# how is wattage calculated in speaker cabs?

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I have a friend with an unknown speaker cab (4x12" speakers)
the impedance is 8 ohms across the cab.
I think two speakers in series are wired in parallel.
Which would make sense thay they are 8ohm speakers.

Two questions
What I'm wondering is there anyway to work out the wattage rating of
a speaker or the whole cab?

If I am able to find out the wattage rating of one speaker what is
the math involved in calculating what the whole cab would be.

Re: how is wattage calculated in speaker cabs?

"Richard Morley"

**  Step 1:   Have a look at the speakers.

** Step 2:  Multiply by 4.

...  Phil

Re: how is wattage calculated in speaker cabs?

Thanks Phil

Couple of other things, if the cab is 300w then what do you think is
an appropriate sized guitar tube amp in order not to drive the
speakers too hard?

I've read that some people believe that using a low wattage tube amp
(e.g. 5 watts) on high rated speakers is not a good idea.

Assuming that the tube amp is robust and when turned up does not
overheat or damage any components is this true?

Would a small tube amp be fine?  Also for distortion from my
understanding as long as your driving the tubes and not overdriving
the speaker your fine.  Is that correct if not what are the best
rules to go by?

Re: how is wattage calculated in speaker cabs?

"Richard Morley"

** 100 to 120 watt rms rated amps -  ie with 4 x EL34s or 6L6s.

Cos an over driven tube amp produces far more *effective power* into guitar
speakers voice coils than bench testing with dummy loads indicates.

** Works fine,  IME.

Makes a very small amp sound massive.

** See above.

** Look at what others are *successfully* doing and don't stray too far away
from that.

Erring on the side of caution is never a bad idea.

Rules to live by.....

....  Phil

Re: how is wattage calculated in speaker cabs?

You might be able to have some confidence in the nameplate if the speakers
are speciality musical or hifi, from a *reputable* manufacturer, but I
recall endless discussions in times past, in mags such as EA, about what
power actually means. There is 'peak' power, which is based on peak values
of voltage and current, IHFM power rating, which IIRC is the power that an
amp can deliver as a short transient, while the supply caps are at full
charge, and no doubt plenty of others. The hifi industry is stacked with
trickery and junk being sold as prestige equipment. I bench tested a 50/50
watt rated solid state amp a while back that a guy was trying to use as a
music amp, it was struggling to deliver 3 watts RMS continuously.