Conference napalm::guitar

    
    
    Hi Greg,
       I would say each speaker output should drive an 8 ohm load. That
    is, two cabinets with the speakers in each configured so each speaker
    jack sees 8 ohms. Hope this helps.
    
                                      Mikey Z.

    
    I will have to diagree.  If the amp has an impedance switch, then
    the total load must be the same as the switch setting.  So two cabs
    must either be 16 ohms, wired parallel (which is how the jacks are
    set up) or 4 ohms wired series.  Or if they are both 8 ohms, set
    the amp to 4 ohms and wire in parallel.
    
    There *are* some amps which have speaker jacks marked for specific
    impedances (like some MESAs) because they have the jacks wired to
    different taps on the output transformer.
    
    

    The Mesa documention on my amp says that you can plug the builtin
    speaker into either the 8 ohm or 4 ohm output without hurting the
    amp.  They even recommend that you experiment with that.
    
    Not sure if this is a good idea for other amps though.
    
    	db

    I got a little confused reading .2  Here's my crack at it.
    The speaker jacks on the back of the amp are wired in parallel.
    (I BELIEVE this is true; anyone care to confirm it?).
    If you connect one 8 ohm cab to each jack, you have two 8 ohm
    loads wired in parallel.  This means the amp sees a 4 ohm load
    (two 8 ohm loads in parallel = 4 ohms) so you need to switch the
    impedance selector on the amp to 4 ohms.  This is how it works
    on Marshalls and I would expect Hiwatts to be similar.
    
    One note of caution, while Mesa-Boogies may be designed to tolerate
    mismatch impedance loads, on a Marshall its the quickest and surest
    way smoke the output transformer.  Some amps are very sensitive
    to this, I would guess that Hiwatts are.  I'd call up a dealer who
    knows for sure before I did anything.
    
    /rick

    Thanks, Rick!  That's exactly what I was looking for.  I didn't
    know how the two jacks were wired (parallel or series).  Unfortunately,
    I don't think there's even a Hiwatt dealer in Co. Springs.  I would
    presume that it would be very similar to a Marshall, though, so
    I think your information is correct.
    
    gh

Another related question:

I want to run a 1x12 16-ohm cabinet off of my 8-ohm output. Is there some way 
to match the impedance - a switch of some sort that I put in line? 

And if so, any recommendations, and an idea of the price involved would be 
appreciated....

Alan S.

    not to be facisist ( or however you spell it ) I think the only 
    acceptable alternative is to replace the speaker with a 8-ohm.
    What kind of speaker is it?  

>    not to be facisist ( or however you spell it ) I think the only 
>    acceptable alternative is to replace the speaker with a 8-ohm.
>    What kind of speaker is it?  

I hope that isn't necessary!!!

Its a Celestion, 70-watts.

Alan S.

Call Celestion, or a music shop that sells/services them.  You should be able to
change the tap in the crossover (which I hope it's got, fer yer speakers sake)
from 16 to 8 ohms.  Thers should be solder lugs on the xfmr, and ya just change
the one that the jack is connected to.

			GONZO

    
    
    
    	I'd probably buy a 50 watt 16 ohm resistor (Allied) and put it in
    parallel with the 16 ohm speaker to create an 8 ohm load.  I was
    thinking about doing something like that to my CRATE cabinet until (at
    the last minute) I bent down (you guys know how hard it is for me to do
    that) and found out that the speakers are 4 ohm a piece.. heh heh.. one
    smokey amp avoided...

    I don't mean to be rude, Steve, but I think there are notes in here
    somewhere that say such a fix will do more damage than not, 
    but Gee I got no idea where they are.
    
    Back to being rude...

    
    
    	Thanks!!  Not rude, helpful, appreciate it... I just never
    understood howcum...  a load is a load right??  duh...

> a load is a load right??

No. Impedence (AC resisitance) varies by frequency. Beyond that, I'm lost
but I suspect a dummy load would probably require some type of RLC circuit
(whatever that is).

ccb - staying in Software for today.

Does that mean that the only way around this problem is to change the speaker 
input itself? There's no little box that will change it in-line??? (Hmmmmm.... 
do I see a market here??? 8^)

Or, since I'm running 2 of them, I suppose I can just wire them in series. But 
I'd prefer the option of only having to run one of them instead of always 
having to use two.....

alan

>There's no little box that will change it in-line??? (Hmmmmm.... 
>do I see a market here???
    
    I'd think there would be a huge market for impedence matching devices. 
    I had (have) a use for one.  
    
    There's also probably a good market for a simple series/parallel box
    that you can plug some number of speakers into and have it wire them in
    the perscribed manner.  An example would be if you had an amp with an
    8ohm output (only one) and two 16ohm speakers you needed to plug into
    it (in parallel for an 8ohm load.  Yeah, you could make your own
    parallel Y cable, but if you had a little box you could plug them into
    with standard speaker cables, wouldn't that be desirable?
    
    Greg

    
    	Re .10
    	Even if you got the correct load impedance with that
    	scheme, the volume of the second speaker would be only
    	one-fourth the volume of the main speaker. You won't
    	even hear the second cab unless you put you ear up to it,
    	so what would be the point? The only way to go is to get
    	a speaker (or speaker cab) with the same impedance as the
    	main speaker.
    	   Amps with a transistor power stage can generally handle
    	strange loads, but a tube amp has an output transformer to
    	match it to a specific load and you'd better get it right
    	or you'll be looking at a major repair. A good 8 ohm guitar
    	speaker costs less, and is alot more available than your
    	average an output transformer.
    
    	Neal

OK, so far I can either: 

a.) I live with it the way it is (8 ohms power amp out into 16 ohm speakers).
    This won't hurt anything (its a transistor amp, no tubes), but it will
    hurt the amps performance, right? (I only get 50% of the power?)

b.) I can replace the speakers with the 8-ohm version (not an option right
    now because of $$$ - they ain't cheap!)

c.) I could wire the two 16-ohm cabs in parallel, and everything would be 
    fine. Question - is there a way to split the 8-ohm output signal, again 
    in-line, so I can send a 16-ohm output to each of my cabinets??? 

adv<thanks>ance,
alan

    
    Somone said a while back that impedance varies with frequency, how true
    but it can never drop lower than the DC resistance The formula is:
     Z = R + (XC + XL) where XC & XL are the capacitive & inductive
    reactances. If XC & XL become zero (as they do nearly at resonance) you
    still have R.  With valve output stages (as somone pointed out) you can
    run into problems if the impedance goes too high as it will be
    effectivly be reflected at the tranformer primary which is HT & will
    cause arcing in the primary windings.  So it is safe to put load
    resistors in cabs providing they'll take the power & youve got the
    values right....but what a waste!
    
    Cheers
    		Pete.