I've "been there" with the aluminium sockets and cured the instability (in
my case a VHF ocillation) with a VHF series tuned circuit from one of the
cathode/filament pins to chassis (or from a cathode/filament pin to the
grid grounding strip?) From memory I used about 3 turns of 14g wire, 1/4"
dia, in series with a small (15pfd max) air spaced ceramic trimmer
capasitor with just enough capasitance to kill the oscillation. Sorry for
the uncertainty, it was built for a friend years ago and my 'RAM' is a
little rusty!
This amp was a "2 hole" lowbander (160M-20M), so I don't know what the
effect of this series tuned circuit would be at the higher bands. I tried
several different parasitic suppressors in the plate leads but they didn't
help in this case.
FYI ... I used the "coaxial" cathode/filament input tuned circuit from the
old Eimac 3-400Z article, beefed up with 3/8" copper tube and a big fat
inner conductor to handle the 42A filament current of the pair of 4-1000's.
I found the 50 ohm tapping point was about 1 turn down from the tube.
This circuit worked like the article said it would, and several other
amplifiers have been altered to use this form of input tuned circuit.
Results ... 5.2kV HT on the anodes, 120W drive from TS-830, 3000W RF into a
dummy load ;-) with hardly a glow in the plates ... it runs as cool as a
cucumber.
73 ... Bob, ZL1RS
At 14:36 10/08/98 EDT, you wrote:
>
>
>One very important fact in the stability of the 4-1000A amp in "grounded
grid"
>is to use the plastic Eimac socket and run the 3 grid connections through the
>side slits in the socket to chassis ground. This requires only about a 3/4
>inch long lead. I use copper strap about 3/16 inch wide. If you USE the tab
>connections hanging down from this socket and run connections to the chassis
>you have an increased lead length of about 2 inches more. This additional
lead
>length AND the use of the aluminum socket yielding about the same lead length
>will cause a large amount of hair pulling in trying to stabalize the
>amplifier.
>good Luck,
>Terry - W6RU (ex W6TG N6UR)
>
>
>In a message dated 8/10/98 7:22:56 AM Pacific Daylight Time,
jono@enteract.com
>writes:
>
><< >Greetings to the Group,
> > I've just come into possession of an Eimac 4-1000A,
> >glass chimney,
> >and matching socket and am looking for a proven HF amp design to press
> >them into service. If anyone knows of any good designs in past ARRL
> >handbooks etc. PLS let me know. I'd be happy to pay duplication costs if
> >the designs are viable.
> >
>
> Hi Steve,
>
> As someone who is fighting this tube right now in an amp design, I would
> say that if this is your first HF PA design, it's not going to be easy.
> I didn't believe others about this at first, but the 4-1000A has some
> very pesky resonances that tend to make it unstable especially on the
> higher bands like 10 meters. It also needs very high voltage to get
> adequate gain and lots of drive. Lots of guys use 5 to 6 KV. At that
> voltage you get around 12 dB of gain. Lower voltages give somewhere
> around 10 dB of gain. Lots of guys that have them (including myself) use
> the bigger rigs like the FT-1000 that can put out 200 watts.
>
> On the bright side it's an extremely rugged tube and can take a lot of
> abuse. It's also kinda cool to see those plates glow bright orange! But
> it likes lotsa cooling. --SNIP--
> >>
>
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>
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