On Jun 3, 2006, at 10:41 AM, Gudguyham@aol.com wrote:
> In a message dated 6/2/2006 5:47:24 PM Eastern Daylight Time,
> r@somis.org 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
r@somis.org
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