Greetings, Morrie--
I built an SB-220 in 1977 and have used it regularly until lightning damaged
it three months ago. It is on my workbench now and will be repaired as soon
as the spirit moves me.
Rich Measures has a website with much information on the SB-220. The URL
is:
http://www.somis.org/ , particularly at http://somis.org/sb220ci.html
A man named Harbach prepared many modifications for the SB-220. When his
health prevented his continuing in business, another ham picked up the job
and supplies those modifications now. He is moving his home presently, but
when he settles in his new abode, you can see his offerings at the following
URL:
http://www.harbachelectronics.com
According to Rich Measures, the Heathkit engineers used a special
transformer core in the filament/bias transformer to limit the inrush
current to the tubes and nothing further is required in that regard.
The SB-220 requires the transceiver to switch about 130 VDC at about 100 ma.
This requirement exceeds the ability of many modern transceivers and can
damage them. The Harbach modification for this is called the Soft-Key
module. I recommend it or any similar circuit that permits the transceiver
to switch low voltage and low current.
Rich Measures tells how to lubricate the cooling fan motor. When I bought
my SB-220 some 25 years ago, I bought a spare fan motor. I have not needed
to use that spare motor because I lubricate the motor annually. The cooling
method devised by the Heathkit engineers is very effective, even though it
looks so simple.
I recommend two other modifications supplied by Harbach. First is a new
power supply board - I believe he calls it a "Rectifier/metering board." It
provides better metering accuracy, a better method of supplying bias voltage
to the tubes, and newer generation diodes in the high voltage supply.
Secondly, he supplies a new bank of electrolytic capacitors with bleeder
resistors. Newer electrolytics are better quality than older ones and the
bleeder/equalization resistors can be of higher resistance and thus produce
less heat.
The 3-500 tubes need to be protected from VHF parasitic oscillations, some
say. I installed the "parasitic suppression kit" from Harbach. I believe
that Measures also supplies such a kit.
-------------------------------
Now, my recommendations:
I really like my SB-220. It is a workhorse amplifier. I operate only CW
and use Ten-Tec transceivers with QSK, but my SB-220 is not a QSK amplifier.
I keep the drop-out delay in my transceiver set to the longest time -- about
one second -- and just operate normally. What happens when I begin to
transmit is that the SB-220's antenna relay energizes and stays in that
state until I pause for a second. I do not lose my QSK ability during a
transmission because sufficient received signal bleeds through the antenna
relay of the SB-220 to allow me to hear all but the weakest signal during my
transmission. To me, there is an advantage in doing it this way: received
signals AND noise are attenuated during the transmission and thus my shack
is quieter. The disadvantage of doing it this way is that at the beginning
of a transmission the antenna relay switches "hot." 25 years of doing it
this way did not damage my antenna relay (but the lightning did).
My insurance company bought a Ten-Tec Centurion amplifier to replace my
SB-220 after the lightning storm. I drove down to Ten-Tec a month ago and
picked up the new Centurion. It is still in the trunk of my car, new in its
boxes. I am more excited with the prospect of being able to repair my
SB-220 than with my new Centurion, even though the Centurion is a fine
amplifier.
Some people will sell an SB-220 for less than $400, but to me, an SB-220 in
good condition with new tubes (assuming the tubes are of good quality) would
be worth perhaps $550 or $600 US dollars. I would not sell mine (even in
its damaged condition) for $700. I've always heard that in continuous
commercial service the 3-500 tube has an estimated life of 20,000 hours.
That is many, many years of amateur operation!
73, Mike N4NT
----- Original Message -----
From: "ZL2AAA" <zl2aaa@paradise.net.nz>
To: <amps@contesting.com>
Sent: Monday, November 03, 2003 2:52 PM
Subject: [Amps] SB220
Hi All A local amateur has the opportunity to purchase a SB220 with new
3/500's what are the pro's and con's for this amp and what is a reasonable
price .Tnx Morrie
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