>
? 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.
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