[Amps] cathode driven tetrodes

[John Lyles]

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 at contesting.com wrote:
> From: Fuqua, William
> Sent: Wednesday, October 31, 2018 4:22 PM
> To: Jim Thomson ;amps at 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
>