[AMPS] SB1000 parasite question for Tom and Rich+

[measures]

>
> > <<  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.

€  Mr. Rauch's statement about grid-resonance is the way it is.  
When the supposedly grounded  grid  is resonant, Pandorra's Box is 
undoubtedly open.    

>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.

€  The source of the VHF seed-voltage is the anode resonance.  Every time 
the anode current changes, a damped wave VHF signal is produced.  With a 
SB-220, the damped-wave ringing takes place at 120MHz.  Under the right 
conditions, intermittent regeneration is possible.  
>
cheers, Ian

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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