OK, it has a PI input. I suspect there was originally a swamping
resistor from grid to ground and the PI circuit is intended as a
low-pass filter matching device to compensate for the grid-cathode
capacitance. I used a similar circuit in my 4CX1K amp.
The idea is that the grid-ground capacity adds to the output capacitor
of the PI circuit. This allows the input circuit to operate over a wide
frequency range, as long as it is below the cutoff frequency of the
filter. But you need the swamping resistor.
Is there any indication that a large non-inductive resistor was removed?
The neutralizing capacitor should not be where you have it. In that
position it is an "un-neutralizing capacitor" and will increase the
tendency to oscillate! Take it out.
My previous advice was based on my assumption that the input tank
circuit was a parallel-tuned circuit. The PI doesn't lend itself to
bridge neutralization as I suggested. But if you had a 50 ohm swamping
resistor, it would probably be stable without neutralization.
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/
On 27 Jul 2015 17:29, Roger wrote:
Schematic posted here:
http://ai7rr.webs.com/apps/photos/photo?photoid=199229397
73, Roger
AI7RR
On Sun, Jul 26, 2015 at 10:54 PM, Bill Turner <dezrat@outlook.com> wrote:
Roger, it would help if you could draw a schematic and post it on a
website somewhere.
73, Bill W6WRT
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