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From: R L Measures <>
Date: Sat, 3 Jun 2006 13:48:50 -0700
List-post: <>
On Jun 3, 2006, at 10:41 AM, wrote:

> In a message dated 6/2/2006 5:47:24 PM Eastern Daylight Time,  
> writes:
> 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 line.  Perhaps this is what you meant?

Mr. ____?:
   The original Tune C was rated at 3kV.  Later, a 4.5kV unit was used.
h will do it.

> 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 there 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 burned 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 were 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.
> 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 know 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).

  Heath's tuning instructions are reagent-grade Bandini.

> Since I have autopsied dozens of SB-220's I have found at least   
> 80% of them had one or more badly burned contacts.  That's 8 out of  
> 10.

Were the 10m contacts the most often burned?

> 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.

1.  Pi-networks are never operated at resonance, they are operated  
close to resonance.  Low-pass Pi-networks resonate a bit lower than  
the operating freq and high-pass Pi-networks resonate above it.

2. When a SB-220 is overdriven, the potential across the bandswitch  
increases only slightly because 3-500Zs are emission-limited.

> It is very difficult to examine both sets of contacts on an SB-220  
> to see if they are 100%.

An inspection mirror and a penlight will do the job.

R L MEASURES, AG6K. 805-386-3734

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