ORIGINAL MESSAGE:
On Fri, 22 Feb 2013 11:18:14 -0600, Mike wrote:
Earlier handbooks, etc, show plenty of amps with different tubes with no
parasitic suppressor at all, and that's what I'm going to try.
REPLY:
Remember your oscillator basics: RF energy has to be fed back to the
input
circuit in the correct phase to sustain amplification. IMO, the best way
to
suppress VHF oscillation is to make the input circuit a very low impedance
at the frequency of the VHF parasitic tank in the plate circuit. If the
impedance is low enough, it will swamp out enough RF energy so that the
tube(s) will not oscillate.
Probably the most important rule in keeping VHF impedance low i to keep
all
wiring in the input circuit as short as possible. One of the worst things
you can do is to run coax from the back of the chassis where the tube is
up
to a band switch on the front panel. Sections of coax like this act as a
transmission line transformer and can exhibit very high impedance at VHF
frequencies, just what you don't want. Instead, mount the input bandswitch
right next to the tube socket. Yes, I know this means having two separate
band switches but it goes a long way toward stabilizing your amp.
I have built several amps with this in mind and NO parasitic suppressor in
the place circuit. All were unconditionally stable, even with the antenna
disconnected.
Bill, W6WRT