some fenders (read: OLD fenders) can take a mismatch like this: 4ohms [head] --> 2ohms [cab] ... it's one of those things that "isn't advised", but if you do it for a show/practice, the amp won't immediately die ... i am pretty sure the same is true for most mesa's - i.e. 8ohms [head] --> 4ohms [cab] will be "ok" for that time when you're "in a pinch" - but to do it long term (all the time) would be a bad idea ... and i'm pretty sure i've read that mesa's can handle a 1-step mismatch in either direction ... they are pretty beastly amps ...

and so we're all on the same page:

no load:
bad! for tube amps
"ok" for solid state amps

head --> lower impedance cab:
bad! for solid state amps
"ok" for SOME tube amps

matching:
is ALWAYS the best policy

__________________
gear for sale - PM if interested ...
scott's gear ~ hard rock --> metal

my best memories were forged in basements and garages ...

Alright, let's get to the hard stuff

More questions that should be addressed:

Within the same cab ~ Speaker watt mismatches/ ok or not?
Within the same cab ~ Speaker Ohm mismatches/ ok or not?

When mismatched which speaker will blow 1st (if any)?

Is it necessary to have much more capacity within the cab vs. the amp? i.e. 100 watt amp ... is a 100 watt cab OK or or not?

A 4x12" cab can handle how many watts if it has one 25 watt Greenback in it and three Vintage 30s?

__________________
Home Thread

Fat Strat into a Soldano, always.

I didnt see this in the thred so I guess I will throw it out there.

The forumla for find impedence in parallel circuits is 1/a + 1/b = 1/c.

A = first speaker

B = second speaker

C = final impedence

so if you have 2 8 ohm cabs,

1/8 + 1/8 = 2/8 = 1/4

final impedence = 4 ohms

So when things get a little hairier, like one 4 ohm speaker and one 8 ohm speaker, you can still figure it out

1/4 + 1/8 = 2/8 + 1/8 = 3/8 = 2.66 ohms.


Now, finding inpedence in series-parallel cabinets.

It is basically two sets of speakers wired in series, that are wired in parallel.

So, it will go like this

first set is two 8 ohm speakers

second set is two 4 ohm speakers

so the first set has an impedience of 16 ohms, and the second on has and inpedience of 8 ohms.

that is the series part.

The next part you apply the parallel forumla to it.

1/8 + 1/16 = 2/16 + 1/16 = 3/16

16 divied by three is 5.33 ohms.

__________________
Call me Jem, or Jeremy.

Homophobia is gay.

you mean (for example) using 2 v30's (60W) + 2 g12t75's (75W)? -- yeah, it's ok - but you'll have a cab that can handle 240W, not the 270W you might expect ... the cab is only as strong as its "weakest link" ...they're ok, but they'll create some weird loads ...the ohm mismatch (between speakers) won't cause damage - so if you're talking about the wattage "mismatch" - i don't think you can say really ... i'd imagine it's one of the lower wattage speakers - but i wouldn't be surprised if by some "miracle of science" the higher wattage speaker would go 1st ...i don't think it's completely necessary - a "good idea" maybe, but not a stedfast requirement ... didn't hendrix + townshend overshoot their cabs handling?100W ... the 25W greeenback x 4 ...

and incase anyone is wondering - it's spelled I-M-P-E-D-A-N-C-E ... ...

__________________
gear for sale - PM if interested ...
scott's gear ~ hard rock --> metal

my best memories were forged in basements and garages ...

Good work Scxtty!

The only thing is the ohm mismatched ... I was told that while in paralllel that it may cause a disbalance as the amp tried to compensate for the different loads. Dunno.

__________________
Home Thread

Fat Strat into a Soldano, always.

More good information.

I didn't put this in the original post, because most players are going to try to stick with the same impedance on multiple speakers.

But here it is if they need it

__________________
Darrell "Dimebag" Abbott 1966-2004
Rest In Peace. :'(

2007 Schecter C-1 Classic
Mesa Dual Rectifier
Mesa Recto 4x12
Carvin UH5000 wireless system
Digitech Valve FX processor
Behringer FCB1010

Mikey's Rig

Proud member of the MESA family

Say you're runing a 60 watt amp on a 25 watt greenback and a 75 watt G12T ... I've been told that the higher watt speaker will blow 1st. Dunno why so that's why I asked. Weird.

__________________
Home Thread

Fat Strat into a Soldano, always.

Possible because the amp doesn't have sufficient power to drive both speakers efficiently, and the voice coil on the higher watt speaker heats up to the point where it quits working. I've been told it's better to overdrive a speaker than underdrive it, because that's what will happen. Of course, that was with car audio.

Or, there's not enough power to counteract the natural retraction of the speaker, and the speaker "slams" into the magnet, eventualy destroying the voice coil.

Just a couple of stories I've been told.

__________________
Darrell "Dimebag" Abbott 1966-2004
Rest In Peace. :'(

2007 Schecter C-1 Classic
Mesa Dual Rectifier
Mesa Recto 4x12
Carvin UH5000 wireless system
Digitech Valve FX processor
Behringer FCB1010

Mikey's Rig

Proud member of the MESA family

this is not true with guitar speakers. the less power you put to them, the less they will heat up. if both speakers are the same impedance, the lower rated one will blow first with increasing power beyond its capacity, but then the other speaker will either be mismatched to the amp, or the circuit will be open altogether. this is why I like using all 16-ohm speakers in my 4x12's, wired in parallel . if one blows, the others are all still in the circuit.

I've heard this as well, but have never seen any proof.

this is a definite possibility, however, I think the wire in the voice coil would melt long before the speaker would fail in this way. the cones of guitar speakers just don't have enough travel in them to allow this at reasonable power levels.

__________________
My Rig:

http://www.elkvis.org/rig/

Kinda half right, this stuff all more applies to PA's than guitar:
Really what people mean is that it is easier to blow speakers with amps that are too smal than too large. Don't get me wrong, if you run a huge wattage amp into a low wattage speaker, it will probably burn up the voice coil eventually.
Now, the problem with having an amp that is too small is this: If you have, say a 25W amp into any speaker, and you are trying to cover a 10,000 seat arena, it won't be loud enough. You will be be running all the levels on the mixing console as high as you can get them, but still it won't ever be loud enough. You will start clipping something, giving it more signal than it can handle. This generally happens at the amplifier. If you put too much signal into an amp, it won't have the ability to handle it anymore. Higher end amps, like Crowns, will start shutting down for safety. Cheaper amps will clip, which means they will take the sine wave has too much amplitude and clip off the top, making it look flat, almost like a square wave. When you clip a signal really hard, it actually ends up being a DC signal, which will destroy the speaker.
The for live sound, for PAs is to get the biggest amp you can afford, and use your ears. If you hear the speaker slapping around or clipping, turn the stuff down or you will destroy gear.
More than you probably ever wanted to know, but it applies more to PAs, which have different signals than the program material going through PAs. I have no experience with heads and cabs, and matching them, but I don't think very many companies would build a speaker cabinet for guitar that would be destroyed by normal use with normal heads and amps.

__________________
for sale:
check my fs post:
http://guitargeek.com/chat/showthread.php?s=&threadid=69753

mesa f-50 combo, $800 shipped
fostex MR8 digital 8 track recorder, $175 shipped

I see.
Maybe that's what the guy meant, I just didn't understand him right. I know guitar speakers are a different realm than audio drivers and such.
I didn't really know the answer to Motorhead's question, I was just throwing out theories.

__________________
Darrell "Dimebag" Abbott 1966-2004
Rest In Peace. :'(

2007 Schecter C-1 Classic
Mesa Dual Rectifier
Mesa Recto 4x12
Carvin UH5000 wireless system
Digitech Valve FX processor
Behringer FCB1010

Mikey's Rig

Proud member of the MESA family

That's not the formula I have... To find the combined parallel impedance of two units with impedance IMP1 and IMP2, the formula is

IMPn = (IMP1*IMP2)/(IMP1+IMP2)

Are these mathematically the same thing? Anyone got a proof?

Great idea for a thread.


Oh, and impedance would be more accurately characterized as applying to VARIABLE current, because if the load fluctuated on your speaker from zero to some nonzero point --positive or negative-- but not to the other side of zero, the speaker would still make sound. The speaker's position in space is set by the amount and direction of current. Impedance is, really, a measure of how fast that current is able to change direction -- the inertia of the signal, if you will. More loops on your output transformer give more inertia, so that's why the OT has multiple ohm taps.

__________________
my rig's thread - my playing style

those formulas are interchangeable.

__________________
for sale:
check my fs post:
http://guitargeek.com/chat/showthread.php?s=&threadid=69753

mesa f-50 combo, $800 shipped
fostex MR8 digital 8 track recorder, $175 shipped

Just wanted to mention That Fender made they're heads (pre-1977) for 4 ohm so that you COULD use them in 4, 8 or 16. The further you go up the less "fender" clean headroom and volume you get. Lots of players prefer an 8 ohm load because it tames the SHowmans down and makes them break up earlier and sound less sparkly... using a 16ohm works just fine and can sound EXCELLENT (especially with old bassmans), but there is almost no "fender" signature sound left.

__________________
Tyler

Guitar rig with recording samples.

Youtube Sample of My Sound

I found a Marshall MkII JMP on the internet. It is a 76' 50 watt master volume. There was a mod done in the back for the speaker impedence selector. It can handle 4 & 8 ohms.

MY QUESTION:
I have a 1960ax, which is a 16 ohm cabinet.
Can I change the head back to 16ohm so I can put the two together? Will it cost a lot?

OR

Will it be safe to just plug in?


Cab (16)>>>>Head(8 or 4)

***I recently posted this on another thread but thought it fit yours better

thanks for the help!!!