>>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?
>
The answer is a semi-yes. Eimac/Varian writes about neutralizing a
push-pull GG amplifier in
*Care and feeding of POWER GRID TUBES*, p.83 & 84
" 5.4 NEUTRALIZING GROUNDED-GRID AMPLIFIERS
- Grounded-grid triode amplifiers offer an attractive alternative
to the more usual grid-driven amplifier. The control grid is
operated at r-f ground and serves as a shield to capacitive
currents from the output to the input circuit. Generally,
neutralization is not required until the control grid lead
inductive reactance becomes significant. The feedback from
the output to the input circuit is no longer due only to the
plate-to-filament capacitance. The physical size of the tube
and the operating frequency will determine when neutraliza-
tion is required."
"Two methods of neutralization are commonly used with
grounded grid amplifiers. In the first method, the grids of
a push-pull amplifier are connected to a point having zero
impedance to ground, and a bridge of neutralizing capac-
itances is used which is equal to the plate-filament capac-
itances of the tubes."
"The second method of neutralization requires an inductance
between the grid and ground or between the grids of a push-
pull amplifier of a value that will compensate for the coupling
between input and output circuits resulting from the internal
capacitances of the tubes." ...snip...
OK, but are push-pull grounded-grid amplifiers commonly used? I have
never seen one or seen an article about building one. ... Eimac says
nothing about neutralizing a single-ended grounded-grid amplifier.
thanks, Paul
Rich---
R. L. Measures, 805-386-3734, AG6K
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