I get what's required for output bridging. That's the easy part because you have 4 independent transformers and series connections pose no problems, but parallel connections require the voltages to be the same.
So, something has to be done on the input side, so you have a single level control and a single input I would think.
Typical bridging for solid state amps requires one to invert one input and connect the speaker to both positives. Not with this amp.
It's 100V line to voice coil bridging.
This tube amp has a bridge switch:
https://www.google.com/url?sa=t&rct...TsIZpLGhzRT9y3Q&bvm=bv.74115972,d.eXY&cad=rja
So, the real key is for parallel operation, the output voltages have to be nearly identical.
Tube amps like to be loaded a bit or they do nasty things.
The transformer connections are easy. The input connections bother me. You can see the advantage of a single level control and a single input.
A lot of the stuff I found by searching yields some not so good information.
benb here, **broken link removed** has the right idea, but the discussion is tube vs solid state. benb says the amp would be less bothered, "i.e. lower current, and he's probably right. The real question is bridging transformer coupled amps.
If you can endure the outputs levels are close, then bridging is easy. It;s just paralleling the outputs paying attention to phase. Fuses in each (2) of the 100 V outputs would offer some protection.
Using an AC voltmeter and a signal generator and connecting the outputs in series, you could adjust the input for the least output and then reconnect properly.