Hi Tom,
if I remember correctly, the inherent feedthrough capacitance for a 4cx1000
in a common grid configuration is less than 0.01 pF, formerly 0.00 pf.
In my experience the 4cx1000 stability problems are in the grounding of
screen.
Or one uses the 4cx1000k (expensive and not final since the capacitor may
suffer damages) or one modifies the socket to directly grounding the screen
contacts to amp. chassis, of course doing what's proper to re-establish the
correct polarization for the tube.
Concerning the coaxial line lenght I agree with your comment.
73,
Mauri I4JMY
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <k2xx@swva.net>
Cc: <amps@contesting.com>; "Eric Moore" <emoore@windemullerelectric.com>
Sent: Tuesday, January 30, 2001 7:38 PM
Subject: Re: [AMPS] linear amps
>
> > Collins also recommended that a specific length of feedline should be
> > used between the exciter and the 30S1 input. I have ignored this
> > warning with no ill effects. The 30S1 uses a 4CX1000A in the final.
> >
> > 73, Joe
>
> I've always been puzzled by that recommendation, since unlike the
> 30L1...the 30S1 is stable and clean.
>
> I've read some gibberish about phase shift, but when I look at line
> lengths and network phase shifts none of the phase-shift
> explanations make any sense at all.
>
> Longer input lines can stabilize amps with inherent regeneration,
> like the 30L1 and other unneutralized amps using tubes with high
> feedthrough capacitance, because the longer and lossier line helps
> terminate the PA's input port on frequencies other than where the
> exciter's output tank is tuned.
>
> But if the input circuit has adequate Q and is matched, and if the
> PA has minimal regeneration, the system would be totally
> unaffected by line length.
>
> When you see someone saying "use a line this long" you can bet
> the system has problems of some sort, and that is a band-aid.
>
>
>
>
> 73, Tom W8JI
> w8ji@contesting.com
>
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