----- Original Message -----
From: "2" <2@vc.net>
To: "Steve Thompson" <g8gsq@qsl.net>; " AMPS" <amps@contesting.com>
Sent: Tuesday, December 18, 2001 12:35 PM
Subject: Re: [AMPS] Bypassing Screen of 4cx800a for 50 MHz
>
> >
> >
> >-----Original Message-----
> >From: W9GT <w9gt@home.com>
> >To: Lamb <k7fm@teleport.com>; amps@contesting.com <amps@contesting.com>
To: <amps@contesting.com>
> >Date: 18 December 2001 01:33
> >Subject: Re: [AMPS] Bypassing Screen of 4cx800a for 50 MHz
> >
> >
> >>
> >>Good suggestion Colin. I was familiar with the 4CX1500B (W5TMN) amp in
> >>Bill Orr's Handbook, but never tried the grounded screen arrangement. I
> >>built a 4CX1500B amplifier, but I used a conventional circuit and the
> >>regular SK-800 series socket which had the built-in bypass cap. Really,
> >the
> >>grounded screen may be an excellent way to go and accomplish the desired
> >>results. It is only complicated by the requirement for floating the
screen
> >>and bias supplies below ground.......
>
> >This is an honest question, not a carping comment. Is bypassing the
cathode
> >to ground at -350V easier or less critical than bypassing the screen at
> >+350V?
> >
> A poorly bypassed screen causes instability. A poorly bypassed cathode
> causes less power output. The best bypass uses multiple caps of
> different values in parallel.
>
> cheers, Steve
>
Steve, Rich, et. al...
Surely the best screen bypass technique for a Tetrode is to mount the tube
such that the screen is at chassis (both DC and RF) and then run the cathode
at below chassis? What you then do is bypass the cathode back to chassis
for an RF path (if you get it wrong you get gain reduction).
Your HT pack now needs a seperate B+ and B- lead, B- goes to the cathode
which operates at a negative voltage wrt to chassis. The screen supply is
effectively connected across cathode and chassis (screen supply -ve to
cathode and screen supply +ve to chassis).
With this arrangement you get:
- do not need screen bypass capacitors (they vanish)
- can use a cheap socket
- very good isolation between anode and control grid
- may not even need any neutralisation
The same arrangement was used in the 4CX1000A 144MHz amp in the
1990 ARRL handbook with much success.
Mike G8TIC/M0VHF
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