On Mon, 7 Nov 2005 07:52:44 +0000 Ian White G/GM3SEK
<gm3sek@ifwtech.co.uk> exclaimed:
> 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.
Thank you Ian,
I will peruse your site for these!
I was curious on the 4CX1000A for 2M in the ARRL book. I thought I read
about harmonic issues stemming from the Plate line lengths used. It was
anecdotable but I would like to learn more on this as well. Is there
references to this on your site or available elsewhere?
Shawn
N7LQ
--
Mitchell's Law of Committees:
Any simple problem can be made insoluble if enough meetings are
held to discuss it.
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