I think it depends.

You can use the same wattage speaker as the amp's output (amp's are rated for their clean output), so you could use a Celestion Blue (say) which is 15 watt with your 15 watt. If you only ever play it clean you'll be fine, if you put the amp on 10 you will almost certainly blow the speaker at some point. That's where the 1.5 to 2x rating comes in, valve amps can reach 2x their max clean output when you crank them.

Some speakers compress more when you drive them harder (Alnicos like Celestion Blues) some crunch a lot (Greenbacks). Some people like that and some don't. Almost nobody plays their amps loud enough for this to be an issue anymore. Clean amp with pedals won't do it at all.

My advice, get a speaker that you like the sound of OR get one that you know will give you the sound you want out of your kind of amp and make sure the rating is above the output of your amp, this is usally the case today anyway, there's only a few 15 watt speakers I've seen, most are 25 or above and should be fine in your amp.

Speakers need to work harder to sound better but most importantly, amps just sound better turned up, because for humans (up to a point) louder sounds better.

+1 except for the Blue being unable to push 15W - two Blues in an AC30, traditionally. No problem.
Speaker wattage is mostly about the thermal load the speaker can take. Meaning that any other issues (compression, stroke too long for suspension and voice coil, cone breakup) are taken for granted in guitar speakers. Modern speakers have a higher power rating due to better thermal properties of materials used - my guess would be that the rest is no more able to handle the power than the vintage stuff.
Working a speaker hard, under that assumption, would still mean putting enough power to get the cone to break up. My advice would be the same as ez's: listen and choose, don't worry.

Thanks guys

That's quite useful info, thanks

one of the other things is that a higher rated speaker will be warmer and smoother sounding
With no real breakup...... from what I've read


Xx

Attila,

One thing to note is that you speak of wattage, but not impedance, it's also wise to make sure you buy the right speaker to match your output transformer. Traditionally larger transformers can drive double their load, so 4R with drive 8R, 8R will drive 16R etc, but if you've got a 4R driving a 16R it's going to effect your tone, try get as close to the intended load.

Cheers

Matt

Good post EZ - thanked.

I'm not a fan of speaker breakup - it tends to add a harsh and buzzy edge to the tone. So I usually prefer a higher rated speaker. I do like the compression of an alnico magnet though - it's a very natural and very musical sound to my ear.

Gearhead wrote: two Blues in an AC30, traditionally. No problem.

Actually the Vox amps are well known to be hard on the speakers if you crank them. It's just very few people get to crank them any more. I do also suspect that the newer Blues are actually higher power handling than the rating - I'm not sure, it's a guess. Maybe a Vox aficionado like Vic would know for sure.

Thanks Matt i didn't know that

but that makes sense cause you'll be changing the load Resistance on the power tubes

Peace

matta wrote:
it's going to effect your tone

What if the effected tone from mismatch is one that you like?
.....Treble too harsh?? then mismatch and see. Outcome is often quite naas..

bottledtone wrote: What if the effected tone from mismatch is one that you like?
.....Treble too harsh?? then mismatch and see. Outcome is often quite naas..

There are cleaner ways to do it. Driving up is safer, so as I shared 4R into 8R etc, but what it your amp only has a 16R tap, is it not better to drive into 8R then a 4R? Impedance hasn't been mentioned in this thread so I thought I'd bring it up in relation to wattage. Several speaker cabs today are 16R, driving a 3.2/4R out a 5W doesn't sound great from experience, but your mileage may vary.

I'd start with changing out tubes before dropping a ton of cash on a speaker with a higher/lower impedance than intended.

Cheers

Matt

damaging amp or speaker??
output transformer is a impedance matching component, the ohms of the speaker will reflect on the primary coil of output transformer. the valves can only push what they push. They are limited in the wattage they push. they can't push more, (unless voltage, phase inverter/preamp driving is increased)
In fact the amp will work less.
You can pull valves out (also only if fixed grid bias, or if cathode bias has each valve with its own RC) and have even one valve working in a 100watt intended for 4 valve.

Never run valveamp open, meaning don't pull speaker out. The valves will loose its load and this causes stress on transformer and valves. running a load smaller (4 into 8) will be safer than runing load higher (8 into 4) but these numbers are well in the safe zone. 8ohm over or under is perfectly SAFE.

Plenty amps have switches that remove output valves and as a result the optimal impedance is lost when done so, but not close to danger/failure etc.... (eg. Matchless DC30 has a switch that removes 2 el84's.....with all 4 valves it has a 4k load....when running only 2 valves it sees the same 4k and not 8k... this is the same as having an 8ohm speaker into a 16ohm out. Mesa do this too. even DrZ remedy. I do it on my 30watts)

Several amps also have multiple and selectable taps on the OT to allow you to remove the tubes AND reflect the correct impedance at the same time, but I see your point RE the impedance.

As shared originally, it's just something to consider, to a non-technically minded person it's a fair observation to keep the load within double the impedance of the amp output transformer. I've seen guys drive a 3.2R (orginal Tweed Champs) load into a 16R speaker and it sounds TERRIBLE, they blame the amp for sounding bad, but the tonal shift has little to do with the amps intended design and more the users application.

The damaged OT was in reference to NOT having a load, not the incorrect load, I didn't word it well, you did, and I believe it's a great addition to this thread since they all go hand in hand.

I've seen it time and time again with Marshall amps, in fact you'd be hard pressed to find original OT's in many older models because they were powered up without a load

Again your mileage may vary, just thought I'd point it out since no-one else did.

Cheers

Matt