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After trying a few schemes I have settled on a MAX9814 circuit for
the AGC.

I used the Adafruit board but there are a few variations on eBay with
some probably requiring less hacking than the Adafruit design. I had
to solder a wire on a SMD component to access the CT (time constant
capacitor) pin of the MAX IC, and remove the Electret capsule.

I have attached the schematic and a few pictures of the AGC circuit
as I now have in the rig. The 5VDC is taken from the Raduino board.??

Performance wise I took two sets of measures, one with the AGC turned
down low and one with the AGC turned one third of the way up.

A few notes on the tests. I used an A/B comparison with an FT-817,
with the pre-amp off, receiving??a carrier at 1,500Hz? from local
radio stations (with attenuation). The FT-817 S-Meter seem
notoriously non-constant in the steps between the S-units, but that
is all the reference I have where I am. The AGC voltage is taken from
the CT pin of the MAX9814.

The AGC voltage fluctuates quite a lot, so I tried to record the
average value.

At high signal strength, to determine if the saturation was coming
from the AGC circuit or the uBitx upstream, I would bypass the AGC
and keep the volume down to prevent the audio circuit past the volume
pot from saturating. If the harmonics of the audio disappeared, the
AGC alone was producing distortion, otherwise it appeared at least
before the AGC, and possibly in as well.

Here are the results:

1. "Medium" AGC: input pot turned to about 30% of full scale.

FT817? ? ? ? ?AGC
S-Meter? ? ?voltage(mV)? ? ? ?Notes
S0? ? ? ? ? ? ? ? ? 0? ? ? ? ? ? ? ?
S1? ? ? ? ? ? ? ?300
S2? ? ? ? ? ? ? ?350
S3? ? ? ? ? ? ? ?400
S4? ? ? ? ? ? ? ?460
S5? ? ? ? ? ? ? ?510
S6? ? ? ? ? ? ? ?650
S7? ? ? ? ? ? ? ?750
S8? ? ? ? ? ? ?1,700? ? ? ? ?Large variation. FT-817 S-meter S8
plateau issue? S9? ? ? ? ? ? 2,200? ? ? ? ? Some saturation of AGC
noted (starts to appear in audio FFT, not noticeable) S9+10
?2,460? ? ? ? ? ?Saturation of AGC audible, but not unplesant. S9+20
? ?2,460? ? ? ? ? ?Audible saturation of both uBitx and AGC (harsh
sound).

The AGC kicks in early and keeps the volume pretty constant until
saturation occurs. Saturation of AGC does limits the dynamic range of
receiver.

1. "Low" AGC: input pot turned to about 7-10% of full scale.

FT817? ? ? ? ? AGC
S-Meter? ? ? voltage (mV)? ? ? ?Notes
S0 -S4? ? ? ? ? ? ? ?0? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??
S5? ? ? ? ? ? ? ? 50-200? ? ? ? ? ? (100mv avg)
S6? ? ? ? ? ? ? ? ? 200
S7? ? ? ? ? ? ? ? ? 360
S8? ? ? ? ? ? ? ? ? 500
S9? ? ? ? ? ? ? ? 1,260
S9+10? ? ? ? ?1,800
S9+20? ? ? ? ?2,300? ? ? ? ? ? ? ? ?Saturation of both uBitx and AGC
(visible in audio FFT, but not really audible) S9+30? ? ? ? ?2,400
? ? ? ? ? ? ?Audible saturation of uBitx and mostly of AGC.

This is the most "FT-817 AGC" like, from my perspective, and is what
I have settled for.? I want to use the AGC voltage as an s-meter
input and this setting does produce a gap at the bottom end, but this
is not critical IMO.

In both cases I noted some small "clicks" when the AGC kicked in on
strong sudden signals.?

The maximum gain of the MAX9814 as set in the schematic attached is
of 50dB and requires screened cables in the audio circuit. I
originally had the input and output of the circuit fed to a two core
"stereo" screened cable and I would get feedback. I had to use two
single screened audio cables.

73, John (VK2ETA)