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Re: [Amps] SB-220's, 3-500's, some history

To: <Gudguyham@aol.com>, <amps@contesting.com>
Subject: Re: [Amps] SB-220's, 3-500's, some history
From: "Tim Duffy K3LR" <k3lr@k3lr.com>
Reply-to: k3lr@k3lr.com
Date: Fri, 6 Feb 2009 18:24:44 -0600
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Thanks for sharing this valuable information Lou.

Your email is a keeper!

73!
Tim K3LR

-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Gudguyham@aol.com
Sent: Friday, February 06, 2009 7:34 AM
To: amps@contesting.com
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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