Amp Watts vs. Speaker Watts - Cab Questions

I've seen a number of posts about little 5w amps driving 2x12 or larger cabs, and much of the discussion centers around impedance matching. But what about watts? Can a 5 watt amp drive a 4x12 cab filled with a quartet of 75-watt Celestion G12Ts and get the speakers to break up? Or even four 25-watt "Greenbacks"?

Similarly, if I plan to build/buy a 1x12 or 2x12 cab and drive it with my 15w Peavey Classic 20 tube amp, and I'm mostly going for overdriven/distorted rock sounds (with some occasional cleans), is there some optimum wattage speaker combination I should be considering? Will two 25w speakers be "too much" for the little amp? They would likely be wired in series, if that makes any difference (the C20 needs a 16 ohm load.)

Thanks in advance.

Short answers: Question 1, yes, Question 2, no

Speaker excursion (aka "breakup") is dependent on more than just how much power is going to the speakers, and depends a lot on how much low frequency sound is being generated. If your downtuned and using a distortion pedal in front of the amp, 5 watts will easily drive a quartet of G12Ts into excursion.

The amp itself doesn't care what the power handling rating of the speakers. It's gonna put out what it puts out. It will matter a lot if the impedances are mismatched though, since the power transformers in the amp will expect a certain load.

The overall volume is going to depend on how efficient the speakers you use are. Wes covers this pretty thoroughly in this thread:

http://forums.guitarnoise.com/viewtopic.php?t=13825&highlight=efficiency

Thanks Moonrider. I digested most of that, but let me ask a more basic question which I didn't see addressed:

If all else is equal, then will a speaker rated at 75w have substantially more pre-excursion headroom than one rated at 25w? Assume the same low-wattage amp, same frequency range, same SPL ratings on the speakers, etc.

I should also say, I'm not concerned with getting maximum volume, but I am interested in getting more breakup from the speakers for the given power my amp can put into them. So it seems logical that a lower wattage speaker will break up sooner than a higher wattage speaker, again with all else being equal (which I know is never the case, but theoretically.)

Also, if this is true, then if one mixes speaker wattages in a cab (say a 30w and a 60w in a 2x12 cab), then won't the 60 watter be relatively clean even as the 30 is pushed into excursion?

Yours is an interesting idea Slej; I've often wanted to get speaker distortion added to amp distortion - like the old time guys playing at cranked volumes. Even considered making a little speaker box with low watt 5" or 3" speakers for recording.

Hey Demo, I found some good info about that on this page:
http://www.amptone.com/speakers.htm

Hey Demo, I found some good info about that on this page:
http://www.amptone.com/speakers.htm

Ted Weber also has an excellent FAQ here, quite a few of the questions dealing with speaker breakup and the different factors that cause it.

http://www.webervst.com/sptalk.html

LOL I was just reading that this morning! Haven't gotten through it all yet, but it makes me think twice about building my own cab out of spare wood, vs. buying one from a reputable dealer. Weber's got some interesting low-wattage speakers, too. :)

I built mine but I had a speaker to use, didn't need to pay for it. I also decided an open back cabinet was the way to go. Closed back was dependant on the size and shape and all as to the frequency response and overall sound. I read that for an open back it really didn't matter. Open back was idiot proof so this idiot made one. :P

I also decided an open back cabinet was the way to go. Closed back was dependant on the size and shape and all as to the frequency response and overall sound. I read that for an open back it really didn't matter. Open back was idiot proof so this idiot made one. :P

That's very interesting. I ended up getting a 1x12 cab that is convertible; it's closed back with a separate top 1/3rd panel that can be removed to make it semi-open. (You can just make out the separate panel in the 2nd picture.)

I'm going to put an Avatar Hellatone 30 in it; I've read very positive reviews about this speaker, and several people have paired it with amps similar in size and power to the C20 with great results.

try getting a baby to push a car
thats like trying to get a 15 watt amp to run through a 400 watt cabinet
its impossible it drowns out the sound nearly impossible
you will get more sound running the amp on its own

try getting a baby to push a car
thats like trying to get a 15 watt amp to run through a 400 watt cabinet
its impossible it drowns out the sound nearly impossible
you will get more sound running the amp on its own

That's not quite true. What you describe isn't the effect of a cab's power rating, but of a mismatched impedance.

You might serve yourself well to read through the information in the links posted in this thread. It'll help you understand what your own equipment is doing.

Well as it evolved I'm only trying to get a 15w amp to drive a 30w cab, so the baby easily pushes the car. :)

I do still have my doubts on how a 300w 4x12 box would sound, but for my needs, the 15w PV amp + new cab + 30w Hellatone speaker is a great combination. No more boxy mids; warm bottom end; and the treble is bright but not harsh like before. Amazing what a difference a speaker can make.

I have absolutely no use for overdriven speaker distortion. Generally sounds nasty. I'd rather hear what's coming out of the tubes with only modest "coloration" by the speaker's characteristics. (There's always some of that.) Speakers rated for considerably more than the amp's power are often more sensitive (louder) than lower power rated speakers. And you're not going to blow them by cranking the amp. The only downside to them is increased weight from the big magnets.

Shouldn't amps be rated in decibels? Why do they use wattage? It seems like a smaller rated amp could push more sound around with a bigger speaker than a higher rated one with a smaller speaker, as long as the rating and impedance was right. Since a 40 watt is only a little louder in decibels than a 50 watt, why not just push a couple 12" speakers with the 40 watt amp? I'm not even sure if decibels is a good measure of how much air larger speakers can move around to fill in a large room? :?