As far as my knowledge on radios goes, being a former communications officer, the AGC is usen in the receiver circuit to allow a constant volume of signal to be heard instead of louder and softer signals, it has nothing to do with tuning.
Tuning is performed to match the amplifier output impedance to the antenna impedance. Basically, you're compensating for the length of the antenna by tuning the amplifier via a set of coils to match both impedances which allows for the best tranmitter efficiency. Most (western) tranceivers do this automatically the first time you key-in on the new frequency, the russian counterparts require you to press and hold the tuning button until the system, via its light, tells you tuning had been succesfully completed.