psudo Center channel from stereo

I remember way back something about making a "psudo" center channel from stereo by connecting the center speaker to the L+ and R- on the amp. I am not sure if that is correct and don't want to damage my old Nakamichi amp. Object here is to use the center speaker on "B" and only when watching movies. Don't have a surround system.
Thanks for info from those who remember these things.
There was the "Hafler Hookup" from a long time ago that provided a pseudo quadraphonic sound with two extra speakers and a jumper setup between the main speakers. Could that be what you're thinking of? You can Google or Wiki it.
They had resistors in the one I had. It was to help to keep your amp from having to low a resistance on it. []
The Hafler circuit gives you a difference signal (intended for pseudo-surround) and you want a summed signal for a center.

I tried several different ones over the years for fun. I did have a Dynaco ( Panor owned at time I believe ) that did have a five channel, that included the center speaker. I can't remember exactly what the center channel sounded like, but I don't think it was the matrix sound, like the typical rear four four channel one. These(matrix 4 channel only) can have a lot of ambient, reverb type of sound, depending on the recording. This link I posted above may be it. If you could find the diagram (with resistors to relieve the load ) or one of these, it may fit the bill. []
This guy has some kind of a diagram that shorts the two negatives from the amps outputs. This is risky. I remember measuring power supply voltage between the two minus posts on some amps in the '70s and '80s. I think a friend blew an amp doing this. It might be cheaper just to buy an old Pro-Logic (receiver/processor), as these can be found cheap nowadays. In this guys post, he says to contact the manufacturer before trying his circuit. []
This is is one way:

Another way is to build a simple mixer to combine the L/R signals without shorting them.