Speakers, Ohms and Wattage? I'm clueless....

paulnb57

My first ever Amp head is in the mail, Ive only ever had combos......Im utter at a loss as to what power rating and ohmage i need in a speaker cab....
Its a Jet City 20 watter head into 16 ohms.......

I get for one speaker it would need to be 16 ohms, dont know the power rating though, or even if thats the right term?
But I'm thinking of making my own 2x10 or 4x10 cab
So if the power is shared between two or four speakers Im lost, utterly totally and completely...

Is there some simple to understand reading somewhere so I can gen up....any advice welcomed!

Cheers!

ICBM

If the amp has only a 16-ohm output then you need a 16-ohm cabinet. (ie 16-ohm speaker, if it's a single-speaker cab.) The power rating can be anything above 20W, although 30W or more gives a better safety margin. There's no upper limit, although often a very high-powered speaker will sound a bit stiff and sometimes dull when driven at less than 20W. (Very much less - bear in mind that even for a 20W amp, turning it down to 'home volume' will give a power output of less than 1W usually.)

I'm pretty sure the Jet City 20W amps have both 16 and 8-ohm outputs though, in which case you can also use an 8-ohm cabinet.

If you're using a cab with two speakers, you need to combine the impedances in series (the cab will be double the impedance of each speaker) or parallel (cab half the impedance of each) or in a four-speaker cab, usually series-parallel (cab impedance the same as each individual speaker).

For multi-speaker cabs, the power is shared equally between the speakers if their impedances are the same regardless of the wiring scheme, so the cab power rating is the number of speakers multiplied by the lowest-rated speaker, not the sum of the ratings.

You *can* mix different impedances, but the maths becomes more complicated so usually better avoided unless you have a special reason to!

professorben

Ohms law is your friend here. 

Sporky

Ohms law is a friend to no-one.

A passing acquaintance at best, and you're glad once you've passed.

paulnb57

Amp output
1x16 ohm
or 
2x 8 ohm

So could I plug in one 8ohm cab? Or must it be two....clueless!!

ICBM

paulnb57 said:

Amp output
1x16 ohm
or 
2x 8 ohm

So could I plug in one 8ohm cab? Or must it be two....clueless!!

You can plug in one 16-ohm cab (the 16 ohm jack), one 8-ohm cab (either 8-ohm jack) or two 16-ohm cabs (both 8-ohm jacks - because two 16-ohm cabs gives an 8-ohm load in parallel) - if that makes sense!

Beexter

@paulnb57  if you search online for the user manual for the Mesa Express amps,  there's a really useful few pages at the back showing what is safe and what isn't along with a load of diagrams which may well help you. 

paulnb57

Thanks folks, desperately clueless!

DJH83004

Paul, to keep things nice and simple, why not make yourself a 2x10 cab using two 25watt 8 ohm speakers in series, which would give you a single 16 ohm load. In future you could always make yourself an exact duplicate and as IC says plug them both into the 2x8 ohm sockets (2 x 16 ohm in parallel will give you 8 ohms)

sm55onl

This was posted on a previous, similar thread...hope it helps
http://blog.hughes-and-kettner.com/ohm-cooking-101-understanding-amps-speakers-and-impedance/

sm55onl

Also, this (pages numbered 19 to 24.....or pdf doc page numbers 23-28).....as noted by Beexter, above.
http://www.mesaboogie.com/media/User Manuals/ExpressSers.pdf

paulnb57

Thanks folks all appreciated...

paulnb57

Just be clear in series is.......
Oo bloody fuck! Found it in the Mesa literature, Look! Over here! Old bloke, learning shit!

olafgarten

This is helpful as well
 https://www.jensentone.com/wiring_diagrams

paulnb57

Thanks @olafgarten ;

Maynehead

On a side note, if you've never had an amp head before, NEVER to turn the amp on without a speaker plugged in (more specifically never take it off standby, but safer just not to turn it on).

Otherwise there may be smoke.

paulnb57

Thanks @Maynehead I knew that, but its worth repeating!

Smoke is bad!