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To: Pete Smith <>
From: R L Measures <>
Date: Fri, 2 Jun 2006 14:46:23 -0700
List-post: <>
On Jun 2, 2006, at 5:41 AM, Pete Smith wrote:

> At 08:20 AM 6/2/2006, wrote:
>> In a message dated 6/1/2006 11:55:24 P.M. Eastern Standard Time,
>> writes:
>> The one  supposed test I seen on the net which was very flawed,  
>> was meant
>> solely to  discredit the suppressors. If I were to have ran the  
>> test, I would
>> have ran it  on knowingly squirelly amps
>> Can someone tell me why some SB-220's have stability problems and  
>> why  others
>> don't?  Can we identify the difference between them?   OR......Are  
>> all
>> SB-220's not satable?
> My SB-220, which was evidently built by a very skilled OT, is  
> unconditionally stable.  Construction quality, proper lead lengths,  
> etc. are one consideration.  Another may be the tubes used - I have  
> heard that certain manufacturers' 3-500s have higher gain than others.

Amen, and so far there is no explanation that makes sense.

> My tubes are all Eimacs, still going strong after 30 years.   
> 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, which had the effect of shifting the weakest link to the  
> bandswitch.  Whether any of these changes may have affected  
> stability is beyond me...

Increased Tune-C spacing did not improve stability, but sales of  
bandswitches improved considerably
> I did not mean to imply in my earlier message that SB-220s don't  
> need parasitic suppressors, or that needing suppressors is somehow  
> a bad thing.  This whole suppressor argument has achieved  
> theological status, with about as much hope of being resolved as  
> one of those debates.  Cooler heads have observed that sensible  
> amplifier designers determine empirically whether an amplifier is  
> stable or not, and apply suppressors if indicated.

Unfortunately, the test for fundamental instability does not test for  
parasitic instability.
W7MOI purchased a new Henry 2K-4A.  Nothing at all happened when he  
rotated the tune and load controls with zero drive.  After almost 8- 
years of almost daily use, he announced that the 2k-4A was perfectly  
stable.  Within a week, he heard a big bang and subsequently  
discovered that both 3-500Zs had grid-fil shorts.
> 73, Pete N4ZR
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