Another embarassing admission...

[EtherealWidow]

Well I guess I could follow in some others' footsteps this week and say that I've overlooked something on my amp. So I read the manual wrong. A safe impedance mismatch is NOT a 4ohm head going to an 8ohm cab. The OTHER way around, however, is safe. This may seem obvious, but this is only my 2nd tube amp, so be gentle. I've had it this way for about a month and a half and have cranked it multiple times. Everything SEEMS fine, however now I'm nice and paranoid about tube sockets and the tranny.

 

[Spewnz]

in the manulas it states theat mesa amps are not very sensitive to ohmage mis-matches so fix it and if you dont notice something terrible out of the ordinary then your probably fine

 

[EtherealWidow]

Yeah you know what, I'm thoroughly confused about this. I hear that with Mesa's, it's typically fine because they run really cold. Thank God. But in the manual it says that running an 8 ohm output into a 16 ohm cab is fine. The amp will be a tad darker and the tranny will run a bit cooler. I've been running my head from the 4 ohm jack into the 8 ohm cab. I called Weber this morning and the guy told me that this was really dangerous to do. And now that would sort of make sense. If you're driving 2 x 8 ohm cabs then you would need a stronger signal that was less resistant, so naturally you'd run it from the 4 ohm jacks. And it would be safe mismatch to run the 8 ohm jack into a 4 ohm speaker, right? Am I on the right track now?

 

[elvis]

Most amps are not that sensitive to impedance. The sound may change by doubling or halving the impedance, but it doesn't change the way the amp operates all that much. The main dangers are shorting the output or no load. The former can cause high peak current in the coils, and the latter can cause large flyback voltages in the coil. Both run the risk of high power dissipation, and no load also runs the risk of breaking down the coil insulation if the voltage gets high enough.

If the output tranny is underdesigned, you could run into power dissipation problems with changing load, but I think that's unlikely. Depends on the amp design. Most players have run double or half load on their amps at one time or another, some for years.

 

[EtherealWidow]

Yeah I read an article and it said that halving the impedance would wear the tubes faster, and doubling could potentially cause flyback currents. It's nuts how complex all this is.

 

[EtherealWidow]

Anyhow, I think I'll just run it the way it was meant to be run. 8 ohm head to 8 ohm cab. What a novel idea.

 

[Don]

The manual states that you can do it either way.

 

[MrMarkIII]

Yes, Mesas are tolerant of mismatches, but DO NOT go down to 2 ohms - you're asking for expensive trouble. An example is running two four ohm cabs out of the two four ohm speaker jacks. That's two ohms, not smart.
Going from low to high (amp to cab) is safer. Running an amp's eight ohm output into a four ohm cab is high to low, again, not as safe, but you can get away with it.

 

[elvis]

I don't thnk running 2 Ohms from the 4 Ohm output is any worse than 4 Ohms from the 8 Ohm output. Neither of these is necessary, though, as you can run 4 Ohms from the 4 Ohm output, and the two 4 Ohm cabs can be run in series instead of parallel, as 8 Ohms from the 8 Ohm output. So it's a poor choice to mismatch when you don't need to, but I don't think it's going to break anything. I would stop there though, as running less than 2 Ohms from the 4 Ohm output is risky for sure.

 

[MrMason]

Think of the output impedance on the back of the amp as what the amp expects as a load. So if you plug out of the 4 ohm output then your amp expects 4 ohms, and so it can handle anything over 4 ohms. If you plug a 4 ohm speaker into an 8 ohm out, then the amp is expecting 8 ohms but only getting 4 ohm, so it would be expecting less resistance than it's getting and potentiality overworking itself. Either way, I think it's best to always use the proper impedance.

 

[EtherealWidow]

Think of the output impedance on the back of the amp as what the amp expects as a load. So if you plug out of the 4 ohm output then your amp expects 4 ohms, and so it can handle anything over 4 ohms. If you plug a 4 ohm speaker into an 8 ohm out, then the amp is expecting 8 ohms but only getting 4 ohm, so it would be expecting less resistance than it's getting and potentiality overworking itself. Either way, I think it's best to always use the proper impedance.

That's exactly how I had to start thinking about it.

 

[soundchaser59]

I was always told your speaker load should never be less than the amp output jack. Sure it can be done, some amps will handle it better than others. But a 4 ohm speaker plugged into an 8 ohm jack will cause that part of the output transformer to try to deliver more current than it is designed to deliver, which means it runs hotter than it would if you plugged it into the 4 ohm jack. A 16 ohm speaker will draw less current than the 8 ohm tap is designed to deliver, so it will run cooler and therefore is safer......but it does noticeably alter the tone.

You can think of it like water pipes if you want......a 4 ohm speaker is a larger diameter water pipe (less resistance) trying to draw too much water from an 8 ohm tap narrower pipe (more resistance). Eventually the 8 ohm tap will try to break up or over heat from delivering more water than it is designed to handle (if we assume that the water creates a lot of friction, like current fighting with impedance). With current the 8 ohm does not restrict the flow of current to the 4 ohm load, instead the 4 ohm load will allow or draw more current than the 8 ohm tap is designed to deliver.

I cant even remember who explained it to me that way..... coulda been my Dad for all I know....

 

[bigdawg]

Well I guess I could follow in some others' footsteps this week and say that I've overlooked something on my amp. So I read the manual wrong. A safe impedance mismatch is NOT a 4ohm head going to an 8ohm cab. The OTHER way around, however, is safe. This may seem obvious, but this is only my 2nd tube amp, so be gentle. I've had it this way for about a month and a half and have cranked it multiple times. Everything SEEMS fine, however now I'm nice and paranoid about tube sockets and the tranny.

My manual says that is a safe mismatch... what are you reading??

4 (head) into 8 (speakers) is safe, 8 (head) into 16 (speaker) is safe...

PAGE 11
RE
AR PANEL: Controls & Features (Continued)
SPEAKERS: These jacks handle the output to speaker cabinets or the internal combo speaker in your Express. There is one 8 Ohm and two 4 Ohm jacks here and between these outputs, most popular speaker combinations can be accommodated. You’ll find that different impedance loads change the way your amp sounds and feels - and though it is usually best to match the load you are using to the matching SPEAKER impedance output – you may find a certain mismatch combination to your liking. For example, some players like the tighter, bolder correct-match-response of an 8 Ohm speaker on the 8 Ohm SPEAKER output. Others may prefer the scoopier, more elastic vibe of the 8 Ohm speaker on the 4 Ohm jack[/b]. Feel free to experiment as your Express is capable of running with most any load and producing amazing sound. If you prefer the sound of an impedance mismatch, by all means go with the Tone.
NOTE: Using impedance mismatches in the low direction (i.e. a 2 Ohm load on the 4 Ohm output) will cause the power tubes to wear a bit faster.
COMBO: Express combos use either one internal 8 Ohm speaker or, two 16 Ohm speakers (wired in parallel) to produce an 8 Ohm load. If you wish to use an extension speaker cabinet (we make several matching Express cabinets, see you nearest dealer) you will need to remove the combo’s speaker from the 8 Ohm SPEAKER output and connect it and the extension cabinet (also 8 Ohms) to the two 4 Ohm jacks for a correct impedance match.
HEAD: Express heads can be used with most any speaker enclosure. Most MESA cabinets 1x12, 2x12 and 4x12 are wired to 8 Ohms. If you intend to use a non-MESA cabinet (we we’re sure you’ll find our cabinets superior both in sound and construction) that is wired to 16 Ohms, connect it to the 8 Ohm SPEAKER output.
Now that we’ve covered the features and operation of your Express, it’s time to enjoy the best part…the sound and feel! We wish you many years of inspiration, fulfillment and musical satisfaction from your new Express.

 

[EtherealWidow]

Dude to be honest, I have no idea what I'm talking about anymore with this whole thing.

 

[bigdawg]

the way I remember it:

you can get 4 people in an 8 seater car, but cant get 8 people in a 4 seater car without causing problems.

 

[MikeyV311]

your mesa is built tough man... if everything is functioning correctly, I wouldn't worry about it. As long as the amp hasn't experienced zero load, (no speakers) for a while, or way too low of a load (about 2 ohms or less), all should be well! =)

 

[TimeSignature]

Dude to be honest, I have no idea what I'm talking about anymore with this whole thing.

LOL

 

[edward]

Yeah I read an article and it said that halving the impedance would wear the tubes faster, and doubling could potentially cause flyback currents. It's nuts how complex all this is.

Any impedance mismatch causes additional stress on the output transformer: go one way and you load the primary winding; go the other way and it's the secondary winding ...nature of the beast. Additionally, how much the mismatch determines the stress put on the tranny. So going from 4ohm to 8 is much less difference in loading than 4ohm to 16.

These are electrical "truths" if you will. But all that said, additional "load" doesn't mean "bad" by default. Kind of like saying loading your car up with passengers and luggage is "bad" ...there are built-in design parameters; just don't exceed them.

I've read, for example, of numerous failures of vintage Marshalls' trannies dying if you mismatch impedance (or even look at em funny ). And some of their modern offerings are scarcely better. This is no slam on Marshall; this is simply observation of people's failed trannies on their Marshalls. Contrast this to Mesa's failed trannies. ...cricket...cricket...what? no failures? OK, I'm no tech, but in my over 10+ years of using a variety of Boogies live, full-band practices, at home (and cruising forums like this ) I have not heard of a single failure to any transformer, let alone failure due to impedance mismatch.

Moral of the story: say within the design parameters ...the lines are our friends. Mesa says one-order of mismatch is well within their design (and not to drop below 4ohm final load), and they are confident enough about it to put it in writing and back it up with a warranty. Not to mention their stout trannies clearly bear this out. So I think you can relax and not overthink the matter. Play your boogie and enjoy the tone!

Edward