[Amps] SB-220 reliability and tuning

[Jim Thomson]

Date: Tue, 11 Sep 2012 05:59:43 -0400
From: Pete Smith N4ZR <n4zr at contesting.com>
To: amps at contesting.com
Subject: [Amps] MSB-220 reliability and tuning


This puzzles me. My SB-220 has run for 12 years of CW contesting (in 
the SSB position), with only minimal problems. 1200 watts out on 80, 
down to about 1000 on 10. I added the Harbach high-volume fan, as well 
as a home-brew vacuum relay T/R system, and have a muffin fan above the 
tube compartment to help by drawing air out. I polished the filament 
pins and the mating socket clips, after resoldering filament pins a few 
times.

Power supply components are all original. Tuning is easy on the low 
bands, and even on the high ones I can get close enough in a quick band 
change that I don't risk the hardware before I can tune (using CW 
dits). My secret? An index card stuck to the panel behind the tune and 
load controls, with lines for each band. No turn counters or verniers 
required.

73, Pete N4ZR

##  I know of at least 2 doz folks who have smoked the plate xfmr
in their SB-220.   How you get it to last the weekend, running 1200w
out on cw is beyond me.  It only has a 500va  xfmr in it.  While
dots may have a 50%  duty cycle,  dashes sure as heck don’t. 

##  sure the  index card might work, but the tuning on the higher bands is
still extremely touchy.   Min to max C  is still  only one half a turn. 
Since a 6:1  vernier will solve the  touchy tuning issue, then get one with
a calibrated 0-100  skirt across a 180 deg arc.....and kill  2 birds at once. Get
one for the load as well, then you can log both of em.   The tuning is silky
smooth on the low bands...and acceptable on the high bands.  Heck,  even MFJ
sells the  6:1  vernier dirt cheap. 

##  Put some cool-amp silver plating goop  on both the pins and socket clips,
and the resistance will drop to zero every time..and stay that way. They have to
be spotless clean 1st though, b4 apply the  silver paste.  I use that stuff on everything.
I silver plate CU tubing tank coils with it, and also use it on relays, contactors, buss bar
and everything else you can think of.

Jim  VE7RF