[AMPS] Parasitic suppressors/another question,

[Rich Measures]

>
>On Thu, 8 Apr 1999 17:38:35 -0700 Rich Measures <measures@vc.net> writes:

>>>On Thu, 8 Apr 1999 11:13:44 -0700 Rich Measures <measures@vc.net> 
>>writes:
>>>>
>>>ALL SNIPPED
>>>
>>>>> 
>>>>> This is somewhat of the principle behind a load pull test.  You 
>>>>either 
>>>>> short out or open circuit the output of an amp and then rotate it 
>>
>>>>around 
>>>>> the Smith chart (via Ctune).  Also, the tank inductor generally 
>>>>provides 
>>>>> a large enough reactive impedance to VHF energy that it likely 
>>>>doesn't 
>>>>> matter wether you have a load on or not.
>>>>
>>>>Ÿ  amen. In parasite-arced bandswitches, damage occurs primarily on 
>>>>the 10m and 15m taps.  .  
>>>
>>>
>>>Completely subjective and unproven Rich.
>>>
>>Ä  the photographs lie?  
>
>
>Anthing taken long after the fact is open to question.
>
>
>>> 
>>>Another option which I subscribe to is Peters/Ians  version when it 
>>comes
>>>to parasitics.
>>>
>>>In a real amp that is 100% stable to start with, the switch failures 
>>are
>>>primarily due to the mfg trying to save a few $$  and using a poor
>>>initial choice .
>>
>>Ä  The TL-922 and SB-220 bandswitches have a withstanding potential of 
>> a bit under 6000v at sea level.  The peak RF potential in both amplifiers 
>>is around 2900v.  (see arced bandswitch photos on my Web site and in 
>>"Parasitics Revisited", Sept./Oct. 1990 *QST*)
>>Ä  If such  bandswitch arcing is not vhf-related, why is it common to 
>>see 100% increases in R-supp after bandswitch arcing?  
>
>Simple I believe. The voltage is not an issue in the SB-220 but the
>current is. I have examined almost 200 bandswitches and amps that have
>been run hard on 10/11M show a loss of contact tension. 

€  So it is the loss of contact tension that indirectly causes the 
resistance of the suppressor resistors to substantially increase without 
showing an outward sign of resistor overheating?

>This soon leads to overheating and eventual failure. 

€   only open bandswitch contacts arced.  

>When the rotor heats up it then also
>transfers heat to the 15M contact. It then becomes a domino effect. 
>IOTH, those amps that use positive spring loaded contact pressure such as
>the JV-9000 series do not exhibit that failure mode.

€  The arced AL-82 bandswitch shown in the photo on my Web site is a 
JV-9000.  
>
>I have modified a few SB-220's with a second output wafer and wired in
>parallel with a good or rebuilt original. These amps are still used in
>contests and failures have ceased.
>
>No voodoo suppressors, no parasitics.
>
€  low Q = voodoo?
>
>>>Before the doubters wind up in a snit I suggest that they take a real
>>>look at the specs of the commonly used switches and try to understand
>>>exactly what they are spending money on.
>>>
>>Ä  Better yet, use a high-potential tester and evaluate the bandswitches 
yourself.  
>
>I already have and in-circuit testing does not agree with your results.
>The SB-220 switch in a humid summertime enviroment will breakdown at a
>hair over 3500V. In a dirty or nicotine stained amp the breakdown is
>around 2800V. Reconnect the Tune cap and the breakdown is below 2500V in
>extreme dirty condx.
>
€  A spilt cup of tea will also reduce the breakdown V.  .  .  My 
measurements were mostly done on unslimed, removed bandswitches that had 
vapourized 10m and 15m contacts.  Measurements were taken across the 
remaining good contacts.  

€  It's beginning to look like you were right, Dick.  

-  cheers,  Carl


Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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