> From: Rich Measures <measures@vc.net>
> Subject: Re: [AMPS] 3CX2500
To: <amps@contesting.com>
> Date: Sun, 20 Jul 97 11:48:42 +0000
> >Me and my mates at the LA8G club stn have got our hands on a brand new
> >3CX2500 F3 / 8251.
> >
> This is a medium Mu triode. It is intended for (non-linear) Class C
> service. In Class AB1 grid-driven linear service, the available power
> output would be small indeed because not much peak anode current is drawn
> with 0v on the grid. In Class AB2 cathode-driven linear service, due to
> lack of Mu, considerable RF driving voltage/power is needed at the
> cathode. IMO, it would take roughly 1000w to fairly drive a 3CX2500F3 in
> cathode-driven service.
I know of a local ham operator using two 3CX2500F3's in parallel
grounded grid, and he obtains over 10,000 watts PEP output with the
tubes (I've been there and seen him pin a 10 kW Bird 43P on twenty
meters).
The drive power requirements in grounded grid are high, because the
driving impedance of the tubes is high, but it suits his application
just fine. He did have to mount the grid flanges on an insulated and
by-passed support, so he could apply grid bias. The bias value was
too high to allow normal center tap biasing without excessive
dissipation.
I don't know the details beyond that, and the fact he uses an 8877 as
a driver. If you aren't looking for high power gain at high power
levels the 2500F3 will certainly work. Expect the power gain to be
about five, at full output.
The problem isn't so much the mu, but rather the high driving
impedance the low mu tube offers. In grounded grid PA's, the gain
relates directly to the ratio of driving to anode impedance. If you
ran 6kV on the anode with low plate current, you might easily get
enough power gain (because the anode impedance would be high) for
2000 watts out with a normal exciter, I can ask him how much drive
his PA takes to output 2 kW.
There certainly are better tube choices allowing lower HV for two kW
output with the same drive.
73, Tom W8JI
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