>
>Rich says:
>
>>1N5400 series,3a avg. diodes are around $0.25 each. A string of 'em
>>provides adjustable cathode bias, and can withstand 200a peak during a
>>"glitch" -- a current that will destroy a 50w zener in mS.
>
>They also provide a voltage which falls at 2mV/deg C per diode. So 10 diodes
>will give a bias which falls by 200mV for a 10 deg C rise in temperature. If
>you get a rise of 30 deg C, that's a drift of over 5% in the bias voltage.
>Further, the dynamic resistance is given by 25/I, where I is in mA. For a
>500mA plate current swing (i.e. the DC from no RF in to DC with a reasonable
>amount of output), that's a non-negligible change in resistance.
True enough, however, a change of 5% in cathode bias potential makes
virtually zero difference in the IMD performance of any grounded-grid
triode I am aware of. .
>
>Even so, the effects on the overall IMD are probably negligible because as
>KM1H and K1TA have said, the IMD performance of most modern transceivers
>both will dominate and is appalling!
>
>As far as Type approval is concerned, the FCC is only concerned with
>spurious emissions - that's harmonics and signals far out.
Indeed, which are signals outside the ham bands according to the FCC.
- cheers, Peter
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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