8ohm Guitr Head to 4ohm bass cab (bass guitar)

Probably not, it's usually ok to run an amp into a higher ohm cab, but running into a lower ohm cab is almost never good.

 

Get another 4 ohm cab and run them in series to achieve 8 ohms.

Does the cab have 2 speakers? Maybe they're 8 ohm speakers. You could rewire it to run 16 ohms.

 

This is my Sunn 200s ... my tech did a simple mod and added a selector switch off the existing transformer taps, might be something to look into.

 

Just checked the Solarus schematic, and like many other Sunn amps, there is an unused 4 ohm tap tucked away inside the amp. It's a common mod, if the amp going to be use for bass, to have a tech mod the output jacks so the amp is tapped for 4 and 8 instead of 8 and 16. Adding a three way switch is also a very cool idea.

 

With tube amps it depends upon how the output tubes are loaded. Some amps are better if the mismatch is slightly high, and others are better if the mismatch is slightly low. For example, vintage Fenders are setup to run with the expected load or half the expected load. Mesas are setup to run the expected load or double the expected load. With vintage Marshalls, I think running any impedance mismatch is pushing your luck, as I don't believe the output transformers are all that robust. No idea which way to go with a vintage Sunn.

 

Hence the usually, and almost never.

 

Sorry but you have the qualifiers backwards. The general advice is tube amps are usually safer if the impedance is lower than expected. At the extremes, tube amps tolerate shorts pretty well, but not open circuits. This is why the output jacks on many tube amps short the output to ground when nothing is plugged in.

In this case, since the amp has an unused 4 ohm tap, the best advice is probably to have the amp rewired, so the OP can use the desired speaker without incurring an impedance mismatch.

 

This is not what I’ve been advised directly from Mesa Boogie. If a mismatch is necessary, the impedance should be higher (numerically) than the amp.

 

Sorry Will but wrong way around. You are thinking SS.

OP you will probably get away with it at the cost of a hit on tube life.

BTW Welcome to Talk Bass.

 

The whole point of my post is it's better to consider the specific amp in question and give the correct advice, instead of general advice. Mesa is a bit of an exception to the rule, and I identified your point in post #6 (and also many other posts).

 

Sunns of that era all have 4, 8, and 16 ohm taps on the OT. Two of them are wired to the output jacks, which are paralleled, when both jacks have a phone plug in them both default to the lower impedance tap.
How they were factory wired depended on what cabinet configuration was ordered with the amp.
Until the Hartzell era, you couldn't buy an ala carte amplifier unit from Sunn, every amp came with specific cabinet(s), which is why Sunn had identical amplifiers branded with different names designating the amp/speaker as a unit.

Your "Solarus" was actually a 40 watt 2 X 12 guitar combo amp until 1969, when a "Solarus" became a 40 watt head with a separate 2 X 12 guitar cab, in 1970 Sunn dropped the 2 X EL34 40 watt amps altogether and the"Solarus" became a 60 watt 2 X 6550 guitar amp sold with a 2 X 12 cab, with an available upgrade to JBL D123F's.
The actual amp you have is identical to the Sentura I guitar amp, but the Sentura I came as a 1 X JBL D15S as a combo, after 1969 a separate amp and 115 JBL D15S cab.
The 60 watt 200S, Sonaro, Sorado, Sonic I-40 bass amplifier units are identical except for the labeling. What it was badged/sold as was solely dependent on what cab(s) it came with. The 200S generally brings more money in the market, the amps are identical.

Tube amps are pretty forgiving of miss matched impedance. The ultimate performance of the unit is compromised as far as accurately and efficiently reproducing and amplifying the input signal, however mechanically/electrically the amps are fairly resilient.
That said, as far as which way is "safer" for tube output stages regarding impedance; Leo Fender's amp's jacks defaulted to a dead short when there wasn't a plug in them as a safety measure in case some jackwagon fired one up unplugged. That's real close to zero ohms impedance.

I have a two cab artist endorsement deal 2000S rig that was run hard on tour for many years with a two ohm load on the 4 ohm jack. I mean run HARD, the artist I acquired it from knows of at least a half dozen D140F's it ran through during his tenure on tour. It was well maintained, couple of caps and drifted resistors over the years, factory iron, pots, etc.
Lots of cosmetic battle scars, but tests up to spec on a bench to this day.
Just one of the reasons I get a chuckle from the obviously inexperienced and clueless "me too" tube amp thread posters who complain about tube amp frailty, high maintenance, failure rate etc.
There is no arguing about the weight part though!

 

Not that I like disagreeing with the fine folks at Mesa but according to my tube education this is wrong. The last thing you want is to have is a flashover in the output tubes.

 

I believe the reason is because they don't set up their amps so the tubes are right in the middle of the load line. Assume you have an 8 ohm tap, but the amp is setup so it is really more like a 10 ohm tap. Then running a 16 ohm speaker is less of an mismatch than running an 4 ohm speaker...therefore running the speaker one order high is the accepted mismatch. At least that's my crude way of understanding it . I could be wrong as engineering is over my head.

 

Then in my mind the OPT was not correctly designed. I doubt this of Mesa.

 

I am not aware of, but could imagine a scenario where if asked about a slight mismatch, maybe <1 ohm, Mesa could recommend going higher for a specific situation/amplifier. As I said earlier, tube output stages are pretty resilient. Also speaker loads are reactive, ratings are "nominal", and the output tap only actually sees the perfect load sporadically and for milliseconds at a time during service use.
Still, across the board less is better (read; "less bad") for tube amps.

To me the quote "If a mismatch is necessary, the impedance should be higher (numerically) than the amp." is fundamentally backward as a general reference to a transformer coupled tube output stage amp, and also to me seems very un Mesa-like coming from a Mesa CS person.
In the big picture, as a company the venerable Mesa Boogie has a lot more than a little experience with tube output stages...

Possibly a miscommunication? It seems the highly tube oriented Mesa Boogie has in fact zero tube bass amps currently, and with the class D WD-800, following the path of most other hybrid amp manufacturers, has emblazoned "2 Stage Class A Tube Preamplifier" across the face plate.
It isn't uncommon for a TB member to refer to their hybrid as a "tube amp", @jeff62 were you speaking with Mesa about a specific amp when you got that response?

Which brings to mind another question.
Why do so many transistor amp company's marketing departments and their end users liberally make reference to how "tube like" their units sound, while I can't think of a single tube amp manufacturer/end user that claims/brags that their amp sounds "transistor like"?

 

I think you are making the assumption that setting up the loading of the tubes is precise and there is only one way to do it. This is not accurate. Different designers choose to load the tubes high or low depending upon the results they want. I am sure Mesa has a reason for what they do.

 

From what I have seen, Mesa has been consistent with their guidance on what defines a Safe impedance mismatch with their amps. Having the speaker one order higher than the output tap is what the company advises as a Safe impedance mismatch, for example 16 ohm speaker on 8 ohm tap. Many of the manuals are still available online for all to see, despite the all-tube bass amps being discontinued. I suggest opening the attached Strategy manual to page 27.

I used to assume it was a mistake made by the marketing department, but @agedhorse has confirmed that it is done for technical reasons related to the way Mesa sets up it's output sections

 

Not a miscommunication based on my experience. I received that advise on two separate occasions, once where I did not ask, on both a Mark IV tube amp as well as a Boogie Buster tube amp.

 

These impedance debate threads make me smile. I built a motor drive for a lab prototype that used a signal generator, an unmodified Bogen MO-100A PA amp, and a synchronous AC motor driven from the 115 volt tap on the output transformer. It worked like a champ.

That same MO-100A was also the power amp in my gig rig at the time, along with a homebrew 2x12 and preamp.

To the OP: You'll be fine with this particular setup, and the max volume will be slightly reduced compared to an 8 ohm cab. The reason for the confusion is that the general answer is "It depends on the amp and output transformer design".

We forget that tube amps used to drive all sorts of loads, not just cabs.