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@contesting.com
> [mailto:amps-bounces@contesting.com] On Behalf Of Shawn Tayler
> Sent: 07 November 2005 12:51
> To: amps@contesting.com
> Subject: Re: [Amps] Grounded Screen 4CX1500B
>
> 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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