[AMPS] SB-220 conundrum

[Pete Smith]

At 08:39 AM 1/18/2000 -0800, K2VCO wrote:
>Pete Smith wrote:
>> 
>> 2.  But on 40m, power output (PLATE and LOAD tuned for maximum) is ~1450w,
>> grid current ~250 ma. and Plate current ~900 ma.  There is also a fairly
>> striking difference between the PLATE setting in this condition and the
>> setting for minimum plate current.  If I tune the PLATE for minimum plate
>> current and LOAD for maximum output, the figures look much more like those
>> on the other bands.
>
>An off-the-wall guess:  I recall that with grid-driven amps, a big difference
>between the point of minimum current and maximum output was often
indicative of
>improper neutralization -- in other words, feedback from the amp's output
to the
>input.  Perhaps in changing the relays, you did something which increased the
>output-to-input coupling.  Is the braid of the coax from the input relay
to the
>cathode circuit grounded at both ends?  I could imagine some kind of stray
>resonance in this cable...


Good ideas, Vic.  As far as I can tell, my relay change should have done
little to the situation.  If you know the layout of the SB-220, the three
cables (from the input, to the input tank, and to the output for bypassing
the amp) are all in their original positions and are their original length.
 The input relay has some short "pigtails" connecting it to the original
coax ends, but even when I took them and the relay altogether out of the
circuit the behavior remained unchanged.

73,  Pete N4ZR

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