[Amps] tetrodes operating VHF, common grid

[John Lyles]

4CX5000A isn't a great VHF tube, Eimac 4CX3500A is better due to lower 
cathode lead inductance and shorter height. I did a comparison of the 
4CX3500A in grounded grid (with bias) and grounded cathode, at 100 MHz. 
Built both circuits and tested them. It got published in one of the old 
NAB handbooks, as well as an app note from Broadcast Electronics in the 
early 1980s. I may still have a copy if anyone asks.

These days I design amps with grounded grid tetrodes at VHF. Bias to the 
screen and control grid is still provided, only RF grounded in cavity 
circuits. I get power gains of 25x (~14-15 dB) and decent efficiencies 
of 58-64% using the TH781 and TH628. These are large tubes, but the 
principle is the same. The 628 can run up to 5 amps of screen and grid 
current without damage, with pyrolytic graphite grids.

73
John
K5PRO
> Date: Tue, 18 Mar 2014 19:37:30 +0100
> From: "Peter Voelpel" <dj7ww at t-online.de>
> To: <amps at contesting.com>
> Subject: Re: [Amps] 4CX5000A in grounded-grid

>
> Don?t mix things up with high? grounded grid designs!
>
> The 4CX5000A in that 2m amp is used with 1.2KV screen voltage and -300V grid voltage.
>
> The 4CX1000A with its zero watts grid dissipation works flawless in the Collins 30S1 either.
>
> 73
> Peter
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Fuqua, Bill L
> Sent: Dienstag, 18. M?rz 2014 19:07
> To: Hsu
> Cc: amps at contesting.com
> Subject: Re: [Amps] 4CX5000A in grounded-grid
>
>    Yes, most of the drive power is fed thru to the output.
> However, one problem with these tetrodes is that if both grids have he
> same applied voltage the control grid will dissipate more power than the
> screen grid rather than the other way around. Since there is no way to
> tell how much current each is drawing you can't tell when you are getting
> into trouble.