Incidentally, the output capacitance of a 3CX3000A7 in Gg is 24pf that's a
little bit more than twice the single 8877.
The 38 pf is not "huge" and anyway is the input capacitance.
A paired 8877, less that a single 3CX3000, would get more output capacitance
because of interconnections.
Last but not least, a reasonable PI with a loaded Q of 12 requires more than
the 3CX3000 internal capacitance and a vacuum variable capacitor
(raccomandable with such HV) has generally only 10 pf of residual
capacitance.
73,
Mauri I4JMY
----- Original Message -----
From: "Bob Marston" <k1ta@contesting.com>
To: "G SEVEN" <superiorwavelength@prodigy.net>
Cc: "AMPS" <amps@contesting.com>
Sent: Sunday, March 11, 2001 4:49 PM
Subject: Re: [AMPS] 3CX3000A7
>
> GSeven wrote:
>
> > I thought it would tune around at least 25pf bit its back all the way
out.
> > What should I expect to see with say 100 watts provided the tank is
right?
>
> Answer Not much The 3CX-3000A7 is notorius as being a Pig to drive. The
> spec sheet states that 410 watts is required to drive it to 2.0 Amps of
> plate current. With 100 watts don't expect much more than 500 mils of
plate
> current with the tube biased correctly.
>
> As to 10 meters not tuning that is also predictable. The 3000 has an
> inordinately high output capacitance. 38pf Compare this to the 8877 and
> 4CX-1500A which each have values of 10 pf and its understandable that you
> are not able to tune 10 meters.
>
> Finally the plate voltage that you state you are using (6700 volts) is a
> full 33% over the manufacturers Maximum recommended Plate Voltage of 5000
> Volts. BE CAREFUL !
>
> It appears that Eimac intentionally developed this tune with the objective
> of keeping it out of the hands of Hams and CBers. My recomendations, if
you
> have this Amp running on all bands but 10 declare victory and leave it
> alone. Drop the Plate Supply to 5000 volts and limit the service of this
> amp to 14 Mhz and below.
>
> 73s
>
> Bob K1TA
>
>
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>
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