If you have a vintage Marshall or Traynor or an amp with a similar circuit and you bridge the channels using a patch cord, you can do this mod to internally bridge them.
The amp must use the same tube for both channels, that is, one half of the 12AX7 for each channel.
Simply solder a wire between pin 2 and pin 7 of the tube, then mix the two channels as you like.
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.