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Re: [BULK] - [Amps] HF and 4cx250b's

To: Steve Katz <stevek@jmr.com>
Subject: Re: [BULK] - [Amps] HF and 4cx250b's
From: R.Measures <r@somis.org>
Date: Sat, 17 Jul 2004 06:12:59 -0700
List-post: <mailto:amps@contesting.com>

On Jul 15, 2004, at 7:23 AM, Steve Katz wrote:


4CX250Bs require a regulated screen supply, making the PSU design more
complex.

 However, the screen current is so low in a 4cx250B that it often goes negative, so the tube is ideally suited for using a simple zener-string shunt screen regulator connected to the LV end of the HV bleeder-R.
http://www.somis.org/D-a-10.GIF
This circuit happily protects the tube from any possible failure mode.
--- cheerz, Jerry




They have enormous gain (about 20 dB), so drive requirements are
very low, requiring either a low power exciter or an input attenuator when
used with the typical 100W PEP output rig. Grid and screen dissipation are
very low, so overdriving can be quite exciting (catastrophic). Because
they have a lot of gain into the UHF spectrum, greater care must be used in
component selection and layout to prevent oscillation. They don't lend
themselves well to GG operation. Their IMD characteristics are not as good
as many power grid triodes. With all these things in mind, of course they
can be used. They're very popular in VHF and UHF amplifiers, and have been
for 50 years. A few HF transmitters and amplifiers over their many year
history have been designed and sold successfully, maybe starting with the
Collins KWS-1 (which used 4X150s originally). I think Tokyo Hy-Power (THL)
still uses a pair of them in a 1kW PEP HF amplifier, so if you want
something to emulate, that might be a good starting point. -WB2WIK/6


"There is no reason anyone would want a computer in their home." - Ken
Olson, Chairman & Founder, Digital Equipment Corporation (1977)

-----Original Message-----
From:   jerry [SMTP:jlee@oceanwide.co.nz]
Sent:   Tuesday, July 13, 2004 11:23 PM
To:     amps@contesting.com
Subject:        [BULK] - [Amps] HF   and 4cx250b's

Hi all,

Is there some reason why there seems to be very few HF amps built around
the 4cx250b's? They seem to be cheep and very plentiful.


I am thinking of building an HF amp using 3 or 4 4cx250b's. Is there

anything wrong with doing this?


Cheers


Jerry

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Richard L. Measures, AG6K, 805.386.3734. www.somis.org

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