Why does the light dim at the base sound when I turn the volume up

Hi,

If there is no discernible difference in the audio quality of the receiver as you increase the volume I'm wondering if you need new dial lights.

Does the tuning meter light dim as well?

If the tuning light does not dim it may be that the dial lights are aging and are more susceptible to changes in the voltage supplied to them because of the increased power requirements of the receiver.

Here is an image of the relevant part of the circuit taken from the service manual for your receiver.

Click on image to enlarge for better viewing.

As you can see the dial lights along with the tuning meter light have their power supplied from a separate secondary winding of the input power transformer.