[Amps] More parasitic choke questions

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Wed, 28 Jul 2010 08:06:58 -0400, "Carl" <km1h at jeremy.mv.com> wrote:

>Thats right, I forgot that you use an 8877 which JI says doesnt need any 
>suppressor. There certainly is nothing special or fancy in the AL-1500 
>layout either.
>
>How about showing us your expertise with a 6M 3-500Z amp or maybe one of the 
>Ruskie tubes? Im sure the new builder would be extremely interested in a 
>real project rather than just talk.

REPLY:

Not just 8877s, although that's my favorite. I also have used GS-35B,
GU-43B, 4CX1000A and 4CX1500Bs in homebrew amps with no suppressors.
All worked fine. Notice those are all relatively small tubes with
external anodes which I believe helps with stability by reducing anode
lead length. I mount the tune cap as close as possible to the tube and
thereby keep the VHF parasitic frequency as high as possible. 

My only experience with 3-500Zs was to modify a Heathkit SB-1000 for
six meters. On that one I removed the parasitic suppressor, replacing
it with an L-matching network using the original tune/load caps, and
it worked fine too. 

In all of these, the key to VHF stability is maintaining a very low
impedance in the input circuit at VHF, so that any VHF feedback energy
is swamped out. Basic oscillator/amplifier theory. No voodoo involved.
73, Bill W6WRT