40ish watt amp through 30 watt speaker, dangerous?

[tunakdude]

Hi,
I'd like to run a 2 x el34, plexi-like amp through a 1x12 cab at 8 ohms... trouble is, the speaker that i like best for this kind of amp is the celestion g12h30, and those are rated at 30 watts... but from what i hear often, i get the impression that these ratings might be kind of arbitrary, so i'm wondering, do you think that i'm seriously endangering the speaker by doing this? will i blow it out? i usually don't turn the volume all the way up, but, as you know, by the time the volume knob is at half, it's about as loud as the amp gets...

can anyone offer any advice?
thanks

[R ski]

If the speaker is worthy, here is a sure test.

Wail the amp to speaker for a short session

Stop and if your nose is good, sniff near the voice
coil. If you smell a acrid ordour, then your speaker
will fry in the long run.

[tonezoneonline]

No offense intended but you must be from Harmony Central R ski?
Well maybe just a little.

[Steve Conner]

I don't think R ski's reply is quite as daft as it sounds. (no offence R ski) The output power that actually ends up coming from an amp is dependent on so many things. It could be less than 40w. Or it could be considerably more if you dime the thing.

But then again, when you play, you probably don't wail continuous loud notes for a whole hour, so the speaker gets a chance to cool down in between guitar solos And, the resonance of the cabinet and voice coil inductance can cause the speaker to show up a higher impedance and not accept all the power the amp is trying to shove into it.

So I think you're as well just to try it, and if it dies of overheating, go and get a higher powered speaker, or a second 1x12", or modify your amp for lower power, or whatever. I had a 50 watt amp with 2 EL34s and it turned out that changing them for 6L6s knocked the power down to about 30w. It also changed the tone of the amp greatly, but in a way I liked a lot. (I'm not too sure why it did that, given that most amps with a pair of 6L6 tubes are rated at 60w.)

[Ray Ivers]

Well, I'll tell you one thing... the original Greenback reissues are definitely 25 watt speakers IME. A '74 Marshall 100 watter destroyed one of mine (in a brand-new 4-12 cab) with three loud - but not dimed - power chords. Based on this experience, if it were my G12H30 I wouldn't put more than 20 watts though it - but YMMV.

Re R_ski's post - if anyone doubts that the nose can be used to identify speaker problems, try opening up an old Marshall 4-12 cab to replace a blown original speaker between the 3rd and 4th sets - or a W-bin compression chamber to replace a fried 600W 18" near the end of 'Techno Night".

Ray

[Mark Hammer]

Speakers blow for a number of reasons. One reasons is simply mechanical duress. If you've seen "Back to the Future", you've seen the part where Michael J Fox turns up, plugs in, and blows the cone off the thing. A sharp high amplitude transient can provide mechanical stress that exceeds what the unit can handle. Doesn't happen often but it can happen. Obviously, running a 40W amp into a 30W speaker will never provide such stress if the volume control is set to 1 and you feed the amp normal signals.

A second means of destroying a speaker is fuse-like action. To be light enough to move that fast, the voice-coil wire has to be thin. Pump enough current through a wire and it heats up like the wiggly stuff in the walls of your toaster. Pump more current than that, and it goes pffft like a fuse. In some respects, a speaker voice coil IS a fuse set up to barely avoid burning out.

The third route is friction-based heat. Normally, the gap between the voice coil and magnet is such that the spider holds the voice coil away from the walls on the inside of the magnet structure. The voice coil goes back and forth but never touches those walls. Of course, the farther away from the walls, the less efficient the speaker is, so if you want something that responds well, you have to take the risk of running that voice coil closer to the magnet. Many companies will fill the gap with somethng I don't quite understand called ferro-fluid, which can serve as a kind of lubricant and also dissipate any heat build-up that comes from touching/rubbing the sides of the magnet structure. But there are limits to what can be done, and the further out the cone moves from the resting point, the less effectively the spider guides its movement. Push it harder and you WILL get rubbing. The extent to which that rubbing translates into heat build-up and risk to voice-coil is a function of things like frequency (more high frequency content means more heat buildup in a short period by virtue of more friction ), heat dissipation strategies within the magnet structure or around it (like heat fins), and ultimately the composition and wire gauge of the voice coil itself.

So, it is certainly possible to use a 30W-rated speaker with a 40W rated amp, but obviously you have to do the things that will avoid all three types of stress to the voice coil.

If it is the case that the amp sounds great when pushed a certain way, as does the speaker, but the amp push and speaker push are incompatible, then maybe the thing to consider is some type of power attenuator on the output. It doesn't have to be much. You might want to consider some of the power attenuators out there like the THD Hot Plate or Scholz Power Soak. If your output transformer has a tap for 8R and another for 16R, consider picking up a suitable 8R power resistor, wiring it up in series with the speaker, and using the higher impedance tap.

[Steve Dallman]

My girlfriend runs her Mesa Dual Rectifier into an old PV PA cabinet which is ported and now houses a Vintage 30. Even playing outdoors without the amp miked, it's held up well. I will mention that we don't play at ear splitting levels though. We're both too old for that and desire to keep the hearing we have left.

She also runs two Randalls on occasion. One is a 50 watt tube head and the other a 100 watt solid state. She uses a 4X12 with two greenbacks. The bottom speaker openings are left empty as per Kevin O'Connor's "detuned cabinet" theory and it sounds better than it did with 4 speakers. She only uses one or the other head. The speakers have held up for several years now.