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[AMPS] parasitics

To: <amps@contesting.com>
Subject: [AMPS] parasitics
From: measures@vc.net (Rich Measures)
Date: Tue, 21 Apr 98 11:07:45 -0800
>
>>It seems to me that even a 100% increase in Rs would not make much 
>>difference in the performance of a VHF parasitic suppressor. 
>
>Depends on the amp. The Clipperton L and the Ameritron 811 series do not
>like much of a value change. The SB-220 is still happy at 100 Ohms.
>
The suppressors in the three appear to be quite similar.
>
>>>With overstressed 2W carbons for R, I agree. But with 5W metal oxide
>>>available at a fraction of the cost of 2W carbons why not change when 
>>the
>>>cover is off? If you want to spend the time measuring the R of an
>>>antique, why not just upgrade at the same time?  I dont even bother
>>>wasting my time to measure the old ones any longer.
>>>
>> Is it a waste of time to measure the resistance of a 
>>seemingly-critical 
>>resistor? 
>
>In my case...yes. I have already measured enough of them to see the
>pattern. Since time is money I cant see wasting it. With Mouser selling
>5W metal oxides at $ .49 it makes no sense to unwind the L measure the R,
>rewind the L and put it back in the circuit only to wonder when you will
>have to do it all over again.  
>
Wind, unwind?  I unsolder one end, clip on the DMM.   We are talking 
maybe half a minute.  If one knows how the resistor was damaged, an 
instant replay may be avoided -- i.e., if Rs failed from 28MHz operation, 
Ls needs to be decreased a bit;  if Rs seemingly failed due to VHF 
parasite, maybe it would be a good idea to reduce the VHF-Rp of the 
suppressor?  

>
>> .  .  There are two types of damage to Rs.  It is my opinion 
>>that when there is cosmetic damage and less than a 50% change in 
>>resistance, 28MHz operation is usually the reason.
>
>Cosmetic damage and any increase up to 10 times or more is usually from
>CB or 10M use. The increase depends how many hours or years of abuse. 
>
10-4 good buddy

>>  However, if there 
>>is little or no cosmetic damage, and a large increase in resistance, 
>>higher frequency energy is the culprit.  
>
>During the debug of a new amp I agree. In a well behaved amp that just
>went bang I disagree.  Take a 2W carbon and subject it to a 100X overload
>for a few milliseconds at DC. You will see the same results. 

How does Rs receive a 100x overload in any amplifier, new and old, where 
Rs is paralleled with an Ls that is a virtual short at HF?
>.........


cheers
Rich...

R. L. Measures, 805-386-3734, AG6K   


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