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Post subject: Replacement Speaker Question
Posted: Sun Feb 19, 2012 9:22 am
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Looking at a replacement speaker for an 18 watt amplifier and I don't understand the impact of a few variations.

My current speaker is 50W and 8ohm.

I could obviously match that and figure the replacement will do fine but how would choosing a different wattage or ohm speaker impact the sound and amp?

Example: What if I installed 35 watt or a 16 ohm as a replacement speaker?


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Post subject: Re: Replacement Speaker Question
Posted: Sun Feb 19, 2012 10:33 am
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The lower the wattage of the speaker, the more easily you get speaker distortion. For some, speaker distortion is desirable, so they can use a speaker that is close to the wattage rating of the amp. Obviously, if you want clean sound with headroom, use a higher wattage speaker. At some point, increasing the wattage of the speaker will not help any more. Example, a 50 watt speaker is probably plenty for an 18 watt amp for little-to-no speaker distortion.

The impedance of a speaker (ohms) is matched to the amp to get the most power from the amp, not always the best sound as taste varies, but usually loudest. Sometimes a mismatch, such as using a 16 ohm speaker on an 8 ohm amp, or an 8 ohm speaker on a 16 ohm amp, can result in a more pleasing tone, and no harm in trying different ohms values.

One other parameter to consider is speaker efficiency. Rated in db, like 98 db or 103 db, indicates how efficient the speaker is, and the higher the number, the more efficient. So, in the same amp, all else being equal, a 103 db speaker will give you more sound than 98 db speaker.

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Post subject: Re: Replacement Speaker Question
Posted: Mon Feb 20, 2012 9:36 am
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To anyone with an all-valve amplifier:

STOP!

DO NOT mis-match impedances! DO NOT DO NOT DO NOT! You will blow out your Output Transformer, possibly one or two of your output tubes.

If the cab puts out an 8-ohm speaker path, you need to match it with an 8-ohm speaker load.

Yes, you can go with a higher-wattage speaker. The Wattage calculation is an estimate of how much output power the speaker can safely handle and perform reliably. You can go a bit over that if you have to, but you run the risk of doing small damage to the speaker with a current spike and may void any warranty you might have on said speaker. A good rule of thumb for most guitar players is to get a speaker than can handle 150% of the RMS power rating of your amp. If you're in a 4x12" speaker cabinet, then you'd want each speaker to be roughly 1/4 of THAT.

Example: I have a Marshall JVM410H, a 4-channel 100-watt RMS all-valve guitar amplifier head unit that I pair to my Marshall 1960A Lead speaker cabinet loaded with Celestion G12T-75 speakers. The cab is running in a matched 16-ohm configuration and has a total power handling rating of 300 watts. I could safely use a 150-watt amplifier (like a Mesa Triple-Rectifier) on that cabinet. Each speaker can handle 75-watts safely.
If you have a 40-watt amp, you could get away with a 60-watt speaker.

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Post subject: Re: Replacement Speaker Question
Posted: Mon Feb 20, 2012 5:35 pm
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FFX, this is not a universally true statement. The impedance of any speaker is only a NOMINAL value, found usually with a pure 1kHz sine-wave @ 1watt. The speaker's impedance throughout the frequency range will vary from make-to-make and cab-to-cab. It's partially why speakers sound so different. Anyhow, here's a little blurb from RG Keen. Who knows a lot more about these things, than me:

" For tone? Try it several different ways and see which you like best. "Tone" is not a single valued quantity, either, and in fact depends hugely on the person listening. That variation in impedance versus frequency and the variation in output power versus impedance and the variation in impedance with loading conspire to make the audio response curves a broad hump with ragged, humped ends, and those humps and dips are what makes for the "tone" you hear and interpret. Will you hurt the transformer if you parallel them to four ohms and hook them to the 8 ohm tap? Almost certainly not. If you parallel them and hook them to the 16 ohm tap? Extremely unlikely. In fact, you probably won't hurt the transformer if you short the outputs. If you series them and hook them to the 8 ohm or 4 ohm tap? Unlikely - however... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

Extended A: 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.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.

For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes and leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load. "


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Post subject: Re: Replacement Speaker Question
Posted: Mon Feb 20, 2012 6:34 pm
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Excellent post BMW2002Ti

I have found the sweet spot for my Twkr is 16 ohm amp setting into an 8 ohm 1x12. The sound is crisper and clearer, more articulate and kind of springy sounding if that makes sense, just great. I am also reminded of all of those little C600 pushing 8 ohm ext cabs quite nicely.

Rare (ever?) to see a blown OT around here, and likely would be due to using amp without a speaker connected, or even a blown (open) speaker. I can certainly understand the concern for something electrical, but there is so much myth around guitar amps and speakers, and I would bet that many of the supposed Marshall "problems" might wrongly be attributed to an impedance mismatch. People tend to blame that which they don't understand, and impedance matching is a perfect example.

Image

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---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Replacement Speaker Question
Posted: Tue Feb 21, 2012 8:25 am
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You should know the only "tap" I know about is the one at my local establishment that says Yuengling Lager on it.

A little more background is probably in order. I obtained one of those Crate V18 amps dirt cheap and find the sound is not pleasing. I've replaced the preamp and power tubes already and didn't find much improvement and since the speaker appears to be really cheesey (has some kind of plastic coating on it) thought speaker change would be something a novice like myself could do next. Amp is very dark and needs something to brighten it up. Just looking to move it in the right direction...and not willing to spend a lot. Probably $50 max and would love to find a used speaker in the $30 range that would liven it up a little.


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