2 Tube Guitar Amps into 1 speaker ?

[Chuck H]

From what I can remember Peavey Classic 120/120 in mono mode is running both channels in parallel at half the impedance so maybe yes, you can run two amps in parallel but under certain circumstances.

I don't have that schematic, but it's likely that since both OT's are in the same chassis Peavey opted to parallel the OT's at their primary windings (good) rather than at their secondary windings (not good).

[GainFreak]

Actually they paralleled the secondaries but they are running the power tubes from the same preamp/PI. Below is the schematic.

Classic 120 120 schematic.pdf

[J M Fahey]

Yes, power tube grids are in parallel.

So NOW it is absolutely guaranteed that current through tubes is in perfect phase from DC to a couple MHz ... try that with separate amplifiers fed a guitar signal at the preamp input .

[christarak]

Waste of time speaking to stones, let the amps eventually explode, that will *technically* explain him why.

Oh Juan! You made me laugh!! "speaking to stones"...love it

[drewl]

Bogen tube power amps were designed to be paralleled together for more power.

They call it Cathode paralleling. But they are identical amps being run together.

[loudthud]

I just Googled it. Turns out there is a Metal band called "Speaking to Stones" and they have a CD available on Amazon

[Jazz P Bass]

Bogen tube power amps were designed to be paralleled together for more power.

They call it Cathode paralleling. But they are identical amps being run together.

I like that way the they 'jumper out' the 10 ohm bias read resistors.

[g1]

The 2 examples given (PV Classic 120 and Bogen MO100) show the great lengths they go to in order to ensure that both matched power amps and OT's get the exact same input signal from a single preamp.
Like JM explained,

it is absolutely guaranteed that current through tubes is in perfect phase from DC to a couple MHz ... try that with separate amplifiers fed a guitar signal at the preamp input .

This is pretty much the opposite of what was presented in the first post, where the owner wants to blend 2 different signals from different preamps and poweramps.
It is the blending of dissimilar signals that is the problem, which causes the amps to work against each other, and will likely result in failure of one or both amps.