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[AMPS] SB1000 parasite question for Tom and Rich

To: <amps@contesting.com>
Subject: [AMPS] SB1000 parasite question for Tom and Rich
From: 2@vc.net (measures)
Date: Thu, 3 Aug 2000 15:37:02 -0700
>
>> I just got a mail from a SB1000 owner who gets the normal 750 -900 watts
>> out with his old used tube. But wÌth two different new EIMAC tubes he only
>> gets around 200 -250 watts out and both tubes tubes get red. 
>
>Sounds like bad tubes.
>
>> parasite suppressor that changed from original value and now with new
>> tubes with higher gain at VHF the linear is parasiting. Right?
>
>Very very doubtful. The way you can find a parasitic is to key the 
>PA without drive so it has idling current, and rotate the tuning 
>control through it's range while looking for a sudden appearance of 
>grid current or an increase in anode current.
>
Yet another old wives' tale. This is how you test for instability at the 
operating freq.  
>.....
 To generate the requisite damped wave anode circuit ringing, it takes 
transient anode currents.  

>> sure,  but I think he said the ceramic switch needs replacing and he tried
>> to get a replacement from Tom? A burned switch is "normal" in a SB1000
>> that has parasites if I remember right in earlier SB 1000 discussions on
>> the net.. I know Rich knows a lot of this fenomen, dont you?
>
>No, the burned switch comes from the fact Heath did not include 
>an anti-corona washer on the high voltage lug, and the relay 
>sequencing may be wrong. They also used a transformer with a bit 
>poor voltage regulation, which doesn't help.
>
Owners who have replaced burned bandswitches and added the anti corona 
hardware have reportedly continued to experience intermittent open 
contact bandswitch arcing.  The fix that seems to work is:   reduce VHF 
amplification.  

>A parasitic will virtually never damage a bandswitch. 

Please see the collection of toasted bandswitch photographs on my  
Website.  Is the Internet the best thing since sliced bread, or what?

>Underloading 
>the amplifier will, as will driving it with an exciter with spikes when 
>the loading control on the PA is too far closed.
>
>Change the switch, adding the anti-corona washer, and check the 
>relay sequencing on the TR relay. Make sure the PA is loaded 
>heavy enough, especially if the exciter has transients.
>
?  Tom recommends over 500mA of anode current -- no fooling.  When is Tom 
going to show us the Eimac letter which supposedly gives the ok to 500mA?
>
later, Tom

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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