(non pedal)speakers impedance and wattage

[garfo]

So, my amp works 55 watts at 4 ohms.
I can select from 4/8/16 ohms working operation.
If I use a speaker that is rated 35 watts 8ohms, does it take the load of the amp working on 8 ohms?

[angrykoko]

You want to match the amp and speaker.  If your speaker is 8 ohms then use your amps 8 ohm out.

Remember that if your using more than one 8 ohm speaker (in series or paralleled) then your now at 16 or 4 ohms depending on which.

Is it only a single speaker or is this a 2x or 4x configuration?

[garfo]

It's a single speaker, I always thought that an amp that has minimal load of 4ohms, in this case 55 watts, when working at 8ohms would consume less wattage, about half of it.
I want to replace the speaker in my Bugera v55 with a Jensen C12 series, the reason why I wanted to put a 35 watts it's because the 50 and 100 watts versions are double the price.

[artstomp]

...i guess its safe to install a 35watts 8ohm speaker on a 55watt amp rated at 4ohms...because the amp will see doubled resistance (in this case 8ohms) and will cut in half its driving power to somewhat around 25watts...meaning the 35w speaker can still handle the 25w power of the amp...but in practice it's ideal to double the power rating of the speaker against the power rating of the amp.

...but according to you, there is an impedance selector on the amp..4/8/10ohm...chances are it has an impedance matching output transformer...so i guess the amp still drives at 55watts regardless of the impedance selected...

...the trick i guess is to set the amp's impedance to 4ohms...so your 35w/8ohm speaker dont get busted...i hope it helps...

[angrykoko]

Yeah I would worry that it's not going to sound so good either with the impedance mismatched. 

But what your asking about is out of my league.  If no one here is able to help try your question over at ax84.com, some really sharp amp builders there.

[RobA]

You can blow the output transformer using mismatched speakers.

http://www.geofex.com/tubeampfaq/taffram.htm

[frankie5fingers]

Not all transformers are the same.  More importantly, not all Output Transformers are the same.  The taps on the OT are set up to use different portions of the OT windings.  Not all transformers are wired to accept 4, 8, and 16 Ohms; some only for two, or even one, of those loads.
Using those taps properly allows the OT to operate withing the spec it was designed for and to "meet" the resistance it "expects" from your speaker(s).
Is mismatching OK?  Probably... at least with no more than a 1-2 mismatch.
Is mismatching advisable?  No.
Is mismatching recommended?  Definitely not.
There have been cases of flyback voltages, heated windings, blown OTs, speakers and other mishaps arising from a mismatch.

[DutchMF]

Mismatching can be done without any damage, but ONLY when your speaker load is higher than what your amp wants to see, ie: a 4 ohm amp with an 8 ohm speaker etc..... If you reverse this, your amp is expecting to 'fight' the speaker more than the speaker is willing to give back, and thus most likely not being able to find a good place for all it's power, or..... fry! Hope this makes any sense.

See post below, I'm doubting now.... 

Paul

[RobA]

I actually believe that either direction is a bad idea, but from the page I linked to earlier [ here is the direct non-framed link http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#matchspkr ]

It's almost never low impedance that kills an OT, it's too high an impedance.

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.

This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.

[DutchMF]

Mmmmmm, this goes against all the stuff I ever thought was true.... I'll have to go and read up on this, as I'm in serious doubt now. I wouldn't dare say you (or your source) are wrong RobA, but I don't see the harm in a higher speaker load than the amp wants/needs. As said before, research time! In the mean time, scratch my previous post. Wouldn't want anybody damaging their gear because of me.... I'll be back!

Paul

[angrykoko]

I'm taking a leap here and guessing your speaker is blown?

Why not find a used speaker until you can afford the correct one?

Even ask at local music stores to speak with the amp repair person see if they have anything laying around that would fit the bill for cheep.

[frankie5fingers]

I just reread your post garfo.  You're not asking if it's OK to mismatch the head and speaker are you?  Am I correct that you want to know if the amp, rated at 55 watts at 4 Ohms is within the 35 watt range of the speaker if you run it at 8 Ohms?
If that's the question the answer will really depend on the amp and the speaker.  Many (most) guitar amps use an OT with at least two taps for a couple of speaker choices.  Some are 4 and 8 Ohms, some 4, 8, & 16.  A few have only one, like some old Fenders with a 4 Ohm tap only.  Anyway, that's guitar amps.  There are also some solid state types that allow for bridged (mono only) use of the 16 into 8 or the 8 into a 4 Ohm load.  Assuming you have a normal guitar amp, using a different impedance load won't result in a lowered wattage (by much anyway), your 55 watt head is going to be 55 watts, whichever OT tap you use to match your speaker.  The speaker, if rated at 35 watts may or may not be rated higher for a "peak" power spike.  In all likelihood though, if you see it's rated 35 watts, you don't want to exceed it often, or by much.  Without more info, that's all I have.

[RobA]

... I wouldn't dare say you (or your source) are wrong RobA ,...

Hey Paul, you can say I'm wrong any time you like. I won't mind . I'm not going to pretend that I know this stuff well or that I've actually sat down to do these computations myself. The only reason I linked to that page was to raise doubt. But, there are things we do know for sure. Everyone seems to be in agreement that a 2:1 factor is safe. My problem with that is that the amp designers probably thought that too. It's a common rule of thumb in engineering. Who knows how far they fudged in their designs. The next thing is that the amp is really most likely designed to sound best and deliver power at its most efficient point if the impedance is matched at both sides of the transformer. And lastly for me, transformers aren't really all that straight forward. I'd have to learn a whole bunch of theory before I was confidant that I had it down. Someday, I may try to tackle that. For now, given that the amp is most likely going to sound the best at its designed load and that output transformers are damned expensive, I'm going to error on spending a few more bucks to get the right speaker for the job.

Rob

[garfo]

Yeah, that was it.but apprently from what I understand a valve amp doesn't work as a solidstate amp in matter of power dividing from 4 to 8 ohms.So, anyway my conclusion was to just get a speaker rated 100watts, and problem solved

I just reread your post garfo.  You're not asking if it's OK to mismatch the head and speaker are you?  Am I correct that you want to know if the amp, rated at 55 watts at 4 Ohms is within the 35 watt range of the speaker if you run it at 8 Ohms?
If that's the question the answer will really depend on the amp and the speaker.  Many (most) guitar amps use an OT with at least two taps for a couple of speaker choices.  Some are 4 and 8 Ohms, some 4, 8, & 16.  A few have only one, like some old Fenders with a 4 Ohm tap only.  Anyway, that's guitar amps.  There are also some solid state types that allow for bridged (mono only) use of the 16 into 8 or the 8 into a 4 Ohm load.  Assuming you have a normal guitar amp, using a different impedance load won't result in a lowered wattage (by much anyway), your 55 watt head is going to be 55 watts, whichever OT tap you use to match your speaker.  The speaker, if rated at 35 watts may or may not be rated higher for a "peak" power spike.  In all likelihood though, if you see it's rated 35 watts, you don't want to exceed it often, or by much.  Without more info, that's all I have.

[DutchMF]

For now, given that the amp is most likely going to sound the best at its designed load and that output transformers are damned expensive, I'm going to error on spending a few more bucks to get the right speaker for the job.

My thoughts exactly!!

Paul