Ok so, I was at a rehersal a couple of weeks ago and thought I'd ask this here.

Guitarist in my band was convinced that with tube amps it's ok to mismatch as long as the amp is lower and the only thing it'll do is make the volume lower?

I thought it could cause some damage, who's right here?
Bass Gear:

Mensinger: Speesy
Fender Precision 1989 (CIJ Rosewood)
Fender Steve Harris (CIJ)
Lakland J Sonic 5
Epiphone Explorer
Maruszczyk (custom) Jake

Ashdown CTM 100
You are. Mismatching impedance is a crap shoot. May be fine or may not. I think most people will say double or half is good. But I would match if possible.

The speaker higher than the amp is a truism only for SS amps. Having no speaker attached fits his requirement of amp impedance lower and is well known to be an issue. Having the impedance of the speaker higher causes voltage spikes on the primary side of the xformer leading to internal arcing and malfunction.
Last edited by fly135 at Jun 4, 2014,
If you don't play at gig volumes it doesn't matter, but "technically speaking", you could overload and destroy components if you crank the volume.
Peavey 6505+ Head (Left)
Line 6 Spider IV HD150 Head (Right)
Marshall 1960A 4x12
Ibanez RG T42DX
Agile Hornet Pro 727
Depends on the amp. A robust output transformer can handle a 2:1 mismatch (i.e. 8 ohm head into 16 ohm cab) just fine and some can do a 1:2 mismatch (8 ohm head into 4 ohm cab), but some transformers like the Drakes that Marshall uses are notoriously picky about impedance and may blow if you mismatch. Using a lower volume will lower risk and the mismatch will make the amp slightly quieter.

So you're both right depending on the circumstance. A lot of amps are perfectly safe to mismatch, but if you're not sure about your OT and you have a choice, there's not usually any good reason to mismatch.
The simplest explanation I can give you is how one tech explained it to me.

Think of your cabinet like a big yellow bus. Think of it's passenger capacity in ohms (16 people = 16 ohms, 8 ohms = 8 people)

If your head is set at 8 ohms (that's 8 people lol), you can fit it in either sized bus. If your head output is 16 ohms there isn't enough room in the 8 ohm bus. Just asking for damage.

Of course there is a lot of technical jargon not explained here. But for the most part this is the rule in very childish terms you can remember. There are always some exceptions.
Last edited by cheesefries at Jun 4, 2014,
Pretty much what Colin said. Some mismatches are safer than others (I can never remember which so I don't bother ), and better quality transformers can handle them better. But if you don't know for sure, the safest thing is matching.
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