Rich says:
>The fly in the ointment is that solid-state radios use bandpass
filters >which introduce various reactances at the output. Thus, there
may be >bands where XC1 will need to have a reactance of other than 25 ohms.
Rich is making a very good point here. Although we shouldn't forget that
even amps with tuned inputs (eg 30L1) had a specified length of coax to
connect them to the driver, the idea being that the amps pi input and the
drivers pi output and the coax added up to specific number of degrees on
each band. Some solid state exciters will also go unstable with certain load
impedances eg 50 ohms in the band you're operating in, and high SWR on
another band.
And just because the optimum load impedance is 50 ohms, it doesn't mean that
the source impedance (ie the internal impedance of the driver) is 50 ohms.
73
Peter G3RZP
> ----------
> From: Rich Measures[SMTP:measures@vc.net]
> Sent: 17 April 1999 04:23
> To: amps@contesting.com
> Subject: Re: [AMPS] Need to pick your brain...
>
>
>
>
> >
> >Greetings:
> >
> >I'm trying to help a friend (WN0X) do some maintenance on his SB-220.
>
> >The tuned input for 80M-20M yields a decent input impedance, but on
> >10M and 15M it's really(!) way off... to the point that on 15M, the
> >actual resonant point is somewhere around 11MHz(!) and it's almost
> >that bad (and in the same direction) on 10M.
>
> ? A Pi-network (which is two L-networks in series) is typically resonant
> slightly below the operating frequency. A 21MHz tuned-input Pi-network
> that is resonant at 21MHz, will not work acceptably at 21MHz. At
> 24.9MHz, maybe.
> >
> > ..........
> >Any thoughts on the subject? HeathKit used the SAME inductor
> >for both 10M and 15M tuned inputs,
>
> ? The 10-15 m coils' powdered iron slug gives a wide range of L.
> Unfortunately, the Q of all the 220's tuned inputs need to be increased
> to approx. 2 (XC1=25 ohms) to make the SB-220 compatible with solid-state
> radios. This means that each C-in and C-out needs to be increased, and
> each L needs to be decreased. . . The fly in the ointment is that
> solid-state radios use bandpass filters which introduce various
> reactances at the output. Thus, there may be bands where XC1 will need
> to have a reactance of other than 25 ohms. (For info on reworking and
> adjusting the 220's tuned inputs, see Nov. - Dec., 1990 *QST*, "Circuit
> Improvements for the Heath SB-220 Amplifier" // orig. manuscript
> available on Web site)
>
> >just different caps. on the input and output of the pi-nets.
> >
> ? To increase the Q to approx. 2, turns need to be removed from the
> inductors where backing the slug does not reduce L enough to do the job.
> .
>
> - later, Tom
>
> . . . and it's prudent to oil the fan motor every two years.
>
>
> Rich...
>
> R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
>
>
> --
> FAQ on WWW: http://www.contesting.com/ampfaq.html
> Submissions: amps@contesting.com
> Administrative requests: amps-REQUEST@contesting.com
> Problems: owner-amps@contesting.com
> Search: http://www.contesting.com/km9p/search.htm
>
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|