Amps Speakers & Ohms - An 8Ohm minimum amp and a Marshall 19

[sameoldthing]

hey
i have a Roland GC-408 amp which runs at 60W normally and supposedly 80W with a speaker box attached. I bought a Marshall 1960AV cab the other day and to my horror i found that it could only run 8 ohms in stereo (and my amp is mono). There is however, the choice of 16 Ohm Mono or 4 Ohm Mono, and the 16 ohm is not loud at all.

Is it safe to run the amp into the 4 ohm plug? the cab can take 280W, would the amps power double with lower impediance or would that just wear out the transformer?

OR, should i try to split the amp output into two 8 Ohm sets to run into the stereo plugs on the cab?

[cerealk]

Hi
Until very recently I was running a Mesa 2.90 into a Marshall 1960 AV in stereo.
Mesa output is 8 ohm and these went into either side of the cab (switched to stereo)

I have also run one 8 ohm output into a 16 ohm Marshall cab with no problems. *I wouldnt advise putting a 8 ohm amp output into a 4 ohm setting on a speaker though.

Cheers

Simon

[CrossingStar]

sameoldthing on 5:26 am on Mar. 19, 2002

OR, should i try to split the amp output into two 8 Ohm sets to run into the stereo plugs on the cab?

When the cab is in 8 ohms stereo it's 8 ohm per side which equals 4 ohms, when used as you described. Same as running it in mono 4 ohms. If you run it with the cab at 4 ohms I guess you would wear out the transformer eventually. Running with the cab at 16 ohms is one option, but you said it's too quiet. Another is to set the cab to 8 ohms stereo & just plug into one side of it. Not using the cab to it's full potential but at least you're safe.

[7 Dying Trees]

Rule of thumb:

Always make sure your speaker is rated the same or higher than your amp.

Tube amps(power stage):
Try your hardest to make sure the impedances match, if you can't, go with above rule unless you want to replace all your transformers and tubes (the blowout won't happen immediately, but one day...)

Transistor amps(power stage):
Always make sure that your speaker is of the same or higher rating. With transistors the load doesn't matter that much, as long as it's more.

Also remember that if you run two cabs (ie, a stack), that the impedance is half that of the cab rating (make sure you use cabs rated at the same impedance, otherwise the calculation becomes trickier).

[lucidness]

Ive always struggles with remembering how to properly hook up a speaker cab to a head. *Ive also found that if you understand what is going on youll remember it better. *Maybe this will help a few of you guys out if you find yourself getting confused like i often do.

when speakers are wired in series the impedence is simply added together. *Suppose you have 2 8 Ohm speakers in series. *8 Ohms + 8 Ohms = 16 Ohms.

when speaker are wired in parallel follow this equation:

1/Req = 1/R1 + 1/R2 + .....

So suppose you have 2 16 Ohm speakers in parallel the total impedence is:

1/Req = 1/16 + 1/16 * ==> 2/16 * * *now take the reciprical to find R equivilance (Req) which gives us 16/2 or 8 Ohms. *

on the back of your amp when you plug a cab into ONE speaker output you have set up your cab in series with you head. *when you use MORE THAN ONE inputs you have set up your cab in parallel with your head. *That is why when you plug 2 8 Ohm cabinets into 1 head the total impedence is 4 Ohms. *

Now you really dont need all this math stuff to hook up 2 identical cabinets but say you have a 16 Ohm cab and a 4 Ohm cab and you want to plug them both in to the same head. *

1/Reg = 1/16 + 1/4 *==> 5/16, taking the reciprical again yields 16/5 ==> 3.2 Ohm! *notice this is actually less than 4 ohms. *this is very important because whenever you have 2 speakers in parallel the total impedence will be less than the smallest speaker ohm value. *

And as stated in other posts be sure the total impedence is the same or higher than what is set on your head. *other wise you amp will overheat. *Another thing to consider is if possible run your head at the highest Ohm value possible. *youre amp works harder at 4 Ohms than at 16 Ohms. *You dont get as much power but you amp will last longer. *(note: having the head at 4 ohms and the speakers at 16 ohms will give you less power than your head and speakers both set to 16!)

hopes this helps

cody * *