>> 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.
>
...snip...
At 6kV, for a ZSAC of 0.5A per anode, the grid bias requirement for a
3CX2500F3 (Mu=22) is roughly -220V. If the -220V bias supply is
connected between the filament transformer CT (cathode) and
chassis-ground, the grid flange can be directly grounded. // Another
labor-saver with such tubes is to put the filament RFC in the primary of
the filament transformer instead of the secondary. This allows one to
use a 2A bifilar RFC instead of a 51A bifilar RFC. ..... The filament
transformer is mounted on a sheet of insulating material, and the primary
and secondary windings are RF-bypassed to the frame of the transformer.
The measured C (typically 60pF) of the tranformer to chassis-ground
figures in the C2 calculations for the tuned input.
An 8877 "driver" is a real eyebrow-raiser, Mr. Rauch. I'm guessing that
the amount of feedthrough power is substantial. In the days of the FCC's
old 1000w DC input limit, there were a few savy amplifier builders who
built 1000W DC-input grounded-grid amplifiers using a low Mu triode,
whose output substantively exceeded 1000w! An ideal tube for such a
project would probably be the 3CX3000A1 (Mu=5).
Rich---
R. L. Measures, 805-386-3734, AG6K
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