[Amps] [Fwd: Re: [Moon-net] 4CX250B in P-P 2m?]

[kf4yox]

  I find it very hard to believe a pair of 250b's will be even close to
clean at 1kw out. At that power lever the control grid is really getting
hammered. A pair of 250r's will do about 800 out with 2400 on the plate
loaded. To get 1 kw out with clean rf a bigger tube is the only way. A pair
of 4cx400a's with 2500 plate loaded 375 screen . 360 ma resting current will
deliver 1200 out in ab1.  I have run a pair on 2 meters for years now. Also
to have it run clean a shunt regulated screen supply is a must. I use Ian
Whites tetrode boards on my vhf and uhf amps. I also run a k2riw amp with a
pair  of 250b's on 70 cm. Forget about more than 600 out without
overdriving. It is more like 500 out. it is hard to cool  the 250 series
tubes due to the small radiator. A pair of 8930's can be cooled more
effectively. Good luck   73   Sam KF4YOX

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]On
Behalf Of Dr. David Kirkby
Sent: Friday, March 31, 2006 7:42 PM
To: amps at contesting.com
Subject: [Amps] [Fwd: Re: [Moon-net] 4CX250B in P-P 2m?]


Oops, I mis-sent this. Forwarding on.


Winfried Kriegl wrote:
> Why not consider to use 2 x 4CX350 tubes ? With the same setup and
> construction effort it´s no problem to get an easy kW out.
>
> 73 Win DK9IP/KH7CD

Whilst 4CX250A/F are rated more than 4CX250Bs, they are probably less able
to
take serious abuse. They are not designed for class C, with a grid
dissipation
of 0. So I suspect you might wreck the grids on them.

I think as Ian says, using a bigger tube is more sensible.


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