ORIGINAL MESSAGE:
On Sat, 22 Aug 2009 16:32:58 -0500, "k5jv" <k5jv@kingwoodcable.com> wrote:
>
> It is not an easy job to design a parasite suppressor that work
>from 1.8 MHz to 30 MHz.
REPLY:
A better approach, IMO is to build a circuit that is stable without any
parasitic suppressor at all. Tubes that are physically small such as the 8877
lend themselves to this approach. The Ameritron AL-1500 uses that tube and has
no suppressor. My homebrew 8877 also has no suppressor. Perfectly stable on all
bands and modes. The secret is to eliminate parasitic resonances in the cathode
circuit. By doing this you keep the cathode impedance at the parasitic frequency
low and what feedback there is isn't enough to start or sustain oscillation.
In a really hard case, you can install a series RC of about 33 pF and 10 ohms
from cathode to ground, or even install a series resonant circuit from cathode
to ground, tuned to the VHF parasitic frequency.
The idea is to simply get the gain at the VFH parasitic frequency below one, and
the amp will be stable. It works.
73, Bill W6WRT
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