Mauir said:
"A socket, a wiring and a layout that works for VHF (in a similar
configuration, i.e. Gg and PI) is generally quite stable at lower
frequencies as HF.
Shielding of input and output, avoiding long wire paths and big
distances between components can't be enough to cure all problems but
surely doesn't go in the wrong direction.
Back in the '50's, the 6AK5 was used as an rf amplifier in receivers and it
had a propensity to have parasitic oscillations. The best way to avoid
those oscillations was to pay particular attention to each detail in the
building stage. Most important was to isolate the input from the output. I
remember the shielding used to isolate the input from output was much
greater than most present day linear amplifiers. First hand experience
taught me it was better to design it right in the first place from an
isolation standpoint than to retro fit it with shields made from old tin
cans.
It is tempting to tell ourselves that the amplifier is grounded grid and
bypass good construction techniques because feedback is less of a problem.
The fact that parasitic oscillations are still with us is proof that
grounded grid is not a cure all or excuse for sloppy design and
construction.
Uhf amplifiers rarely have parasitic problems, yet use no parasitic
suppressors. That should tell us something about good design. We should
probably review a few uhf designs before building our next amplifier and
save a few headaches.
73, Colin K7FM
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