Can any multi-channel amp be bridged?

I have a 6 ch amp (100 wpc) - I want to do only stereo for
this application. Can I bridge it for 300 wpc in stereo, or
will I only be able to use two of the channels & let the other 4 be wasted? I haven't bought speakers yet, so I may
buy only bi-wired, but this does limit my choices, and probably ups the price, and will still only utilize 4 of the 6 channels. Any info would help!
With some you can, with some you can't and with some you shouldn't.

You should consult each manufacturer and/or owner's manual individually. For instance, the 2 channel ATI AT1502 and six channel AT1506 could be bridged into 1 and 3 channels respectively, but the 5 channel AT1505 couldn't be.

Then there is the issue whether you should do so even if you can, because bridging will most likely half the impedance of the amp and that could put extra strain on it.

A few years ago I bridged an NAD 214 and it greatly shortened the life of it. The service tech told me even though this was allowable mechanically, by design it greatly increased the stress on the amp.

I'm sure there are many amps out there that can run in bridged mode easily because they are designed for handling very low impedances.

Hopefully, A'Gon members more enlightened them me will answer your call. Good luck!
Krell TAS can be bridged. Look on their website for how many configurations, but I know it can be bridged from 5x200 to 2x800.
Gunbie is right on. Some yes some no some it's amp suicide.
Maybe slow and painful, or instantaneous with smoke and flames!
You MUST find out from the manufacturer if the particular amp can or cannot. (and you CANNOT assume because one amp of a manufacturer can be, that all can be: fatal mistake!
The reason (simplified) is that the electronics must be symmetric in some special way for it to work.
You would be better off bi-amping then bridging. Just make sure that your speakers don't have a common connection in the crossover network. Just because you remove the "jumpers" that connect the tweeters and woofers on the outside of the box doesn't necessarily mean that they aren't still connected inside the box via the crossover circuitry. Some bi-wiring terminals are there strictly for looks. Sean
One of the main limitations of a power amp is its ability to deliver current. If you only use two channels of a six-channel amp, it will be able to supply a lot more current than if it were driving multiple channels.

Even where it's possible to bridge, bridging compromises current delivery and damping factor and therefore sound quality. Bi-amping or tri-amping is best, with one channel per woofer and one per tweeter/mid.
PS. If your amp manufacturer intended bridged operation, there will be a bridging switch and directions on the amp. It's possible to build an external bridging circuit, but I wouldn't recommend it.
When you bridge a stereo amp, you drive each individual amp with the same signal, but one signal has inverted phase. Note that this makes for a more steady draw from the plus and minus power supplies. Some amps (like the Dynaco ST120 of long ago) only meet their specified power rating, both channels driven, when the signals are out of phase. A bridged power amp is operating in a "balanced" configuration. If that's good for preamps, why not for power amps?

I have not used amps in "bridged" mode, but I did do something very similar for many years. I reversed the wires of one channel at the phono pickup, with corresponding reversal of the speaker wires, and then bridged my stereo amp with a center channel speaker. Since most of the signal even in a stereo record, is common mode (monaural, horizontal groove modulation) what I was doing was very close to what today is called bridging. The results were much better than I had any right to expect. Perhaps it was because phase coherency of the three-speaker set was guaranteed by all being driven by the same amp. No amp ever complained about this setup.

So, my conclusion is that, although all the downside comments seem reasonable, bridging can work well. I am sure that 8 ohm speakers are preferable to 4 ohms, but frankly, I didn't have a problem even with 4 ohms. Go figure.
Eldartford, what you were doing with the center channel is not "bridging". In bridging (also called monoblocking, monobridging or summing) a stereo amp becomes a single-channel (mono) amp with much higher power (usually about 3X the rated power).

The setup you described with the third speaker is called a Hafler surround circuit. In a Hafler circuit, the third speaker is fed a left-minus-right signal. Anything that appears equally in both channels is cancelled, and what is left contains a lot of out-of-phase ambient information.

The Hafler circuit was used in the early surround-sound Quad systems with LP. I used one for quite a few years for movie sound, and it sounds fantastic with certain types of music. The Audio Research surround sound system is actually a Hafler circuit. There's a reviewer at The Absolute Sound who uses a Hafler setup for all his listening. NAD gives directions with their stereo amps for implementing a Hafler circuit, but it can be done with any stereo amp. All you need to do is connect the third speaker across the positive terminals of the stereo amp and connect to the positive and negative terminals of the speakers. Properly implemented, the ambient channel should be run at 10dB below the mains.

I've often thought I should go back to a Hafler system for movies. It has some advantages over Dolby and DTS.
Audiobomber...Also called the Dynaco matrix multichannel setup, and they sold hardware to facilitate its setup. They derived a center front channel by using a Y connection of the front Left and Right with the return through the center. What I did was to make the fronts the back by reversing the phase of one stereo channel. If I played a monaural source, my setup would be exactly the same as "bridging". The center was driven by the differential amp outputs, with the two front speakers driven by the single ended outputs. I preferred my setup to the Dynaco Y-connected center because I don't like the idea of putting speakers in series.
Other advantages were easy control of the center channel volume (all it takes is a "blend" pot), and easy derivation of a low level "rear" signal, Left plus Right, (plus because one signal is inverted) so that the rears can be driven with a separate amp. The only difficulty was getting the phase inversion for other sources, like FM Tuner.
easy derivation of a low level "rear" signal, Left plus Right, (plus because one signal is inverted) so that the rears can be driven with a separate amp

I see you've heard of the Hafler/Dynaco circuit. Great stuff! But I'm quite certain the Hafler movie surround setup is L+R for the front center channel and L-R for the rears. Just like Dolby Pro-Logic, but without the phasey "steering" that craps up DPL sound.

I disagree that your experiment had anything to do with bridging, since there is only one single channel in a bridged stereo amp. It's a totally unrelated concept.
Audiobomber...Of course the required center speaker signal is L+R, but there is more than one way to make this happen. One way is to mix the low level L and R signals, and drive a separate amp. Another way is to make a Y connection of the three front speakers, so that the returns from L and R are tied together and then connected to the High terminal of the center speaker, whose Low goes to the Amp "common". My way was to invert the phase of one channel (eg: Right) and then to bridge the amp with the center speaker. In this case the signal that the center speaker receives is (L-(-R)), which of course is L+R.

Think about it some more and draw a picture (again I wish that the Audiogon site had some way to include a diagram in a post). My setup actually resulted in a lower impedance load on the amp than mono bridging, because in addition to the center speaker across the two amps, each amp was also loaded with a speaker, L or R. But, somehow I never had a problem.

Back in the very early days of matrix multichannel I did a lot of experimentation, and discovered lots of interesting things that never got incorporated in commercially marketed equipment. I even created my own logic-assisted system before these were on the market, using a DBX expander/compresser. Most of the problems with logic-assisted matrix systems can be overcome if the process is digitally implemented.