[Amps] Duobanding a SB220 for 6m and 4m

Jim Garland 4cx250b at muohio.edu
Thu Aug 18 05:23:12 PDT 2011

Hi Ian,
You didn't mention modifying the plate choke, but I'm guessing that was an
oversight. You'll need one with fewer turns than the original  (roughly 10
percent of the inductance), and you'll also have to check it for series
resonances near the two bands.  You might also want to change the HV bypass
cap at the base of the plate choke to, say, 100 pF or so. I'll be interested
to hear how well the SB220 works on 70 MHz. (I didn't know there was a ham
band at 4m! Is that for ZS only? Makes me jealous.)
Jim W8ZR
> Message: 1
> Date: Wed, 17 Aug 2011 12:14:07 -0700 (PDT)
> From: Ian Roberts <itrmail2003 at yahoo.com>
> Subject: [Amps] Duobanding a SB220 for 6m and 4m
> To: 'Amps Amps' <amps at contesting.com>
> Message-ID: <1313608447.436.YahooMailNeo at web120615.mail.ne1.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
> G'day,
> My SB220 is currently operating on HF and 6m (10 band was pinched and
realigned) but
> was not optimised - gain is around 7x on 6m.
> This exceeds the max in ZS when driven by an IC-746 so there was no
incentive to optimise
> the output power.
> The input tuning was changed and on the output a piece of copper tape run
to the tune cap.
> Details fuzzy - it was done 11 years ago during the last solar max...
> Possibly there was insufficient inductance on the output.
> Stability is good and there are no problems.
> However, the 70 MHz transverter to be used outputs only 30W and the
intention is to up
> this around 9-10x in the SB220.
> I am happy to say goodbye to HF with the SB220.
> Please advise on the following:
> 1. The grids will be grounded using copper tape.
> 2. The suppressors will be changed to around 33 ohm/2W carbon film with a
couple of turns
> of thin copper wire. Or should I just rewind the existing suppressors with
fewer turns?
> 3. The 10m output coil will be replaced with one of about 2-3 turns with
6m tapped at the
> hot end and 4m tapped around 1.5 turns.
> 4. The TUNE capacitor will replace the LOAD capacitor.
> 5. A 33 pF wide spaced variable (used) will replace the TUNE capacitor.
> 6. The existing T/R relay will be retained - I have a used Jennings 26.5v
vacuum relay lying
> around.
> 7. A "pulled" CRL 850 anode capacitor will replace the existing stock
anode capacitor. I
> have several CRL 850's? ranging in value 12-140 pF and believe these are
good for 5 kV
> and the RF current.
> 8. To switch between 6m (10m) and 4m (15m) the band switch will be
retained for better or
> worse.
> 9. 15m input realigned to 70 MHz.
> End of proposed mods.
> Advice appreciated,
> 73.
> Ian ZS6BTE

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