Help with Music Man head/cabinet issue

[Low End Theorist]

Ok I have an early 80's Music Man 120B head and a Music Man 412B cabinet.
I really love the sound but wanted a little bit more out of it volume-wise, and wanted to get a matching Music Man 115 cabinet.
Well, I jumped at the chance to get a 115RH cab but just found out it is a sixty five and not a 130 or 150.
Did I totally screw up and get something that is completely unusable with this setup?
I don't know much about the 120B head other than it's supposed to be 80 watts through one cab but goes up to 120 watts when using an additional cab?  Will this head destroy that 115?

Any Music Man specialists out there and/or head/cabinet/wattage experts?

I'm a little slow when it comes to this stuff.

Thanks!
Mark

[Psycho Bass Guy]

Unless you're just driving the piss out of them, both cabinets should be just fine. The major caveat is how the amp will perform. It is a solid state amp with a minimum rated impedance of 4 ohms, which means if your 412B is already 4 ohms, you can't use the 115RH without a special series-wired speaker cable connecting them together.

[Low End Theorist]

Ooh I'm not sure what the 412B is as far as ohms go.  I'm going to look online and see if I can find out.  The head does say 4 ohms minimum and only has one speaker out.
The 412B has two inputs but I'm assuming one can be used to connect the extension cabinet?  Is the special series-wired speaker cable something I"ll definitely need and can find easily at most larger music stores?
Sorry to be such an ignoramus about this stuff.  I appreciate the help.

[Low End Theorist]

Ok so both the 412B and 115RH "Sixty Five" are 8 ohms each.
Does this mean it's safe to use both cabinets together with this head?

[Highlander]

Running the speakers as they are will result in a lower than 4 ohms resistance - the connection PBG is on about is like a "daisy chain" - the resistance must add up to 8 ohms - cable/speaker+/spk-/cable/spk+/spk-/cable - all in a line - if you use it "as normal" you will get a 2ohm resistance and an overdriven amp - normal connection will "pair" the speakers together.

It would be possible to get a techie to wire in a socket into one of your cabs to enable you to do this if you are unsure - not to complex if you can use a multimeter and a soldering iron...

[Low End Theorist]

So if I take this head and plug it into the 8 ohms 412b cabinet then out of the that cab I run a speaker cable into the 115RH 8 ohms cabinet - this adds up to 2 ohms?  I never really understood ohms and how they work.
These heads were meant to drive two music man cabinets.  What ohms amount are you saying my cabinets would each need to be in order for this to work without soldering things?

[patman]

isn't 2 8 ohm cabs in parallel equal to 4?

[Low End Theorist]

Is that right?  Running the cable from the first cab into the second cab would be running them in parallel or in series?  I don't know any of this stuff.
I read somewhere that if they were different ohms then the add up to a much lower ohms.
Since both of these cabs are 8 ohms wouldn't the total be 4 ohms?

[Highlander]

It does half the resistance if in parallel so if they are 8 ohm cabs and the wiring does connect them in parallel that would make it 4 ohms total loading...

If there are two speaker ports on the amp you would need to know what they are doing or how they run on into the system - most gear warns you what it does, but a lot is taken as "given" - ie you didn't know... what a shame... I don't have a lot of experience with amps, per-se, unlike our electronics guru, Mr Campbell but if he doesn't have the answer try one of the specialist amp sites...
Certainly I would presume the secondary connector would be in parallel, normally...
A budget test meter is a very useful tool to have in the "gig" bag, as is a "gas" soldering iron and some solder...

Discretion (knowing the answer for sure by using a meter) is most certainly the better part of valour...

[Low End Theorist]

Thanks HERBIE !
The amp has only one speaker out and is marked 4 ohms minimum.
I've read the head puts out 80 watts @ 8 ohms (into my 412)
and puts out 120 watts @ 4 ohms (apparently if I use both cabs daisy chained)

Would that 120 watts be split evenly between cabs?
Like 60 watts to the 412 and 60 watts to the 115?

[Highlander]

If both cabs are 8 ohms then the power draw will be the same across both of them, in theory, Mr Lowend...

Wattage is a power value so it is the "total" of the sum of the two...
I'm trying to think of a good analogy... two differing value filament lamps on the same circuit... one is 60w the other is 200w... you know which one will be brighter and will use more power as a given... no need to think about it... you also know that something that "kicks-in" causes all the other things to "dim" slightly on some "closed" circuits... ie a house...
Crudely speaking it is the same with speakers, but with some speakers (hifi) you need to have balancing circuits to make it all work, just so...
About 30 years back I was running a pair of beaten-to-excrement Sound City 4x12 columns with my DR103 and the sound was just... so... perfect...
There was this Ampeg SVT 810 in the studio that was up for sale and I tried it with the Hiwatt - sounded terrible...
Some stuff works... some just don't...
PBG is definitely the best guy on the site for the specifics of this so if I've strayed or if he has a better analogy I'll happily defer to his expertise...

[Low End Theorist]

Right that's a good way to put it!
Thanks so much!!

[Psycho Bass Guy]

The 412B has two inputs but I'm assuming one can be used to connect the extension cabinet?

Yep. That's a jack wired in parallel. 99.999% of all cabinet inputs are wired this way. Even if your amp had another output jack, it would still be the same connection electrically in most cases (some tube Music Man and Fenders from the 70's had a series-wired extension jack.)

Is the special series-wired speaker cable something I'll definitely need and can find easily at most larger music stores?

Series speaker cables are easy to make, but you'll never see one in a music store. Besides, based on your next post, one is not necessary.

Ok so both the 412B and 115RH "Sixty Five" are 8 ohms each.Does this mean it's safe to use both cabinets together with this head?

You'll be fine.

[Low End Theorist]

Thanks PBG!!  I really appreciate the help!