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My objective is to create the classic rock distorted tone/sound for my electric guitar all while maintaining relatively low levels of volume, so I can practice at home.

To that effect, I'm getting a 1W tube amp head that connects to a "4-16 Ohm" speaker. Rather than purchase an existing speaker cabinet, I've decided to create a custom one with a 12" speaker.

Having decided on cone size, there appear to be 2 major parameters for choosing a speaker: impedance and power handling. This is where I'd like some advice. For example, the 12" ET65 is listed at 65W and is available in 3 impedance configurations, 4, 8 and 16 Ohms. I've read that the power of the speaker must exceed that of the amplifier, so any power > 1W would be fine with this amp head. But what's the difference between 15W, 30W and 65W speakers on a 1W head? Also, I've read that a lower impedance speaker will be able to achieve higher volume on an amp that supports multiple impedances. Is this true? Will a 16 Ohm speaker force the amp to distort at lower volume than a 4 Ohm speaker? This would allow me to keep the volume lower.

With regard to my original objective of getting that classic rock sound at low volume, which would be the optimal combination for my 1W amp head: a 16 Ohm 65W speaker, a 16 Ohm 15W speaker, a 4 Ohm 90W speaker, or some other combination of those two parameters?

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    Could you add an example or three of "that classic rock sound"? There are many, many different guitar sounds used in music often called "classic rock". Many of them were actually created using pedals, instead of amp distortion. Or are you more interested in understanding how to get the lowest volume, and not as much about the exact tone? – Todd Wilcox Mar 14 '17 at 15:03
  • Man, at the risk of stereotyping, I guess classic rock to me would be ACDC. I'm not too concerned about sounding like a specific artist, rather, I'd like distorted dirty tone at low volume. – Megatron Mar 14 '17 at 15:05
  • Please don't throw rotten fruit & veg at me for suggesting this... but have you considered a modelling amp [& even headphones, you'd be amazed at the sounds you can get out of an iPhone these days with the right 'amp'] You aren't going to get the acoustic feedback you'd need to achieve a convincing 'pure loud amp, no pedals' sound if you're not generating enough real noise for it to affect the guitar as you're playing it. – Tetsujin Mar 14 '17 at 19:43
  • @Tetsujin Already have one. Looking to try something new - tube with a 12" – Megatron Mar 14 '17 at 20:22
  • Okay, just thought it worth a mention [phew, no rotten fruit ;) – Tetsujin Mar 14 '17 at 20:23
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I'm gonna hit you with three sections here, so please forgive my lengthy answer:

Disclaimer

All of these numbers are sort of approximations and simplifications. That's because audio equipment, like guitar amps, operates across a range of frequencies, and all electrical and electronic components (to a greater or lesser extent) respond differently a different frequencies. So we're going to proceed with an understanding that a "1 Watt amp" has a rated output of 1 Watts into some nominal load impedance.

We say "nominal impedance" because the impedance of a speaker is different at different frequencies, so first we pick an appropriate frequency at which to measure the impedance, and then we round to a reasonable power of two (4, 8, or 16 ohms). It's more like different speakers are in different impedance "classes" or categories.

Now that we have a nominal load, we can connect the amp to that load, and measure the power that the amp can drive into that load at a certain frequency. That's the rated power output. It's a rating of the approximate power in a specific situation. Assuming all your ratings are calculated in a similar way, you can compare different power ratings to get a sense of the power of different amps, but don't spend too much time trying to perform exact calculations based on rated power output or nominal impedances.


The Answer

Now that we agree that we are talking about simplifications and approximations, I can proceed without feeling like I'm being deceptive or imprecise.

When you double the load, you halve the power dissipation. If you know the impedance at which the power output was rated, then you can calculate the (approximate) output power at a different load. If we assume that the "1 Watt" amp was rated into a 4 Ohm load, then it will put out approximately 1/2 Watts into an 8 Ohm load and approximately 1/4 Watts into a 16 Ohm load.

Ok, good, so then the 16 Ohm load will be 1/4 as loud, right? No, not even close. First, dividing the power output by 4 is a drop of about 6 dB in power, which translates (except see the next paragraph) into about a 6 dB drop in acoustic intensity, but you need to drop by 10 dB to be half as loud.

But that's assuming that the different loads have the same efficiency, which is the measurement of how electrical power is turned into acoustic power. So it might be that the 16 Ohm speaker is so much more efficient than the 4 Ohm speaker that it's actually louder! But that's unlikely. Still, it might not be 6 dB quieter, or it could be more than 6 dB quieter.

TL;DR (too late!): The 16 Ohm speaker is generally your "quietest" option, but it might not be that much quieter when all is said and done. Better to choose the speaker that sounds best.


Further Reading

Some points to ponder:

  • Classic rock "distortion" is usually a lot less gain than most people think it is. AC/DC sounds pretty gainy, but it's really not.
  • Many classic rock distortion sounds are more about pedals than amps (Jimi Hendrix, Jimmy Page, Stevie Ray Vaughan), although some sounds are pretty much all amp (AC/DC being probably the best example).
  • Tone chasing will drive you insane, because even with identical gear you'll never sound the same as another guitarist. Using famous tones as guideposts on your way to finding your own tone is a good strategy though. Just don't get too hung up on re-creating something you've heard.
  • Opinion warning: That 1 Watt amp you linked to seems like more of a gimmick than a low-wattage, high quality tube amp design to me. It looks like the higher end manufacturers are creating all-tube low Watt designs using real power tubes (EL-84, 6V6, etc.) and coming in between 5 and 20 Watts, usually 12 or 15. On the more affordable end of this concept is the Ibanez TSA15H, which also has a Tube Screamer circuit built in (a gimmick but not a terrible one). Fender, Orange, and Engnater also have accessibly priced amps in this category.
  • If you don't already have woodworking skills and/or speaker cabinet building experience, you might find making a 1x12" cab is a lot more involved than it seems.
  • This is very helpful. Would you mind also addressing the concept of speaker wattage? So if I opt for a 16 Ohm impedance, what would be the difference between a 65W speaker and a 15W one? Thanks! – Megatron Mar 14 '17 at 15:45
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    @Megatron The rated dissipation of a speaker is not related to its efficiency or tone, as far as I can tell, besides the fact that it's related to the overall design. One area where there is a correlation is in magnet material. Modern, neodymium magnet based PA speakers are lighter, have cleaner tone, and higher power handling because of the magnet. As opposed to the Celestion Alnico Blue speaker, which has an alnico magnet, lower power handling, and richer tone. The nice thing about the 65 Watt speaker is the cabinet you build from it can be used for lots of different amps, not just this one. – Todd Wilcox Mar 14 '17 at 15:57
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You won't run a 15w speaker (let alone a 35w one) into distortion with a 1w amp. Try a cheap-and-nasty elliptical paper-coned TV-type speaker. A lot of the classic 'practice amps' used these, and they could distort nicely when pushed hard.

The Blackstar amp you mention has a 'speaker emulated output'. This may be perfectly satisfactory for practice at home, using either speakers or headphones. To get what you really crave you need to 'push some air'. Maybe there's no point in spending too much time and money looking for a low-volume substitute.

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    I don't think he's asking about distortion coming from the speaker. He's talking about masterless amps that distort internally but since they have no master volume there's a certain power level at the output where the amp is distorted, and he wants to know how to convert that electrical power to the lowest acoustic intensity (volume) so he can get distortion without loudness. – Todd Wilcox Mar 14 '17 at 15:00
  • Thanks for the reply. This is where I'm a bit confused. Are you referring to speaker distortion or amp distortion? If the amp distorts, would that not still be heard through the speaker? – Megatron Mar 14 '17 at 15:00
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    The classic rock distorted guitar sound comes partly from driving the amp hard, partly from driving the speaker hard. Or you can get pretty close to it with a clean amp and an effects box. In truth, it isn't really satisfying without some VOLUME. – Laurence Payne Mar 14 '17 at 15:03

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