Amp load question

[MikeO]

I have a Marshall 9100 poweramp that has outputs switchable between 8 and 16 ohms. I just picked up a Mesa Recto 4x12 that has an 8 ohm mono input and two 4 ohm inputs. In order to run both sides of the poweramp i need to plug into the two 4 ohm inputs of the Mesa cab, but the Marshall doesn't include a setting for 4 ohms, just 8 and 16. If I run the amp set to 8 ohms per side and plug into both 4 inputs of the cab, can the amp handle it? I know that most tube amps can handle a mismatch in that direction, but I also remember reading that some of the Marshalls from this era (early 90's) didn't handle mismatched loads well and occasionally blew OT's as a result. Could be total BS, I really don't know.

Any thoughts? The Mesa cab is an upgrade over the cab I"m running that rig through at the moment (PV Triple X cab with WGS Vet 30 and BL80 speakers), but I don't want to do anything I'm going to regret.

[Guitarbilly]

An "1 off" mismatch in either direction should be safe for most amps, but it does put a bit of strain in the OT. Some amps are very robust and can handle it just fine. Old Fenders for example, take a small mismatch like it's nothing.

The 9100? A bit risky, IMHO. What I would do is run channel A set to 8ohms into the Mesa 8ohm mono input and run channel B into your other cab also a a correct match (whatever impedance that cab is). That would be a safer way to do it.

[MikeO]

Shit. I was really hoping to stick to one cab. Running out of space, so I was thinking one or the other.

[Guitarbilly]

Stack 'em up

But also, you should be able to run just one side of that amp. Keep the unused side with nothing plugged into it (DON'T use the "Link" input), on standby and with the volume set to zero. That should not be a problem unless you need stereo for effects etc.. but there is no risk.
Or consider getting an 8ohm 1x12 for the other side.

[Chris Z]

https://mesaboogie.zendesk.com/hc/en-us ... r-cabinets

[zozobra]

The cab probably has 8r speakers, so you could replace the backplate and rewire two speakers in each half in series for 16r each side.

[trey85stang2]

Yea rewire the cab I wouldn’t go lower in an ohm mismatch. Higher is generally better than lower but no mismatch at all is safest.

[zozobra]

High you'll fly, low you'll blow only really applies to solid state amps as driving too low a load can destabilise the power amp and it'll oscillate itself to death and burn up.

With tube amps it's more of what does the load do to the reflected primary impedance of the power amp? The OT doesn't have a primary impedance of itself, just a turns ratio, so it depends on the load!

All being well designed, the primary impedance of the OT for the right load and voltages should pass the loadline just below the 0V knee for the selected tubes and it shouldn't horribly intrude too much beyond the max plate dissipation. In an AB push pull power amp the loadline can exceed the max plate dissipation by a fair amount as a tube only conducts significantly for half of each cycle. So, with this in mind, if the load is taken one notch down, the reflected primary impedance is halved, causing the loadline to pass well above the plate curve knee, and right through the max dissipation curve, potentially leading to red plating. Going the other way and notching high will double the reflected primary impedance, which can cause flyback voltages well in excess of the supply voltage during the non-conducting half of the duty cycle for a tube, which can start to nibble through the insulation of the windings of the output transformer primary. Cruelly, as a bit of winding gets weakened by this, the insulation becomes less effective which accelerates the windings shorting and killing the OT.

Now with all this said, it'll depend on the amp. Mesa and fender are conservative with their voltages and OT primaries, so they are much more tolerant to mismatch. Marshalls push their luck with both the voltage and primary impedance, resulting in very silly flyback voltages when notching high.