All: My 1977 SB-220 works great on all bands, including the WARC bands. There
is an issue on 30 meters where there is not enough loading capacity. I added a
two doorknob caps of 35 pf each in parallel across the output capacitor. They
need to be the proper NPO types that can handle the current. Of course, this
results in the loading capacitor being set to a smaller amount on the highest
bands (20, 17, 15, 12, and 10).
As far as the input goes, the tuner in my K3 handles it just fine so I did
nothing different with the input slugs.
One BIG thing I did was to pull out the RF chokes and capacitors from the grids
of the 3-500Z's and directly ground the grids to the chassis with wide copper
strips.This resulted in the amp being easier to drive on all bands. I get about
1 KW out on 10, 12, and 15 meters and a bit more going lower depending upon my
line voltage. This is with the original tubes from 1977!
One thing to check is to make sure that your parasitic suppressors are in good
working order, i.e., that the scrawny original carbon resistors are not cracked
or way out of specification.
I love my SB-220 and it is my go to amp for most of my operations even though I
also have an ACOM 2000A and AL-800H. The ACOM is used for contesting and rapid
band changing and for 160 meters when something interesting comes on the air.
As far as the soft start issue, I chose not to install it. I ALWAYS turn the
amp on in the low power position. I think that the 26 ufd filter caps don't
create a huge current surge - I also replaced the PS diodes with 3A 1000 volt
units.
73 Dennis W0JX
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