[Amps] Why always one parasitic suppressor per tube?

[Carl]

----- Original Message ----- 
From: "Bill Turner" <dezrat1242 at yahoo.com>
To: "Amps" <amps at contesting.com>
Sent: Friday, February 22, 2013 8:50 PM
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


What were the tubes used besides your 8877 amp?

Carl
KM1H