>
? JIM
>
>..
>>? 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.
>
>Agreed. I'm tempted to rebuild the original board just to be
>able to look at it with the spectrum analyzer, but it probably
>isn't worth the trouble.
? agreed. The feculence that is generated by RF bias switching is hard
to see on a typical spectrum analyzer due to lack of skirt selectivity.
A double-filter SSB receiver works quite well in this application.
>My DSO/analyzer only goes to
>16mhz anyway, (32mhz sampling) so I couldn't test it beyond 20m.
>
>>? 8 - 10 % drop is more likely for a C-filtered PS. .
>
>Okay, then make it 3900/3510 . I found the CC curve sheet for
>the 8877 on a site somewhere, but have no idea what idling
>current (thus first-approximation bias voltage) Eimac recommends
>for a 8877 in a GG AB1 configuration at that anode voltage. Any
>ideas?
? 182mA@3500v.
>
>>? 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.
>
>I took a look, thanks. I don't know what flavor the existing
>output vacuum relay is, but the printing on it is "TCR1G N/O 7404" .
>It doesn't look like a Jennings, but I've made larger mistakes.
>Anyway, your bias switch design looks fine, and would fit on a
>board the size of the existing one. Even 18 or so 3A diodes,
>(tapped down) although I've heard stories of temperature
>instability going that route? (A question, not a challenge.
>Haven't done it that way before)
? I have not observed this problem nor heard of any such complaint.
Henry Radio uses fwd-biased diodes to set the oper. bias in many of its
amplifiers.
>
>>..
>>? mo' complicated is not always mo' betta.
>.
>>? chips and RF don't get along all that well.
>
>True enuf. Using a packaged uC module, the total parts count
>wouldn't have to be much higher, but it would have to be well
>shielded and bypassed. I'll consider both routes thoroughly
>before taking one.
>
good luck, Jim
- R. L. Measures, 805.386.3734, www.vcnet.com/measures.
end
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