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[AMPS] Linear AMP timing

To: <amps@contesting.com>
Subject: [AMPS] Linear AMP timing
From: 2@vc.net (2)
Date: Wed, 8 Aug 2001 18:16:46 -0700
>
>On Wed, 08 Aug 2001 14:37:01 -0400, Pete Smith wrote:
>
>>Well, I don't know, but when the T/R relay on my SB-220 began to stick in
>>the "on" position, it turned out to be the bias contacts that were the
>>worst burnt.  Maybe a sign that they were trying to open too soon?  
>
>_________________________________________________________
>
>The contacts must be opening more than just bias, most likely the actual
>cathode current.  

//  The Ic is normally 0.15A - which should not crispy-critter 10A 
ontacts.

> Since this is DC, you should be able to suppress the
>arc by placing a cap and resistor in series across the contacts.  The
>best value has to be determined empirically.  I'd try a .1 uf in series
>with 100 ohms and then tweak the values for minimum visible spark at the
>relay contacts.  You should be able to get the spark down to barely
>visible.

//  Good idea for arc quenching.  However, 0.15A should not be a problem. 
  My guess is that the problem is related to the use of V-cutoff bias 
during RX.  I would try converting to resistor-cutoff bias.  This done by 
removing the +110v PS from the bias contacts, and moving one end of the 
100k-ohm [R27] resistor from the NC contact to the NO contact.  This 
converts the bias to resistor-cutoff.   Also, with the SB-220's original 
V-cutoff bias ckt, when a 3-500Z fil,/grid shorts, the (unfused) filament 
transformer will melt down unless the 220 is quickly switched off.  
However,  with R-cutoff bias, nothing bad results when a tube shorts. 

cheers   


-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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