In the early 1980s while I worked for Broadcast Electronics in Quincy, I
ran a series of tests using a 4CX3500A, which is a shortened version of
4CX5000A (lower inductance connections and higher upper frequency).
Using the same output circuit (cavity) I built two input matching
circuits, one for common cathode and one for common (grounded) grid
operation. On both the screen grid was held at a constant DC bias, but
was well bypassed (and neutralized) for RF. This was at 100 MHz. The GG
configuration had a wider overall bandwidth, which was expected and was
the point of the experiment, to prove whether this was more beneficial
for an FM transmitter that carried stereo 38 KHz subcarrier and
subsidiary communication (SCA) on a higher frequency subcarrier. The
power gain was about 3 dB less with GG, Other than this, there was no
unexpected consequence or change in stability. Since this was a class C
amplifier, I biased the grid toward cutoff at all times. So the power
gain was not as high as could be in AB1 or B.
These days all of the high powered tetrode amplifiers that I build at
work are grounded grid, cathode driven. These giant tubes are intended
to be run this way, with the grids both biased at normal voltage but
well decoupled from RF so that they are at RF ground. The RF voltage is
all applied to the cathode to grid resonator. Power gain is typically 14
dB in class B, and the input tuning is very broad, and insensitive to
thermal changes (such as change of Cg-k as the tube heats up). The input
Z is quite low. I use concentric coaxial cavities for input and output
circuits. Neutralization is still required. For grid driven (GK) mode,
it would be too easy for a big bottle to break into oscillation I
believe. Lots of interelectrode capacitance there.
An internal paper can be found on the BE website maybe, in the white
papers but I think you have to create a password. A summary of the work
is also included in the 7th edition (1985) of the National Broadcast
Association handbook, which can be found starting around page 88 here:
https://www.americanradiohistory.com/Archive-NAB-Engineering/NAB-7th-Edition/3-NAB-7th.pdf
Since this work was not for a linear amplifier, as they say, your
mileage may vary....
73
John
K5PRO
On 11/1/18 6:05 AM, amps-request@contesting.com wrote:
From: Fuqua, William
Sent: Wednesday, October 31, 2018 4:22 PM
To: Jim Thomson ;amps@contesting.com
Subject: Re: [Amps] Super Cathode Driven Tetrodes
Do you know anyone that has had first hand experience in running 4CX5000
tubes in conventional grounded grid configuration? I have some that I would
like to try perhaps. They have 75Watt control grid dissipation and 250 Watt
screen grid dissipation.
73
Bill wa4lav
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