----- Original Message -----
From: "Steve Thompson" <g8gsq@qsl.net>
To: <amps@contesting.com>
Sent: Tuesday, December 18, 2001 6:51 PM
Subject: Re: [AMPS] Bypassing Screen of 4cx800a for 50 MHz
>
>
> -----Original Message-----
> From: Lamb <k7fm@teleport.com>
> To: Steve Thompson <g8gsq@qsl.net>; amps@contesting.com
> <amps@contesting.com>
To: <amps@contesting.com>
> Date: 18 December 2001 12:47
> Subject: Re: [AMPS] Bypassing Screen of 4cx800a for 50 MHz
>
>
> >Steve asked:
> >
> >"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?"
> >
> >You were probably always causing problems for your teachers - asking
> >difficult questions just to see if you could stump them.
>
> Only when I wanted to understand something better. Interestingly,
sometimes
> my questions elicit no response at all.
>
> >The 4CX250 has 4 cathode pins. So, if you are cheap and do not want to
buy
> >the proven, but expensive, socket with the built in screen bypassing,
then
> >it should be easier to bypass the cathode - since you have 4 shots at
it -
> >to get a low inductance bypass.
>
> Indeed, Rich and Mike made much the same comment. The view appears to be
> that it's not as critical as screen decoupling. That's what I had thought,
> but imperfect cathode decoupling puts a reactance rather than a resistance
> in the way. The volts across the reactance will reduce the gain, but it
also
> offers a point where signals can get fed back to the grid. I wondered if
the
> phase element might be troublesome - in practice, I guess it isn't.
>
> Steve
>
Its true that the reactance of the cathode decoupling (or is it coupling hi
hi :-)
was considered a problem and the author of the 144MHz 1990 ARRL
design spoke of using 220pF DM19 Silver Mica capacitors with precisely
1/4" long leads argueing that this was series resonant at 144MHz and was
semi-critical in the design/implementation, however I later had
corrospondance
with two hams in the USA that had built the beast and they said that the
make, value and mounting arrangements of the cathode (de)coupling caps
were non-critical :~)
The 144MHz 1990 ARRL design has around 25-26dB gain and one that
I used back in 1987 (Mick G1EZF's) was unconditionally stable, needed
(had) no neutralisation and produced >900W out driven with an IC202S
barefoot.
If I were going to build a single Tetrode amp on 6m or 2m I'd consider
running the screen at deck - RF simplicity at (slight) HT complexity (ie.
need for a B- lead).
Mike G8TIC/M0VHF
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