W8JI says:
>
>>I've been mostly unsucessful using ferrite beads, but that doesn't
>>say they won't work.
>
>G3RZP says : I've never had success with ferrite beads as parasitic
>suppressors in tube PA's, either.
A complication is that there are several types of ferrite material to
choose from that are rated for VHF attenuator service. Also, there are
instances where multiple beads are needed. The material type should be
matched in accordance with the VHF anode resonant frequency in the
amplifier, as measured with a dipmeter at the DC blocking capacitor.
...snip....
>Strangely enough, I've also never had repeatable success with them as
>parasitic suppressors on dual gate MOSFETS, either.
1/8w 50 - 100 ohm resistors seem to work pretty well.
>As other people appear, or at least claim, to have made them work, I
>guess it's just me.
>
I doubt it, Peter. Unless one has access to test equipment like a
Hewlett-Packard Model 4191A RF Impedance Analyzer, designing a VHF
parasitic suppressor is pretty much guesswork. The goal is to make the
parallel-equivalent resistance (Rp) of the VHF-resonant anode circuit as
low as practicable at the point of resonance.
Unfortunately, no ARRL and no Eimac publication gets into VHF suppressor
design. A book that does is: Dittrich, H. F.; *Tubes for R. F.
Heating*; N.V. Phillips Gloeilaampenfabrieeken--Eindhoven, the
Netherlands. Section 5.8, 'Parasitic Suppression Circuits' starts on
page 96. Suppressor design is also discussed in *Amplifiers* at:
http://www.vcnet.com/measures
Rich---
R. L. Measures, 805-386-3734, AG6K
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