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

[R.Measures]

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

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

=F0  However, the screen current is so low in a 4cx250B that it often=20
goes negative, so the tube  is ideally suited for using a simple=20
zener-string shunt screen regulator connected to the LV end of the HV=20
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=20=

> when
> used with the typical 100W PEP output rig.   Grid and screen=20
> dissipation are
> very low, so overdriving can be quite exciting (catastrophic).  =20
> Because
> they have a lot of gain into the UHF spectrum, greater care must be=20
> used in
> component selection and layout to prevent oscillation.  They don't =
lend
> themselves well to GG operation.  Their IMD characteristics are not as=20=

> good
> as many power grid triodes.  With all these things in mind, of course=20=

> they
> can be used.  They're very popular in VHF and UHF amplifiers, and have=20=

> been
> for 50 years.  A few HF transmitters and amplifiers over their many=20
> year
> history have been designed and sold successfully, maybe starting with=20=

> the
> Collins KWS-1 (which used 4X150s originally).  I think Tokyo Hy-Power=20=

> (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 at oceanwide.co.nz]
>> Sent:	Tuesday, July 13, 2004 11:23 PM
>> To:	amps at 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=20
>> 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