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Date: Sat, 3 Jun 2006 13:41:30 EDT
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In a message dated 6/2/2006 5:47:24 PM Eastern Daylight Time,  

Finally,  there were some changes made during the SB-220 production  
> run  that may have affected stability; one I have heard of is that  
>  the spacing of the tuning capacitor was increased to prevent  
>  arcing,
It is NOT my experience that this was ever done.  Every SB-220 I have  ever 
seen from one of the earliest to the latest always had the same plate tune  
variable cap, however they did change the loading cap somewhere along the  
Perhaps this is what you meant?
Speaking of the SB-220 bandswitch.  It always seemed to me that the  contacts 
on these switches were marginal at best for a 1 KW output amplifier,  however 
I do believe that Heath designed it as 1KW DC input amplifier (legal  limit 
at the time).  That said, the manual tune up instructions only called  for a 
maximum of about 500 mills of plate current.  So at best the output  would be 
about 500-600watts.  At this rating the contacts may be OK,  however we all 
and the reports from others here on the reflector tout their  SB-220's going 
1200-1500 watts output.  That is 2 times the amount Heath  called for 
(according to their loading instructions).  Since I have  autopsied dozens of 
I have found at least  80% of them had one or  more badly burned contacts.  
That's 8 out of 10.  Since the SB-220 has  redundant contacts, many amps still 
worked on the bands that had a bad contacts,  but the slightest mishap in 
loading (ie too much drive out of resonance) would  result in a nice arc at the 
contact since the current capacity of the contact  was now 1/2 what it was.
It is very difficult to examine both sets of contacts on an SB-220 to see  if 
they are 100%.  One really needs a small mirror like a dentist uses to  see 
the contacts and check them for burns.  I'll bet that alot of you out  there 
who have SB-220's that were bought used probably have a contact that is  
somewhat compromised and does not really know it.  In the same time period  
where other brands of amps using 3-500's which had MUCH heavier bandswitch  
contacts.  I have worked with many of those amps too.  The likelyhood  of a 
bandswitch contact was near ziltch on them.  It is obvious that  many run the 
Sb-220 to 800 to 1000 mils of plate current.  In order to  report outputs of 
1200-1500 watts this is surely the case.  The contacts  are simply too wimpy 
for that kind of current, especially on the higher bands,  which by the way 
the contacts that were burned.  Most of the time it  was the 10 meter 
contacts that were burned (or totally gone!) then the 15 meter  or both.  
Rarely did 
I ever see a bad 80-40-20 meter  contact.
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