Nice advertisement Lou, business must be slow down there <G>
However with over 250 SB-220 family 6M conversions and many straight
repairs since the 60's I concur with 99% of the observations.
All the Amperex are graphite and I have a NIB pair of the Eimac
graphites in my tube collection as well as a pair of Russian prototype
3-600Z's there were never released to market.....they didnt work very
well.
Meanwhile, back to work, finishing up a LK-800A 6M conversion.
Stay warm, I see global warming has frozen another elderly homeowner.
Carl
KM1H
----- Original Message -----
From: <Gudguyham@aol.com>
To: <amps@contesting.com>
Sent: Friday, February 06, 2009 8:33 AM
Subject: [Amps] SB-220's, 3-500's, some history
> 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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