Decibels (db) are a ratio between two power levels. It is a very common measurement used to describe loudness since a very large amount of ratios can be described using the decibel. It’s also a good thing to keep in mind that a decibel is one-tenth of a Bel. A full Bel is perceived as twice as loud to most people. So, in order to compare two different output powers, we must first make sure both powers are expressed in wattage (or the same unit if comparing voltage or SPL). The basic equation for this is:
Where P1 and P2 express the output power of the two different sources. It’s also very important that both output measurements are taken with the same load on both amplifiers. Taking 30 watts at 8 ohms is different than 30 watts at 16 ohms, so in order for us to get a proper figure out, both powers need to be measure with the same load on the output.
Let’s take a look at a 50 watt head vs. a 30 watt head. First we take 30 divided by 50 which gives us .6. Then we take the log of .6 which is .222 and multiply it by 10 which gives us 2.22 db. So a 50 watt amplifier is only 2.22 db louder than a 30 watt amplifier. For reference, twice the output power is equal to a 3 db change, and 3db is barely enough for the human ear to discern the change.
One more for fun, we’re going to compare a Fender Deluxe Reverb that has a nominal output of 22 watts to a Marshall 100 watt Super Lead. Now we all know the Marshall is louder but after running these through the equation we find that the Marshall is only 6.576 db. Considering that 10db is a nominal figure for what most people hear as twice as loud, then the Marshall is not even twice as loud as the Deluxe. Surprised?
0 db increase = same power
3 db increase = 2 x power
10 db increase = 10 x power
20 db increase = 100 x power
30 db increase = 1000 x power
40 db increase = 10,000 x power
Choosing an Amplifier Based on Wattage
Now that some of the technical jargon is explained, let’s talk amps. There has been a trend toward lower wattage tube amplifiers in the past few years and for good reason. Let’s face it; most of us are not playing in front of 5000+ people on a regular basis, if at all. That coupled with the trend of a lot of modern music to move away from the arenas and into smaller, more intimate venues has changed the needs of a lot of professional guitarists. If you’re rolling into the club that seats 100 people with a 100 watt Mesa stack, you’re probably going to get some strange looks. The fact of the matter is, most 15 watt amplifiers are going to produce enough sound to be considered LOUD.
So why are there 100-200 watt guitar amps?
First of all, back in the heyday of rock music, the sound systems that were used were not as high quality as what we have now and did not reproduce sound quite as well. A 100 watt amplifier was necessary in a lot of cases to carry the guitar sound in larger venues.
Second of all is clean headroom. A 15 watt amplifier and a 30 watt amplifier are going to be really close in volume, they have approximately a 3 db difference in level which is just at the level that the human ear can hear the difference but the 15 watt amplifier is going to start saturating and distorting earlier.
For someone who uses the amplifier for overdrive and the volume of the guitar for attenuating between clean and dirty, a 15 watt amplifier is going to be loud enough for most venues, perhaps even too loud for some. An 8 watt combo is an ideal amount of power for the volume knob player. It’s loud enough to get by the drummer and if the venue is big enough, they’ll most likely have it mic’d anyway so there’s no need to drag out the half-stack. For the country player or someone who uses pedals for their overdrive tones, a 30 watt amp might be more suitable. Even a 50 watt amplifier is going to be a good bet but getting past 30 watts is not always necessary. Take an AC30 and crank it to the point of saturation. It sounds great but man is it LOUD!
The trend toward lower wattage amplifiers has another upside. The iconic guitar sounds we all want to achieve in a large part come from the 60’s and 70’s. Due to their lack of high powered public address systems, these guys needed to crank their 100 watt amps up to be heard and pushing them to their limits is what generated that sweet tube saturation and tone that we all lust after. This day and age, cranking up a Marshall stack is not only going to garner dirty looks and hands over ears, but it may result in not being invited back to that ultra-hip club it took two years to get a gig at. The smaller wattage amplifiers allow us to push our amps to their limits at a volume level that is comfortable for both you and the band as well as the audience and the soundman. If it’s not loud enough, the soundman will make it louder. You don’t have to be the loudest instrument on the stage (drummers have that luxury, beating on things and all) to be heard in modern venues.
One last note before I wrap up this long-winded article. A lot of low powered combo amplifiers come in a 1×12 configuration and even though it might be loud, it just doesn’t have the push of a large 4×12 cabinet. If you want to make your low watt amp sound bigger and be perceived as louder, grab an extension cabinet. A nicely made 2×12 extension cabinet can make all the difference in to that thin sounding Deluxe Reverb. A larger enclosure and the extra driver allow more air to be moved and will give more of that satisfactory punch and “big amp” feel to the little guy.
If you’re still unsure about this, then by all means keep that 100 watt Plexi, but try this. Go to your local music store and find the lowest wattage tube amp they have and hook it up to a Marshall 4×12. A Vox AC4TVH or Z.Vex Nano Head would be ideal. Set the AC4 on 1 watt mode and crank it up. You’ll be surprised by how quickly the sales representative comes in and asks you to turn it down. You might also be surprised at how good it sounds.
Hopefully this article has helped spread some uncommon knowledge about the wattage rating on amplifiers. Next time you look at that boutique combo that’s rated at 8 watts don’t be so quick to dismiss it as “too small for live use” and plug it in. It might just surprise you at how well it keeps up and the fact that you can crank it and make the tubes work for their money results in sweet, singing tone that tube amplifiers are known for. Thanks for reading folks.