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

[Bill Turner]

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On Mon, 27 Jul 2015 07:29:08 -0700, you wrote:

>
>Schematic posted here:
>
>http://ai7rr.webs.com/apps/photos/photo?photoid=199229397
>
>
>
>73, Roger
>AI7RR

REPLY:

I agree with Vic's comments 100%.  

The only way to neutralize with a pi-input circuit that I know of is
with a coupling coil from plate tank to input tank, which I would NOT
do. Too finicky to adjust and highly dependent on the tuning and
loading of both circuits. This could be made to work in a commercial
amp which operates on a fixed frequency with a fixed antenna, but for
ham use I wouldn't. 

I would remove the pi-input circuit and replace it with a simple LC
circuit with a link coupled input. Then do the bridge neutralizing as
shown in any ARRL handbook.

Better yet, why not convert it to grounded-grid input?  It takes more
drive but is far more stable and should not require neutralizing at
all. Nearly all modern amps are grounded grid for good reason. 

You can still use a pi-network input and adjust it for minimum SWR
while running full power. The pi-network should have a Q between 2-5
for good linearity. I have no idea what the input impedance of four of
those tubes in GG is, so some research or experimentation would be
necessary. 

Good luck and keep us posted. 

73, Bill W6WRT