Amp head and cab ohms mismatch

Lestratcaster

I'm sure this topic has been discussed before, but here goes anyway.

So yesterday I lent my 6505+ head to a mate who's Laney amp is broken. He uses a Line 6 Pod Pro and was using my head to power the cab.

The cab was a 2x12 8 ohm. For some reason I forgot to change the switch on the back of the amp when helping setting up from 16 ohms so it was running at a higher load than the cab. 

I'm not overly clued on the science behind impedance but I've always followed the rule they should either match or the cab ohms should be higher than the amp.

Have I risked damage to my amp head by not setting it to 8 ohms? The set was fine and nothing blew up lol.

ICBM

No, it will be fine.

First, with a valve amp the impedance matching is only really important if the amp is cranked into power-stage overdrive. At any less than that, it really doesn't matter that much. Unless he was playing at quite stupid volume, and had no available clean sound on the Pod at all, it's very unlikely that it was into power-stage overdrive. There will be a small loss of power and change of tone, but no more than that.

Second, with a valve amp it's actually better to mismatch into too *low* an impedance than too high - this is contrary to popular wisdom, I know. In fact, a valve amp will usually tolerate a complete short with no damage if it isn't being really driven hard, with no damage other than possibly stressing the valves a bit. An open circuit is far more dangerous, which is why some amps (eg old Fenders) have a shorting switch in the main speaker jack.

Third, any damage is almost always obvious, usually a severe loss of power or a farty, crackling type of distortion depending on whether the transformer winding has been completely shorted or if it's just arcing, or blown valve(s) and fuse. If it's not doing anything like that it's very unlikely any harm has been done.

(Solid-state amps are the other way round, and it's entirely true that they shouldn't be run below the minimum impedance - which is probably where the idea that it's bad for valve amps comes from.)

Lestratcaster

He may have switched it when I went to help the other guitarist on the other side of the stage, but I don't think he knew it was 8 ohms.

There wasn't any noise from his rig although I did notice the tone was slightly different maybe due to the "mis-match" of ohms. He has an amp model setting for this main rhythm sound and uses the 6505+ red channel into the cab. Doesn't really use cleans.