>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 type of suppressor used makes virtually no difference in the
anode-resonance frequency.
>>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.
... amen to that.
>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.
Since the grid and screen are reasonably well grounded, it sounds to me
like the feedback path is the normal anode to grid feedback C within the
4-1000A(s).
> Some general rules for neutralizing a GG
>amp is to keep all leads short as possible in both in and output
>circuits.
However, the amplifier is apparently oscillating at the anode resonance
of 62MHz -- not at the operating frequency. Neutralization works at the
operating frequency - not at the anode-resonance.. .. . Has anyone ever
seen an article in an ARRL publication, or in an Eimac publication, about
neutralizing a GG amplifier?
My guess is that the next logical step is to use a dipmeter - set to the
wave-meter position - and try to determine the actual frequency of
oscillation. However, it's best to start well clear of the anode circuit
to avoid damaging the dipmeter.
>....snip...
Rich---
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|