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

[R.Measures <r@somis.org>]

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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