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running 8 ohm speakers with a 4 ohm amp?

Lonesomedave

Lonesomedave

May 13, 2011
783
30
Nashville, Cats
#1
hi, guys,

got another stupid question (or to quote Chris Berman of ESPN, "There are no stupid questions...only stupid people who ask questions.")

actually 2 questions:

1) what happens if you run an 8 ohm speaker with an amp with a 4 ohm output?

2) what happens if you run a 4 ohm speaker with an amp with an 8 ohm output?

essentially the same question, just reversed.

for purposes of this question, let's presume it is a tube amp, say a sunn 2000s or marshall vba 400 for example.

would it work? would it damage the amp? would it damage the speakers? would it sound like s**T?

just wanted to know, as I have heard it is ok to run higher ohm speakers with a lower ohm amp, but not the reverse.

thanks,

/s/ Dave
 
#3
For solid state amps (even with tube preamps):

Running an 8-ohm cab from a 4-ohm-rated head is just fine. You'll typically get a little more than half the rated power of the amp, though.

Running a 4-ohm cab from an 8-ohm-rated head would result in excessive current draw, and could damage the amplifier. Don't do it.

For tube amps (tube power section):

It's best to run cabs whose impedance is the same as the output impedance rating of the amp. You might be able to get away with running a cabinet with a lower impedance (kind of opposite of SS amps), but I wouldn't recommend it.

I'm sure some of the tube guys will weigh in on this.

Jimmy?
 
#4
I have run my old all tube Traynor YBA 3 for about a year on various 8 ohm cabs (its a 4 ohm head, i just didnt know that all year) i didnt know so i ran it at 8, never had a problem. I played a lot of shows and practices, it was on just about every day.

I am sure its not ideal, but i doubt it will kill anything.
 
#5
Actually, I think it does affect tone to a degree. Tube amps not being run at their proper impedances always feel a little tighter and a little less tubey to me. It's not real noticeable but I do notice it. Anyway, I've done it with rentals at low volumes, and it's been fine, but with my own gear I would never do it! It can stress tubes and transformers. Won't happen overnight, especially if you don't play loud, but that kind of stuff can take its toll eventually, and I see no reason not to run stuff at their proper impedances.
 
#6
As Munjibunga told you that's basically what happen using 4-8 ohm amps-cabs.
The 8ohm cab solution it will be more helpful if you have a powerful amp but you need to use a small cab for a small-medium gig, so you can just pick a any of the smaller cab out on the market now, like a 110, 112 or 210. It will be much better than use a combo ;-)
Cheers. Enrico
YouTube
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WebSite
 
#12
Tube amps are pretty resilient, and it’s not like impedance is an exacting science relative to the application being discussed here. There is a reason the term “nominal” is used in reference to a cab/driver’s impedance rating;, in actual use the load that any specific cab shows the output stage of an MI amp is all over the place.
While a tube amp will perform its best* with the load impedance matched with the output stage, I think it’s generally accepted that a 100% mismatch is tolerable within the physical parameters of most quality tube amps.

Unrepresented said:
I ran my SVT-II 4 ohm into an 8 ohm cab for years. Maybe I was lucky, but I never had any catastrophic failures during this time linked to the mismatch. I eventually upgraded cabs mostly because I figured I was pushing my luck.
Click to expand...

Now that’s refreshing; an actual first person example of the thread subject!
At the risk of further breaking with the TB tradition of posting “common wisdom” hearsay and/or regurgitated, hyperbolized factoids from TB’s resident technical “armchair quarterbacks” as gospel, here’s another real world, first person example from the other side of the impedance mismatch fence:

Before I bought it, one of my personal two cab 2000S rigs had begun life as part of an endorsement deal, toured multiple times in the 70’s; long hard road use/abuse… with a 2 ohm load on the 4 ohm OT tap. The original road tech for the rig told me that while he realized the situation wasn’t ideal, it was, well… an endorsement rig; they had backup, and the factory would air express another one anywhere they needed in an emergency.
He said they changed tubes a lot but the unit never failed, other than its appetite for 6550A’s it was a rock.
It’s still fine, original trannies still at stock spec. Of course a 2000S is an overbuilt boat anchor, way under rated as far as output vs. component potential, and a higher strung, higher output amp that stresses components more may not perform as well or as long.



*As far as the design specification; what any particular individual considers tonally “best” is subjective, just listen to some of the clips linked here on TB… ;)
 
#13
edbass said:
Now that’s refreshing; an actual first person example of the thread subject!
Click to expand...

I suppose that's fine if you accept anectodal examples as proof of something. Because you can cite two examples of not damaging a tube amp from mismatched cabinets doesn't negate the designers' recommendation not to mismatch impedances. You are ignoring any examples of disasters related to it. Also you seem to have ignored unrepresented's final comment: "I eventually upgraded cabs mostly because I figured I was pushing my luck."
 
#14
Munjibunga said:
I suppose that's fine if you accept anectodal examples as proof of something. Because you can cite two examples of not damaging a tube amp from mismatched cabinets doesn't negate the designers' recommendation not to mismatch impedances. You are ignoring any examples of disasters related to it. Also you seem to have ignored unrepresented's final comment: "I eventually upgraded cabs mostly because I figured I was pushing my luck."
Click to expand...

Actually, the point of my “poke” was that I generally don’t “accept anectodal examples as proof of something” and that I find actual first hand experiences refreshing as compared to the usual plethora of anecdotal advice on technical issues so readily volunteered on TB.
For example; as far as mismatched impedance, maybe you can share one of your first hand, non anecdotal examples of “disasters related to it” that I ignored. Here’s one of mine, when I was about 14 I toasted a Gretsch 6154 by forgetting to reconnect the speakers and playing it, wondering why nothing was coming out until I smelled the “magic smoke” it was releasing.

I didn’t mean to suggest that anyone should run their amp outside the manufacturer’s specifications; I certainly don’t, at least intentionally with my own gear. I was just pointing out two real world examples of impedance mismatches, one over and one under, in reference to the original thread topic.
As I mentioned, my mismatched 2000S rig did have an abnormal appetite for output tubes, but I failed to mention that as with Unrepresented’s scenario I remedied the mismatch ASAP.

Anyway, my apologies to anyone else who somehow misinterpreted my post as an endorsement of running mismatched loads on tube amps.
 
#17
Tell ya what. Why not start a thread where people who have actually had tube amp failures from speaker mismatches post. No anecdotes, no beliefs, no acid flashbacks, just 1st person examples. That should answer the OP's question.

My guess is that it will be a very short (and short-lived) thread.
 
#18
Long technical answer here:

Extended A: It's almost never low impedance that kills an OT, it's too high an impedance.

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. A 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.

This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.

For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes and leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.
Click to expand...

A ton more information here.

Emphasis added to sort-of-important parts, but it's all worth a read. I used my SVT-CL with a 5.3 ohm load for a while, over a year at least, with little effect, but that's not really all that far off from 4 ohms. If it were me, I'd always go for correct impedance. Ever seen an amp with a blown OT? Mismatch is a likely culprit.

As far as tonal variance, when you have a mismatch, you're putting a different stress on the output tubes than they usually see, so really the difference is how the tubes are being driven and whether you're in their "sweet spot" range.
 

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