[Amps] Amps Digest, Vol 122, Issue 28

[Mike Waters]

Well said, Bill! That's exactly what I had in mind.

73, Mike
www.w0btu.com

---------- Forwarded message ----------

> From: Bill Turner <dezrat1242 at yahoo.com>
> To: Amps <amps at contesting.com>
> Cc:
> Date: Fri, 22 Feb 2013 17:50:02 -0800
> Subject: Re: [Amps] Why always one parasitic suppressor per tube?
> 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
>