If you bridge the channels in your vintage Marshall or Traynor amp with a patch cord, you can do this mod to bridge them internally.
The amp must use the same tube for both channels, that is, one half of a 12AX7 for each channel.
Simply solder a wire between pin 2 and pin 7 of the tube, then use the volume controls to mix the two channels as you like.
Follow the wire from the input jacks to find the correct 12AX7. It is usually the first one on the end. The tube socket has numbers on it to identify which pins are what.
I've provided schematics for a Marshall Plexi and Traynor YBA1 that show where to make the connections.
You cannot do this on most Fender amps due to the fact that the channels are out of phase. Not sure which ones, but I know the Blackface reverb models and later ones are like this.