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[AMPS] PA-77 musings

To: <amps@contesting.com>
Subject: [AMPS] PA-77 musings
From: 2@vc.net (2)
Date: Sun, 27 May 2001 20:33:20 -0700
>
?  Jim
>
>I recently bought a "basket case" PA-77. (single 8877) ...

>(1) The amp uses 8.2V RF-switched bias.  

?  RF-switching shifts to non-linear bias during soft syllables.  The 
amplifier's T/R bias  should always be in sync with the T/R state in the 
transceiver.  

>With the current combination
>of B+ components, there's 3900V at the anode, (only bleeder current)
>and I'll -guess- it'll drop to around 3700V on SSB.  

?  8 - 10 % drop is more likely for a C-filtered PS.  .

>That 8.2V bias
>@ 3700-3900V puts the operating point rather far up the curve toward
>class A during speech intervals.  Because the bias/QSK/grid trip board
>is blown to smithereens, I'm thinking of raising the bias to 12V or so,
>(TR, rather than RF switched) with a "BIAS/INPUT then OUTPUT" relay
>sequence.
>
?  The high-speed switching circuit shown in Fig, 7B on my web site 
switches in under 2mS and it supplies adjustable bias for g-g operation.  
It switches so fast that sequencing is obviated. 

>(2) Since I'm headed toward a new controller, I'm toying with
>the idea of using a microcontroller, rather than RC constants
>and such.  

?  mo' complicated is not always mo' betta.  

>The question here is since the anode current is
>referenced to B- instead of ground, and the A/D converters on
>the uC board are ground-referenced, anyone have a good idea
>on how to monitor the anode current?  

?  chips and RF don't get along all that well.  

>...

cheers, Jim

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


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