Desperation is the mother of invention, I have heard.
I recently had the opportunity to update a Hunter Bandit 2000C for a
customer. This was a kit built amp that may have even predated the SB-220
since the date codes on most parts are 1967 and the original Amperex
8163's (3-400Z) are dated 1968. The manual is undated.
Anyway, my customer bought it from the estate of the original owner and
brought it to me for a complete checkout and inclusion of WARC bands.
One of the first things my trusty old Measurements Corp. Model 59 GDO
found was a big fat series resonance right at 17M. The original choke
only measures 45uh anyway and 80M output was hundreds of watts down from
the other bands.
I was limited to 5" in head room so the B&W 802 would not fit, altho it
worked fine with the cover removed. I also tried an Ameritron AL-82
choke, which was also physically too big, and it gave real erratic
loading on 10-15M. More on that later.
On a hunch, I grabbed a 3/4" diameter by 4" long ceramic form from stock
(H.H.Smith #2621) and wound 3 7/8" of close wound #28 wire on it. Series
resonances on 10/12M were too close for comfort so I started removing 5
turns at a time. The final result is 3 5/8" of winding for 188uh and
series resonances (installed) of 15.9, 24.1 and 31MHz. Power output
80-10M is 1200-1300W and no signs of overheating or instability.
The choke has been replicated and installed in a SB-220 with similar good
performance. The reasonably high inductance should make it a good
performer at 160M but I have not tested it there yet.
One size does not fit all and anyone wishing to copy this should
understand that it would be wise to GDO it in the intended amp. Start
with 10+ extra turns and move the resonances around to where they are out
of the way. I have no way of measuring the impedances vs frequencies
since my HP-4815A died.
Other attempts to wind a good choke with different form factors never
worked out well; I encourage others to try the above and report results,
particularly in a more diverse sampling of applications.
Getting back to the Ameritron choke. Installed series resonances were
12.7, 16.2, 21.5 and 29.5MHz. Perhaps in the open spaces of the big
Ameritron amps the latter pair of frequencies are much higher. Don't
expect to intergrate Brand A choke into Brand B amp without
testing.....fireworks may result.
73....Carl KM1H
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