Running Mesa Boogie Bass 400+ into 4 ohm and 8 ohm cabs

[nukes_da_bass]

Question for the Mesa Boogie Bass 400+ gurus:
I found the steal of the century at GC last week- an old used 400+ for $400.00! ($424.00 after tax).

It powered up so I bought it right away. I read the manual online and it gives plenty of advice for using two cabs of the same ohm rating but none for mixing different ohm ratings.

I have two cabs at the moment- an 8 ohm Ampeg 115, and a 4 ohm Avatar 2x10.

Right now due to the newborn twins I'm not in a gigging band so don't actually NEED to run both cabs but it would be nice to know if I can run these cabs at the same time and which outputs to use- the Mesa people seem to think everyone is good at ohm math and it seems I've never quite gotten it (the manual says to run two 4 ohm cabs to use both 2 ohm outputs etc).
I used the search feature and came up dry.
Nukes

[Caca de Kick]

No, you have to use two matching impedence cabs: two 4ohm or two 8ohm. Tube amps don't like mixed impedences.

[nukes_da_bass]

Thanks Caca- even tho it is bad news.

Does this mean the problem I'm having with my Eden B300T is because i mixed impedance? Crap!

The Eden is in the shop- I hope i didn't destroy it i wanted to sell it!

<<<really dumb about impedance, but I'll not mix impedance from a tube amp again.

One more question-the Ampeg cab hasn't had an Ampeg driver in it for a million years- I swapped in Peavey Black Widow speakers when the Ampegs just kept blowing up...
If i change the 8 ohm 15 to a 4 ohm 15, does that turn the Ampeg to a 4 ohm cab automatically? I always wanted to throw a Mesa Boogie speaker in that thing (and of course that would solve the problem by matching the impedance of the Avatar cab).
Nukes

[PotsdamBass8]

Now I'm certainly not an expert, but I thought if you were to plug one cabinet into the head, and then the other cabinet into that one, then you would end up with a total impedance around 2.6 or so?

[rockstarbassist]

I know this is possible on SS or hybrid amps, but I haven't been a tube owner myself long enough (400+ also!) to know this for sure.

[PotsdamBass8]

Again, I'm not an expert so someone else please confirm or correct, but I would think that by linking the two cabs together, you're going to end up with one total impedance in the same way that you can buy a cabinet with multiple speakers that when linked together add up to one impedance.

[BassmanPaul]

The impedance of a cabinet is fully dependant on the drivers. You put a 4Ω driver into the 1x15 it will become 4Ω. Sadly it isn't as simple as that. To function at its best a driver(s) and its cabinet have to work together. You already have a driver in there that was not designed for the cabinet. If it's working for you no foul. Finding a 4Ω 15 might be a little difficult and if you do it might just fail in your cabinet.

Running both of your cabinets in parallel will result in 2.67Ω which is less than ideal for a tube amp and is something I can't recommend for bass.

[levis76]

Sounds like you need another 8ohm cab. Run both 8 ohm cabs to one 2ohm input and the 4ohm cab to the other 2ohm input. Thats what I would do.

Congrats on the twins!!

[nukes_da_bass]

Thanks Paul!
Ill just stick to the 4 ohm Avatar 2x10 right now while I spend most of my time with the newborn twins.

Wife on gigging this year..."Bottles are your new basses...Diapers are your new bass amps!"

And the guy who advised on not playing Frankendriver with the Ampeg...good call I'd say, when I am allowed to gig again I'll trade it towards a second 4 ohm cab.
Nukes

[James Judson]

Asking technical advise from musicians is risky at best. You will get lots of "opinions", some better than others. Here is the bottom line: Match impedance (measured in ohms) to protect your amp, match power (measured in watts) to protect yuor speakers.

One gentle mans says your amp does not like mismatched cabinets. Thats BS. Your amp has no clue what is attached to it. The amp only see's an impedance connected and a matched impedance means 1. Maximum power transfer (ohms law) and 2. Amp does not work as hard getting those watts into your speaker.

Running an 8 ohm cabinet with a 4 ohm cabinet is totally fine. Here is the results.
1. Your amp will see a 2.6 ohm load. Is your amp rated to handle a 2 ohm load (most do not)?
2. The 4 ohm cabinet will used 2/3's the power making the 8 ohm cabinet seem like its turned way down.

Ohms law is fairly easy subject but usually misunderstood by musicians. There is a reason electronics schools spend a long time on the subject and many graduates still don't understand it. I recommend going to electronics school if you want to really understand resistance, impedance, impedance matching, watts, inductive reactance and ohms law. All these things apply to your amps and cabinets. Short of the schooling your user manuals usually spell out your parameters for hooking up stuff to other stuff. Look on the internet for user manuals in the case yours are misplaced.

Hope some of this helps.

[Mo'Phat]

From the manual:

Originally Posted by Mesa Boogie Bass 400+ Manual

"SPEAKER JACKS
Two 8-ohm, two 4-ohm and 2-ohm speaker jacks are provided on the Bass 400 Plus. The
amp will produce its full power at all of these impedances, provided that the total
impedance of your speaker cabinets is matched to the output(s) used. (Slight mismatches
are not a major problem, but they will cause slightly lower output and may shorten tube
life.) When using more than one speaker cabinet, remember that the total load impedance
goes down as more cabinets are connected. For example, if you use one 8-ohm speaker
cabinet, use one of the 8-ohm jacks. But if you use two 8-ohm cabinets, they will be
operating in parallel and the total load will be 4 ohms. Therefore, plug each cabinet into a
4-ohm jack. If you want to use two 2x15 cabinets where each cabinet is rated at 4 ohms
(which is how Boogie 2X15’s are wired), connect both cabinets to the 2-ohm Jacks
provided."

I ran my 8 ohm 4x10 and my 4 ohm 2x15 from the 2 ohm jacks and never once had a problem. The volume difference between the 4x10 and 2x15 was acceptable to me back then.

[BassmanPaul]

Originally Posted by nukes_da_bass

Thanks Paul!
Ill just stick to the 4 ohm Avatar 2x10 right now while I spend most of my time with the newborn twins.

Wife on gigging this year..."Bottles are your new basses...Diapers are your new bass amps!"

And the guy who advised on not playing Frankendriver with the Ampeg...good call I'd say, when I am allowed to gig again I'll trade it towards a second 4 ohm cab.
Nukes

A wise decision in my book. BTW congrats on the twins and I assure you, you WILL be able to sleep again.....eventually!