How to Add AGC to 455kHz IF Amplifier for Variable Gain Control via Voltage

Can somebody help me with adding an AGC feature to my 455kHZ IF amplifier ? I need help just how to change the schematic so the amplification varies with a given voltage. 

As drawn, Q7's emitter current is approximately Q7's hFE (DC current gain) times the current in R6, so is badly controlled. If hFE is high enough, Q7 is saturated and you have zero gain.
Insert a 270-ohm resistor (paralleled with a 100-nF capacitor) between the emitter of Q7 and ground. Q7's base voltage now controls Q7's emitter current and hence Q7's transconductance Gm. Do you see why this is true?
Move the top end of R6 from 12Vdc to a variable voltage source Vagc. With Vagc=12V, you have full gain. As you reduce Vagc, Q7's Gm drops, reducing the gain. Do you see why?

I don't really get it why the 270-ohm resistor and the 100nF capacitor are needed to be onest. I mean that Q7's Gm drops if i put my Vagc to R6 with or without the resistor+capacitor.

The 270-ohm emitter resistor's voltage drop adds to the Vbe of Q7, to give the Vbgnd (voltage from base to ground) of Q7. The 270-ohm resistor stabilizes the DC operating point of Q7 against changes in temperature and against changes due to replacing Q7 with another transistor. Try changing the temperature of Q7 in your simulation, and note what happens.

I see it now! Thank you very much mister!

PeterTraneus Anderson, thanks for that.