[Amps] Parasitics & Filament Sag

[Bill Turner]

ORIGINAL MESSAGE:

On Sun, 27 Aug 2006 10:55:55 -0500, "Phil Clements"
<philc at texascellnet.com> wrote:

>Operation on 10-12 meters
>will cause heating of L-sup on these bands if it is properly designed and
>contribute to its demise faster than those of us who don't operate there.


------------ REPLY FOLLOWS ------------

Agreed. For decades, hams have stuck a parasitic suppressor in the
anode circuit because "it's the thing to do", and of course it nearly
always works.

I think a better approach would be to not have a parasitic suppressor
at all in the anode circuit and to focus on making the cathode-grid
circuit either nonresonant at the anode resonant frequency or so
heavily loaded with resistance that the gain at VHF drops below the
point needed for oscillation, or both. 

Some amps already use this approach and I think it is a good one. More
work needs to be done, but for now I see two factors which seem to
work: Use very short leads in the C-G circuit and use a low value
capacitor and resistor from C to G to load the VHF circuit already
there. Long leads can act like a transmission line which in turn acts
like a parallel resonant circuit.

In my homebrew GS-35b amp there is no anode suppressor and I use a 33
pF and a ten ohm in series and so far it seems perfectly stable. I'm
still working on it so this is not definitive, but it is encouraging.
The GS-35b is a low-mu triode and this helps too.

-- 
Bill, W6WRT