> << The fix that seems to work is: reduce VHF
> amplification. >>
> Rich it seems to me that one way to reduce vhf gain would be not to use tubes
> ( very expensive ones) that have max. ratings to 500 mhz on frequencies of
> 30 mhz or below. HANK
Sound practice!
1. Use power device with lowest transfer frequency to achieve the power
required.
2. Pay utmost attention to isolating the input from the output.
3. Pay utmost attention to RF bypassing.
4. Pay utmost attention to layout in input and output compartments to
reduce stray coupling.
5. Don't assume that power supply wiring cannot be part of the
oscillation (RF bypass these components properly).
6. VHF components generated in the input circuit will appear amplified
on the output circuit.
7. A power amplifier device (in fact any amplifier) that shows a
tendency to self-oscillate has to be stabilised, there are several
methods: mistune the input circuit to reduce the gain, mistune the
output circuit to reduce the gain, resistively load the output circuit
to reduce the gain (this is the nichrome wire technique), reduce the
supply voltage, particularly on the output, to reduce the gain.
8. Pay attention to input and output SWR to ensure that reactive
components do not excite the PA.
9. Check that the harmonic content of the drive source does not contain
unwanted frequencies.
Ian ZS6BTE
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