Cleeds, the vintage Dynaco ST-70 is another amplifier that I recall has provisions for operation with the two channels paralleled. I’d imagine there are others, among tube amps that have output transformers.
It is of course physically possible to parallel the channels of any tube amp having an output transformer, by jumpering the appropriate output taps together and applying the same input to both channels via a y-adapter. I would not want to do that without a specific indication from the manufacturer that it is ok, however. One reason being that if a tube or something else were to cause a failure in one channel while the amp is operating the other channel would be attempting to put out a full power signal while that channel would be attempting to force their paralleled outputs to zero volts. Which of course may be unhealthy unless that possibility is specifically addressed in the design. And for that reason among others paralleling the channels of a solid state amp would be out of the question in nearly all cases, since the near zero output impedance of nearly all solid state amps would not provide any limiting of the resulting current flow. On the other hand, though, solid state amps can of course be designed such that they can be bridged.
As Tomcy6 noted earlier in the thread, paralleling the channels results in the load impedance that is nominally optimal for each output tap being cut in half, relative to stereo operation. The MC275 and ST-70 provide 16 ohm taps, so using those taps in parallel would presumably result in an optimal match for an 8 ohm speaker. But if an amp only provides 4 and 8 ohm taps, parallel operation would in effect only provide 2 and 4 ohm taps.
As can be seen in the specs for the MC275 parallel operation will double an amp’s rated power capability. While if a bridged amp is designed with sufficient robustness, in terms of its current capability and thermal management, it can potentially/theoretically result in the rated power capability increasing by as much as a factor of 4, since the output voltage provided across a load by a bridged amp would be doubled (relative to stereo operation) if the correspondingly increased current can be provided and sustained. (For a given load resistance power is proportional to the square of voltage, as you no doubt realize). As a practical matter, though, the increase in the power capability of an amp operated in bridged mode often falls significantly short of that factor of 4.
With respect to the sound quality of an amp operated with the channels paralleled, I’d imagine that a significant factor would be how closely the characteristics of the two channels match. Although I have no particular feel for how critical differences between channels that are typically encountered would tend to be.
Best regards,
-- Al
