[Amps] Grounded Screen 4CX1500B

[G0RUZ_Conrad]

Shawn I operated the 2m amp for several years on eme. There was a problem
with the 3rd harmonic which appeared to be a result of cavity resonances in
the output compartment. I cured it with a 100 ohm large carbon resistor
placed on one wall of the cavity, grounded at one end. At the time I was
mystified by the suggestion from a broadcast engineering friend but it
worked, presumably by absorbing harmonic energy circulating around the
output compartment.

That may be anecdotal but it actually happened  :-)

73

Conrad G0RUZ

PS I can recommend grounded screen operation, the amp was very stable once I
had cured the harmonic problem.

> -----Original Message-----
> From: amps-bounces at contesting.com 
> [mailto:amps-bounces at contesting.com] On Behalf Of Shawn Tayler
> Sent: 07 November 2005 12:51
> To: amps at contesting.com
> Subject: Re: [Amps] Grounded Screen 4CX1500B
> 
> 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
> 
> --
> Mitchell's Law of Committees:
> 	Any simple problem can be made insoluble if enough 
> meetings are held to discuss it.
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