How do you fix this?

[nrb's70gmc]

Anyone with an old car knows about this.
when you rev the moter up the headlights get brighter and so on.

in my case when i have the radio turned up (with amp) heater on with head lights also on.

at a stop light (with heater and lights on) i can see the lights on the radio dim (when the bass hits) and when the volume is really up the radio will turn off.

during the day with no headlights and no heater i can blast the radio no problems.

its sort of funny when im at a stop light the blower motor for the heater slows down and speeds up when i take off.

any way what is the reason this happens in older cars? and what is the difference in new cars?

i have a 140 (internal reg) powermaster alt and the amp for my radio is connected directly to the battery.

i think the specs for the alt are IDLE-80 CRUZING-100 HIGHWAY-140