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@t-online.de>
To: <amps@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@contesting.com] On Behalf Of Fuqua, Bill L
Sent: Dienstag, 18. M?rz 2014 19:07
To: Hsu
Cc: amps@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.
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