# Is a 75W speaker okay to use with a 50W tube head?



## Humbucker (Feb 18, 2007)

I found a Peavey 112SX speaker cab at a pawnshop, it has one 12" Sheffield speaker in it, this speaker is rated 75W. 

Will this speaker be okay in handling a 50 watt tube head?


----------



## Wild Bill (May 3, 2006)

Humbucker said:


> I found a Peavey 112SX speaker cab at a pawnshop, it has one 12" Sheffield speaker in it, this speaker is rated 75W.
> 
> Will this speaker be okay in handling a 50 watt tube head?


Sure! You've got a speaker rated to handle up to 75 watts and a head that will only put out 50 watts.

Actually, true techies can argue about how things are rated but in the real world you can just take things at face value and go for it.

Have fun!

:food-smiley-004:


----------



## Humbucker (Feb 18, 2007)

Thanks for your reply!

I've always been under the impression that a 50 watt tube amp puts out quite a bit more wattage when it's cranked then its 50 watt label would lead you to believe. I would never connect a 50W speaker to a 50W tube head.

But I figure that this 75W Sheffield speaker with one of my 50W heads will be a perfect match IMHO. BTW, the speaker's impedance is 8 ohms and I have 8 ohm speaker taps on both of my 50W tube heads. I guess that I'll be fine with this arrangement.


----------



## Wild Bill (May 3, 2006)

Humbucker said:


> Thanks for your reply!
> 
> I've always been under the impression that a 50 watt tube amp puts out quite a bit more wattage when it's cranked then its 50 watt label would lead you to believe. I would never connect a 50W speaker to a 50W tube head.


I would! 

It's true that tube amps sound louder than solidstate amps. The power actually doesn't change. The power rating of an amplifier can be measured a number of different ways. Music equipment is normally rated in RMS, which in layman's terms is sorta an average.

Speakers are relatively simple devices. You have a coil of wire suspended in a magnetic field, with the coil fastened to a paper cone to move more air. Wire doesn't burn out the instant the audio signal exceeds the rating. What happens in a voice coil is that the power from the amp flowing through the voice coil wire will cause a bit of heat in the wire. This is normal. Any wire carrying current will get warmer. It also radiates that heat away into the surrounding air.

If you crank too much power through the voice coil for the size of its wire it will heat up faster than it can radiate it away. If things get too out of hand the wire will burn out.

This is not an instantaneous thing! It takes a while for the wire to get hot enough to burn out. The signal from making music is a complicated wave with lots of peaks and valleys. During the valley minimums the power ebbs and the wire is cooling. Peaks add heat back into the system.

Good manufacturers of speakers will rate them by running flat out power into the speaker, with little or no peaks and valleys. With your 50 watt speaker that would be a continuous 50 watts of power. No chance for coil cooling, unlike your guitar signal. This is the most harsh way of measuring the power rating you can do. A guitar signal would ALWAYS be less punishing on the speaker!

In short, a proper rating of 50 watts means the speaker can actually handle more power, if its a music or audio type signal. So you should trust the 50 watt rating.

As to why tube amps sound louder, it's not so much that they put out more watts. They may not at all. It's just that they often will handle peaks much better than transistor amps. You hear those louder peaks! The average power is much less. Transistor amps aren't so good with such peaks. Transistors tend to stay clean until a peak is too strong and then they just harshly clip it off. You don't get the full power peak but rather "fuzz" at a lower volume. Tubes will either follow the peak or if they can't they tend to softly "round off" the top of the peak, in effect compressing the signal. That makes it sound thicker and louder 'cuz the average power of the wave is still higher than with simply clipping off the top of the peak like with transistors.

This is part of a phenomenon called "psychoacoustics', or the science of how the human ear works. Tubes suit our ears style better than transistors. For a given measured power a tube amp will sound 2-3 times as loud as a transistor amp! The power is really still the same and you can go ahead and match up a 50 watt head and a 50 watt speaker no problem.

Except for country music! A continuous whine or moan is equivalent to a strong continuous high power peak and even if the voice coil doesn't burn out it may decide to burn itself out to escape the torment. 

Have fun!
:food-smiley-004:


----------



## Humbucker (Feb 18, 2007)

Wow, that's a LOT of good info there Wild Bill, you really know your stuff, thanks!

BTW, you aren't the same Wild Bill over at the Ampage forum are you?

Thanks again!
:smilie_flagge17:


----------



## Wild Bill (May 3, 2006)

Humbucker said:


> Wow, that's a LOT of good info there Wild Bill, you really know your stuff, thanks!
> 
> BTW, you aren't the same Wild Bill over at the Ampage forum are you?
> 
> ...


Yeah, that's me!:smile:

I haven't been active there much over the past year. I've only got so much spare time and when I discovered this forum I thought I should support a Canadian forum as my first priority.

:smilie_flagge17:


----------



## I_cant_play (Jun 26, 2006)

I was wondering about the same thing too recently. So just to make sure, a dimed 100watt amp into 2 50w speakers wouldn't blow the speakers? I always assumed it would because I saw on the celestion website, when you look up the alnico blue which is rated at 15watts, there's a note beside it that says "vintage speaker, careful with volume levels". Immediately I thought of the AC30 which is a 30 watt amp going into 2 15 watt speakers and assumed that it's because of the exact match that there's some risk. Is there some other factor I'm missing here? (like the whole "vintage speaker" thing for example?)


----------



## Humbucker (Feb 18, 2007)

Cool!

You, along with a lot of other techies over at Ampage aswered a LOT of my dopie questions about amp repair over the last few years. Thanks again!


----------



## Ripper (Jul 1, 2006)

I play a 50 watt amp into a 30 watt speaker, I have used it for years. It gets a workout sometimes too but has never failed. Like Bill said speakers tend to be very hardy beasts. Wattage is not only one thing to consider with speakers. 

You need to take a look at the the sensitivity spec for the speaker along with the rating it has for watts. The sensitivity is more or less how much sound comes out for a given input. Most manufacturers list their sensitivity specs in dB (usually somewhere between 82 to 105 dB). They determine this dB rating by driving the speaker with 1 watt of electrical energy and measuring the acoustic output at a distance of one meter. 

Basically what this means is that you can have 2 speakers with different wattage rating and sensitivities that will actually have nearly identical output.
High wattage speakers in general usually have a lower spec rating and lower wattage speakers have a higher spec rating (more efficiency really). Speakers that are more efficient will produce more sound with a smaller input.
Don't buy speakers on the wattage alone. Use your ears and listen.


----------

