Shawn Tayler wrote:
>Hi Guys,
>
>Playing with my 30S-1 has gotten me thinking about a project. I've been
>wanting an amp for 50Mhz for a while and I have 2 4CX1500B's lating around
>as spares. One would make a nice amp on 6M and I think I'd like to do the
>grounded screen arrangement like the 30S-1.
>
>Could any of you comment on the pros and cons? The PS arrangement seems
>unusual to me with the plate supply return on the groud ie. screen and the
>cathode return appearing to go through the bias and screen supplies. I'm
>sure I'm mis reading it. IIRC the Continental 816 I maintained so years
>back was of a similar arrangement, 4CX15000A. But the plate screen and
>grid supplies returned directly through the cathode with all kinds of
>bypassing.
>
>Comments?
There are two separate issues here: RF grounding, and power supplies.
The RF grounding issue is thoroughly discussed in the description of a
4CX1000 amp for 2m that was in the ARRL Handbooks for several years. At
30MHz and below, the conventional screen bypass capacitor in the tube
socket seems perfectly OK, with the cathode grounded to chassis. At
144MHz, the screen bypass cap may not be fully effective, and the
individuals who built the ARRL amp found stability was improved by
bolting the screen directly to chassis ground (though they don't
emphasize that the bypass capacitor problems move over to the cathode
instead). Of course, 50MHz will be somewhere in between. Even older ARRL
Handbooks have 4CX1000 amps using the conventional screen bypass cap and
DC-grounded cathode. Your call...
The power supply issue is almost completely separate from the choice of
RF grounding. DC-grounding the screen does not force you to follow the
whole 30S-1 route, where the "screen" transformer has to supply the
entire cathode current as well. You can DC-ground the screen and then
use a more conventional power supply.
From the power supply point of view, it's best to start out by thinking
of the anode, grid and screen supplies as being completely separate and
floating. Then you join all three at a single point which will be
connected to the cathode of the tube. You now have a "universal" tetrode
power supply that can be used with all types of DC/RF grounding... so
finally you connect it to the RF deck, and it's there that the power
supply finds its DC ground. (There are other details, of course, related
to metering and safety, but that basic principle still holds good.)
There are several papers on this topic on my Tetrode Boards website, at:
http://www.ifwtech.co.uk/g3sek/boards/tetrode/tetrode-3.htm
The QEX article includes a description of the 30S-1 setup, which turns
out to be something of a package deal - if you don't do everything that
Collins did, it won't work so well.
--
73 from Ian G/GM3SEK
http://www.ifwtech.co.uk/g3sek
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