[Amps] Grounded Screen 4CX1500B

[Shawn Tayler]

On Mon, 7 Nov 2005 07:52:44 +0000 Ian White G/GM3SEK
<gm3sek at 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

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