[Amps] Follow-up...re: Time to ask for help.

[Vic Rosenthal 4X6GP/K2VCO]

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 at outlook.com> wrote:
>
>> Roger, it would help if you could draw a schematic and post it on a
>> website somewhere.
>>
>> 73, Bill W6WRT