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julius_deane wrote:Again, I know nothing. I was almost certain 4 ohm amp to 8 ohm cab was ok, but that 8 ohm amp to 4 cabinet was not.

I've heard it argued back and forth about ohm mismatches.

My Understanding:
Running a 4ohm amp to 8ohm cab could (theoretically) result in the OT seeing "flashback" voltages, as the resistance at the cab is too high, rejecting some of the current. The opposite (8ohm amp > 4ohm cab) could put some undue stress on both OT and tubes, as load is too low and amp is working overtime to supply voltage/current.

As stated, a 2:1 mismatch either way is generally safe with a quality amp, however, not recommended. I've yet to see anyone destroy their amp by mismatching, I'm sure it's happened though. Also, mismatches tend to produce shittier tone, don't really reflect how a matched amp and cab would/should sound.

univalve wrote:

julius_deane wrote:Again, I know nothing. I was almost certain 4 ohm amp to 8 ohm cab was ok, but that 8 ohm amp to 4 cabinet was not.

this. amp lower (or same) Ohm, box higher (or same) Ohm is always safe. mismatching with a lower Ohm cab has the effect, that the amp has not enough load. that stresses the tranny. with a decent amp even this is not critical - but i would never advise to do it...

It's a sad state that cheap PA heads can handle any impedance (so that you can chain up a bunch of speakers and monitors, and it can changing while operating) but we don't have that feature in our guitar amps.