more drive, and conclude that
grounding the grids "makes more power". With a 150 or 200W driver you
crank up the power, get the same output as before, but reduced
distortion due to the feedback.
You can imagine the linearity enhancing effect by noting that the grid
current shoots up at the peak of the drive cycle and is very non-linear.
So when the grid current is reaching peak it is shunting the feedback,
i.e there is less negative voltage on the grid opposing the drive,
helping the tube reach the peak current needed to accurately reproduce
the peak -- in other words, better linearity.
The above calculations are obviously an approximation, ignoring that the
reactance of grid-to-ground capacitance changes by 16:1 over the various
ham bands, and the grid input impedance in in shunt with it. But that
is the mechanism in a nutshell.
______________________________________________________________
Clyde Washburn
ClydeWashburn1@...
"It is better to light one candle than to curse the darkness"
------ Original Message ------
From "Paul Kraemer" <elespe@...>
To "[email protected]" <[email protected]>
Date 3/25/2024 3:23:29 PM
Subject [DRAKE-RADIO] grounded grids vs stock Drake
Curious as to what should actually be expected regarding power gain going from the "floating" grids in L4B / L7 vs strapping the grids directly to ground. I see 200 watt increase on one L7. Increases from 1kw to 1200w, same drive level and power supply. Is that believable? Seems a bit high but that is what I am seeing on one amp.
Thanks
Paul K0UYA
--
73
-Jim
NU0C
Jim Shorney