Larry,
Interesting comments. A guy did a 4CX1000 amp write up in Radio
Communication some years back. He had the novel approach to screen
regulation by using 400 1N4148's in series as a shunt regulator, and with a
50 ohm resistor on the input, he had no trouble at all. Maybe a glutton for
punishment - his metalwork was 1/8 inch aluminium, cut from an old locker
pulled from the trash dumpster, hacksawed to size, and draw filed!
What about the old 30S1?I know that was GG, but did people have trouble with
stability?
Guess I should do the sums on stability....the 4CX1000 was chosen because I
gave a lecture at a radio club, and they gave me a brand new socket and
chimney. G4DGU took the XYL and myself to lunch and gave me a 4CX1000. I
visited a customer and was talking about this and the need for a spare: he
opened his cupboard and gave me a 4CX1000. The rest, as they say, is
history.....
Tapping down the resistor can't hurt though.
If the worst comes to the worst, I can always fit 4 4CX250s.............
73
Peter G3RZP
> ----------
> From: Larry Molitor[SMTP:w7iuv@arrl.net]
> Sent: 17 June 1999 22:50
> To: amps@contesting.com; Peter_Chadwick@mitel.com
> Subject: 4CX1000 (was 3CX800)
>
> (Maybe this will make it out on the third try.)
>
> (I sent this out once before, but it didn't make it to the list.)
>
> Peter,
>
> After 20 plus years of struggling (struggling hell, it was had to hand
> combat) with my 4CX1000 amp, I feel compelled to add a few comments to
> this
> thread.
>
> I would never recommend using a 4CX1000. I have decided they are not
> appropriate for they type of service I demand from my amps.
>
> Having said that and considering you are well past that point already, let
> me suggest a few things that might be of interest.
>
> 4CX1000A's are nothing like 4CX5000's or 4CX10000's. 4CXkw's are the most
> twitchy tube I have ever tried to use. Much worse then even 811A's. The
> gain of a fresh tube, or even a better then normal one, is sufficient to
> require neutralization above 20 mHz even with a 12 ohm grid termination.
> Keeping the anode voltage below 2500 will help a lot. At 2200, they are
> fairly easy to deal with.
>
> I tried a 50 ohm termination at first. That was not a good thing with a
> 100w transceiver. Next I tried a 3 db pad in line. That was a lot better
> but the grid "C" is high enough to cause a big difference between the
> drive
> required at 80 mtr and 10 mtr. I finally wound up with a tapped
> termination. 50 ohms presented to the driver, and the grid tapped down to
> around 15 or so. This was the best I ever had. It took about 80 watts to
> drive it on 10 mtrs. Easy thing to do with a pile of carbon rod resistors
> laying around.
>
> ALC circuits were the biggest disappointment. Never did come up with a
> variation that would work well. Gave up early on using that to control
> drive and just used it for protection. Finally even gave that up and tore
> it all out.
>
> I tried various ways of adjusting the screen and the grid bias. Using an
> approach similar to that which Rich suggested, I came up with an operating
> point that gave a small advantage, but the results were not worth the
> effort.
>
> The biggest single problem was keeping the screen and the grid operating
> at
> the selected point when moving around the band. QSY's as small as 50 khz
> required returning the whole thing. I even altered the tank Q, but didn't
> buy much there either.
>
>
> That chassis now holds a YC-156, but before I converted, I finally figure
> out what it took to make the 4CXkw viable. Anode voltage 2500 or less,
> Grid
> and Screen as per Eimac recommendation, adjust the tap on the grid
> termination so that you can never even come close to grid current with the
> exciter full maxed out on any band. This will result in an output power of
> around 800 watts with a "clean" signal and somewhat less sensitivity to
> QSY's. If you are lucky, you might even get away without neutralization. I
> was not lucky, at least with my only fresh tube. Thing would turn into a
> diathermy machine on 12 mtrs at the drop of a hat!
>
> I hope to resurrect the 4CX1000's in the future as a six meter amp. I
> figure neutralized at about 500 watts out running close to class A might
> work.
>
> 73,
>
> Larry - W7IUV
>
>
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