-----Original Message-----
From: Michael Tope <W4EF@dellroy.com>
To: AMPS <amps@contesting.com>
To: <amps@contesting.com>
Date: 10 May 2001 23:41
Subject: [AMPS] 2 x 8877 on 6 meters?
>
>A friend of mine is working on a 2 x 8877 amplifier that
>he has setup to work on 20, 15, 10, and 6 meters. So far
>its working great on 20 thru 10, but he seems to be falling
>the victim of stray C at 50 MHz. His PI network design (Q=10)
>calls for 32 pf of tune-C, whereas he is measuring ~22pf of
>stray for each tube. Eimac specs for the 8877 suggest the plate
>C for each tube should be around 10pf plus whatever additional
>C is added by the layout, so his numbers, although perhaps
>a little on the high side, look like they are at least in the
>ball park.
You need to be careful with manufacturer's capacitance figures, especially
at higher frequencies. Usually Ca is measured using a shielded isolated
fixture, where strays to the outside world are excluded or nulled out to
give a 'pure' value for Ca. I've measured a number of different tubes when
in the chassis, and Ca is usually around double the data sheet value. The
only way I can get the data sheet value is to suspend the tube in mid air on
a long thread.
Steve
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