Unlike many hams who could count on one hand how many SB-220 amps they have
owned in a life time, some have to go to their feet to keep score. Well, I
am now starting to count hairs on my head and thankfully I have not experienced
Alopecia quite yet. All kidding aside, I have had HUNDREDS of Sb-220 amps
though here on my bench in the last 12 years. I have also seen all the sets
of 3-500 tubes that were in them as well. Beginning with the SB-220,
statistically the average used SB-220 has at least one burned or vaporized band
switch contact!! I would venture to say that 7 out of every 10 amps I have
seen
have a bad contact. That is pretty bad odds. If you are looking to buy an
SB-220 you should be real concerned about this!! Often times if the contact
is not vaporized that amp still works because the contacts are redundant.
Sometimes only one contact on one side is bad and on the other side it is
"OK".
As soon as a high SWR is present or some mistuning, you'll here an arc come
from that contact. In the long run, if you use that band a lot, failure is
close by. Those are pretty bad odds. The problem is it is very hard to see
the contacts on the switch and the inside contact is almost impossible
without the use of a bright light and a dental type mirror. Most other
problems
are simple to fix, but this problem is a bummer. Keep this in mind. OK, now
for the 3-500's. The Sb-220 has been around for a long time, most of them
made around the time when we all used "load em up radio transmitters" most
of
them were capable of over 100 watts output, some Drake and Swan radios put
out 250 watts. Even the lower wattage radios did 160 to 180 watts output.
You could kill a pair of 3-500's with this kind of power input levels. SO I
have seen my share of weak tubes. As time went on and the usual solid state
100 watt radio was the norm, you would be hard pressed to destroy a pair of
3-500's with only 100 watts drive in an SB-220. So the odds of getting a full
output set of tubes in an Sb-220 is very high. I'd say in the 90 percentile!!
Well that's some good news. I have also seen many different types of
3-500s. Eimacs, Amperex, and others, including the Chinese varieties. Eimac
in
the early and late 60's made a run of graphite tubes, they looked different
inside, much like a Pagoda instead of the typical turbine fan blade. Amperex
tubes all looked like this. Not sure if all were graphite or not, however I
have noticed in a small population of older tubes of this sort a difference
in bias requirements. Typically the standard 3-500 is self biased at about
2500 volts or less and no additional bias voltage is required to lower the
idle current to about the normal 180 to 200 mills. The Drake L4.L4B had no
additional bias voltage. The SB-220 with about 2900 volts on the plates has
about 5 volts of bias which settles the tubes down to about 180 mills. If you
run the tubes beyond 3000 volts you will need 5+ volts of bias and as high as
13v as in the Ameritron AL-82. Interestingly however, there was "some" and I
have only seen very few old Eimac and Amperex 3-500 tubes that seem to be
self biased at 3000 volts and require no additional bias to lower the idle
current to 180 to 200 mills. At first when I encounter these tubes, I thought
they were bad since the idle current when biased at 5 volts is so low, but in
the long run they are full output. I've used several Chinese brand tubes in
the past and I have always been pleased with them. Their performance is right
up there with the top brands. The Sb-220 itself had some evolution, there
were 2 different types of loading variable caps used, the Sb-221 used a wider
spaced tune cap. The Sb-221 did not have 10 meters unless it was built like
anSB-220 in the beginning by the builder buying the "10 meter kit". The
Sb-221 as it is lacks a 10 meter contact on the band switch so it is no simple
"mod" to make one work on 10 meters. You'll need an extra contact on the
input
switch as well and some input coils. They did due diligence on keeping the
CB'ers from using this amp easily!! Ditto for the later HL-2200 amp. It has
been a rewarding experience for me to have done these hundreds of autopsy's.
I am getting to fell like the Dr. G. of Sb-220's. My morgue is ever
expanding. 73 Lou W1QJ
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