I have the variant, the AGC-1D, on the bench and just finished up the electrical work.
Someone had moved the threshold control to the front panel, and added a release pot. I hate it when people drill new holes in antique audio, cause they think they need some new control, like they are smarter than the original designers.
If you have a look at the schematic, you will see the limiter and the AGC only differ in side chain timing.
One cool thing is the date of manufacture, and the radio station it was made for, are on the model # tag. This one was made in early '69.
HA-100X input, Chicago BO-2 output.
Was surprised to find the B+ is very high. 395VDC after the rectifier, 378VDC on the 12BH7 plates; beyond the ratings but no metldown. 310-0-310 coming from the power transformer.
The original AGC timing had been altered, and was a single 0.68 cap with 4M7 in parallel with a 5M pot with a slug resistor, so release R ranged 250K to 2M5. This timing didn't really work correctly. It had a tendency to let too much by and distort, it thumped a lot, and there seemed to be oscillations at certain settings.
I took this timing network out, and put a DP6T rotary in using the 5 fastest limiter timings, and the original AGC timing in position 6. I put the AGC timing parts back in the original position, and all the limiter timing parts on the switch. This brought the unit around to proper operation. It acts a lot like a sta-level, but with a lot more options in the timing department. The AGC is different from the limiter settings, and I felt almost no one today would use the slower limiter settings on their own. The AGC will cover that territory when needed, and better.
I then noted the threshold pot caused the meter to shift downward when set to 0 volts. Adding 3k9 to the bottom of the threshold pot kept this meter consistent across all settings. As with almost all limiters like this, threshold is also ratio. A 0 volt bias gives lowest ratio and lowest threshold. Highest bias here is 72 volts actual. I have not seen official spec for these amps.
I still need to relabel the faceplate.
EDIT:
done here
guts before
guts after