Mike Schelly wrote:
>
> Greetings to all...I'm trying to get this amp to behave but it seems to
> be a T.G.T.P oscillator. Without drive at about 4KV grid and plate
> current start to flow. The amp is a 20m monoband with vacuum tune and
> load caps. Makes no difference what type or variation of parasitic
> suppressor is used...copper/silver or nichrome. I get a dip at 62Mhz at
> the plate blocking cap. The grids and screens are grounded with .25"
> copper strap thru the sockets directly to the chassis. The filaments
> are in series with a 15VCT transformer, bypassed with .01 micas. Bias is
> provided by 4 series 1N5402 diodes. With any type of tuned input,
> parallel C/L, pi, or T-input I have an oscillation. Feeding the filament
> direct thru a .01 mica....no oscillation, no Ip/Ig without drive. Does
> any tuned input have to be in a shielded enclosure under the chassis? Do
> I necessary have an input/output feedback problem? Any help appreciated
> guys...
Hi Mike,
Sounds like you have a real problem. From your message I conclude your
using GG which should make the amp insensitive, normally. Without
looking at the construction I guess that your getting a LOT of output
back into the input in phase. Some general rules for neutralizing a GG
amp is to keep all leads short as possible in both in and output
circuits. Shield both compartments from each other TOTALLY. Dont forget
to use well shield coax between the exciter and the amp. Make sure all
the RF is going out to the antenna and not radiating in the shack from
poor constuction.
If you have been reading the hand book on neutralization, dont try to
feed the output back into the input. If you was building the amp to be
very sensitive AB1 and putting screen voltage on the tube it would have
to be that way. Since your grounding the screen the tubes should be hard
to drive and easy to keep neutralized by good SHIELDING.
One easy way to test the amp is to make it draw normal ideling
current and turn the plate and load thru its entire ranges. No output
should ever be indicated.
Good luck, some of my first amps did the same thing about 25 years ago.
Ron W4WA
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