How can you change grid voltage in a vaccuum valve?
Input stages of tube radios and also of microphone amps handle so weak AC voltages (less than one millivolt) that altering the operating point of the amplifying tube can create large enough gain variation range without introducing too much distortion to the amplified AC signal.



In the left we have a generic triode amplifier. I have drawn a battery  "Gain control voltage" or shortly GCV. It actually gives the negative Vg. In radios that voltage comes from the signal detector. The bigger Vg is to negative direction, the smaller is the gain. That forces different AM radio stations to have the same output volume.

The gain reduction is based on the Ia vs Vg curve set at the right. You should see that Vg affects more steeply to the anode current Ia when Vg grows towards zero.

R1 is typically 1 MegOhm so the input AC isn't lost to GCV. C1 blocks DC but allows the AC signal to swing Vg around the GCV.

This is too simple for large signals, it can be used only at about 1mV or less input AC voltages.