Mesa Mark IV impedance mismatch?

[jlb32]

Accidentally plugged my 8 ohm cab into my Mark IV's 4 ohm output. Played for a while like that but could tell something sounded off. It sounded weaker and less aggressive. After a while I realized the issue and corrected it. All seems fine but.....

Any damage from this mismatch?

 

[thiswaythatway]

As long as the head is the same or less than the cab you're ok. Only an issue if you plug the amps 8ohm out into a 4 ohm cab.

 

[D-Rock]

Mesa amps are designed to handle impedance mismatches with no adverse affect.

 

[jlb32]

Thanks!

 

[JamesPeters]

As long as the head is the same or less than the cab you're ok. Only an issue if you plug the amps 8ohm out into a 4 ohm cab.

Actually it's kind of backwards from that, if anything (if you value your transformers, sockets, power supply components not getting high voltage flyback damage...as opposed to working your tubes harder).

Please read this and the point below it:

http://aga.rru.com/AGA/FAQs/technical.html#imp-1

(When they say "higher impedances" they mean for instance using a 16 ohm speaker cabinet with the amp's 8 ohm connection.)

That's the reason behind it. It's not as simple as what most people expect.

To the OP:

The sound you heard was normal for giving the amp 2x the impedance it "expected". So there's probably no damage.

As for amps being "designed to handle impedance mismatches"...I'd still avoid impedance mismatches whenever possible. This comes from a lot of experience (as a user, and as someone who's serviced lots of amps with sockets, transformers, etc. which had high voltage flyback spikes damage them). The amp owner always says "it always worked fine this way"...right up until it doesn't.

 

[peckhart]

Yikes, I don't like the sound of that. I have been running my Mark IV I to a 16 ohm cab because it's all I have.

 

[zewango]

Yikes, I don't like the sound of that. I have been running my Mark IV I to a 16 ohm cab because it's all I have.

You can run out of the Mark IV 8 ohm into a 16 ohm cab all day long

 

[JamesPeters]

Yikes, I don't like the sound of that. I have been running my Mark IV I to a 16 ohm cab because it's all I have.

A mismatch of 1:2 isn't usually a big deal unless you're blasting the amp at very high volumes (especially with very high gain and bass), and even so it can be fine. Factor in a dirty contact on a cable (or jack) or speaker connector though and the problem can be compounded. Instead of dealing with the effect of the intermittently "not so great" connection which can temporarily increase the impedance, you're doing it at an impedance mismatch and doubling that effect.

So under "ideal conditions" (good clean connections on cables/jacks/speakers, not playing super loud, etc.) it's not a big deal...but otherwise it may be.

I don't know of an output tube socket which can survive an arc of thousands of volts even if the transformer can survive it. Ever notice output tube sockets with burnt marks on them after a tube failure (usually because of this kind of thing)? Those usually indicate that there's more of that inside the socket itself, making it partially conductive between the pins. That means sockets need to be replaced at the very least. I've even seen high-voltage-spike-eating diodes (a circuit idea to prevent this kind of damage) which have failed under these circumstances...twice.

If people all treated their amps knowing the limits of this sort of thing, a mismatch of 1:2 or 2:1 would probably be fine for everyone. But people push those limits often. Hey, the knobs go to 10 right? That means the amp must be designed to run with all the knobs on 10!

Anyway if your cab is either a 2x12 or 4x12 at 16 ohms, it can be rewired to 4 ohms. If you don't plan to get another cab, I'd rewire your existing one (assuming it's not a 1x12 lol) to 4 ohms and match the impedance to the 4 ohm output of the amp. At the very least you'll hear how the amp was meant to be heard under normal circumstances. I'm guessing you might prefer the sound that way.

You can run out of the Mark IV 8 ohm into a 16 ohm cab all day long

Until it catches fire.

 

[skoora]

I ran 60's Bassman heads into 16 Ohm Marshall cabs for a few years and never had an issue. I bought and installed a Marshall style, stereo/mono jack plate for my single input cab so that I could choose 4 Ohm mono for my Mesa heads. Turned out great.

 

[zewango]

Well catching fire makes it a better show.

Mesa has it in their manuals, Ive never caught fire
I ran my Mark IV that way.

 

[jlb32]

Yeah, I did not like the tone of the mismatch (8 ohm cab into 4 ohm output). I could tell something was off tonally but didn't realize what it was for a while, probably 30-45 minutes of playing under moderate volumes. I think the master and channel volumes were on around 3.

Once I did correct the mismatch problem the amp sounded great and back to normal tonally. Just hoping it didn't do any damage since I love my Mark IV.

 

[D-Rock]

How many Mesa amps have you shitting the bed solely as a result of running an impedance mismatch? I gonna say zero. That's because Mesa has a failsafe in their design to prevent damage.
Sound quality may/can be affected from a mismatch, but a Mesa amp will not be damaged by it. This does not apply to any other amp manufactures though and is unique to Mesa.

 

[JamesPeters]

Well catching fire makes it a better show.

Mesa has it in their manuals, Ive never caught fire
I ran my Mark IV that way.

They also talk about biasing a bit...unusually...in their manuals.

To be fair, if the amps are biased on the cold side that's part of why it can handle a mismatch like this.

Yeah, I did not like the tone of the mismatch (8 ohm cab into 4 ohm output). I could tell something was off tonally but didn't realize what it was for a while, probably 30-45 minutes of playing under moderate volumes. I think the master and channel volumes were on around 3.

Once I did correct the mismatch problem the amp sounded great and back to normal tonally. Just hoping it didn't do any damage since I love my Mark IV.

No, it's probably fine.

How many Mesa amps have you shitting the bed solely as a result of running an impedance mismatch? I gonna say zero. That's because Mesa has a failsafe in their design to prevent damage.
Sound quality may/can be affected from a mismatch, but a Mesa amp will not be damaged by it. This does not apply to any other amp manufactures though and is unique to Mesa.

I haven't seen a failsafe in the amps or on the schematics. There are things in the design that can contribute to making it less likely to fail under those circumstances, but even so it's not perfect.

And no that's not unique to Mesa. I could say the same thing about a number of other amp designs.

Do you want to bet that I've never seen sockets with high voltage arc damage or a blown output transformer in a Mesa? Look either way it's "he said she said", but you can either take some advice (which is easy to follow) or ignore it. If you think I'm a bad source, fine, I can accept that.

I ran 60's Bassman heads into 16 Ohm Marshall cabs for a few years and never had an issue.

You see, this is what everyone says about this when they bring me an amp which needs a new output transformer, and/or sockets, when the output has a flyback voltage spike. It always works fine, sometimes for 20 years on multiple amps...until the day it doesn't.

It's not as though I'm saying it *will* happen, and not that you can say it *would* be more likely with some amps compared to others to the point you can count on it. But I'm being serious about this, how everyone's all "but it was working fine..." and then I show them the test of the output transformer as they look on in wonder.

So what I am saying: if you're careful, maybe it'll be fine. It's not something I think is worth the risk though when it's easy to match impedance by rewiring a cabinet or swapping speakers with someone else if necessary.

 

[dfrattaroli]

I used to play my Mark IV out of it's 8ohm tap to a 16ohm Bogner cab. There's no issue what so ever. Very clearly stated on the Mesa site.

 

[D-Rock]

Yup, accidental, incidental or intentional impedance mismatch will NOT harm a Mesa.
OP, you're Mark IV will be just fine.