Watts vs. Volume
Clash of the Titans
Discussions about amplifiers frequently visit the subject of how big an amp needs to be for a particular application and how loud is an amp of a given wattage. In the interests of expedience I have decided to write down what I know about the matter and have it available for future reference.
How Much Difference Between
Amps of Various Outputs?
Doubling the wattage of an amp increases its volume by 3 decibels. 3 db
is a very slight increase in volume, certainly nothing dramatic.
For most people 3 db is the slightest increase in volume that they can
perceive. In other words, a 50 watt amp next to a 25 watt
amp would, IN THEORY, be almost the exact same volume, assuming that the
speaker efficiency was identical. It takes a 10 db change to be
perceived as a doubling in volume and 10 db requires 10 times the power.
If you look at Fender's classic lineup of amps you can see this born
out. The Champ was about 5 watts, the Princeton was 12 watts, the Deluxe
Reverb was 22 watts, the Super Reverb and Vibroverb were 45 watts and
the Twin was 85 watts. Each step up was roughly double the wattage.
Of course there are many other factors which would determine clean
headroom. As an example, a 40 watt Custom Vibrolux Reverb breaks up
early because of no negative feedback circuit. I doubt that a CVR can
stay clean any longer than a Deluxe Reverb, but that's a design feature.
It trades clean headroom for beautiful natural overdrive at relatively
low volumes.
Reliability of Wattage Ratings
Wattage ratings are not 100% reliable, 50 watts does not mean 50
clean watts. I once helped a neighbor install a stereo in his car. It
was rated at hundreds and hundreds of watts but when you read the fine
print those watts happened at high levels of distortion. In reality the
stereo was barely any louder than the 28 watt car stereo I had owned
years before.
The key figure is wattage at THD (total harmonic distortion). The
industry standard has been to measure output at 5% THD, but this
practice is not universally adhered to.
By exaggerating the output of an amplifier an manufacturer can claim an advantage over a competitor but that claim is probably far from valid in the real world. In the world of tube amps there should be very few surprises. Most class A/B amps with two 6V6 output tubes have an output very close to 22 watts, most amps with two 6L6s are around 45 watts and so forth. If someone is claiming significantly more output it is likely that they are either exaggerating their claim or pushing the tubes far beyond the design limits. This might work and it might even provide some gnarly distortion but in my opinion it's also likely to mean a short-lived amp.
The Often Forgotten Role of the Speaker
Equally important to any of this is speaker efficiency. Speakers are rated at db per watt as measured at one meter. If you replace a 96 db/watt speaker with a 99 db/watt speaker it has the same effect on ultimate volume as doubling the power of the amp. Watts can seem to be cheap, an 85 watt Twin doesn't cost 4 times as much as a 22 watt deluxe but the difference in volume is certainly not a factor of 4 either. In order to get an amp that seemed 4 times as loud as a 22 watt Deluxe Reverb you would have to have 2,200 watts. Suddenly watts don't seem so cheap any more.
On the other hand high efficiency speakers are actually a very inexpensive way to increase volume. If clean headroom is the desired effect buying a higher efficiency speaker might just do the trick. There are instrument amplification speakers in a range of efficiencies so it pays to research before you buy. Usually this is noted in the specifications under sensitivity.
The study of such information certainly goes much, much deeper than this simple web page. Electrical and acoustical engineers make a living from their in-depth understanding of these things but hopefully the information I have presented here will be helpful in understanding the specifications of amplifiers and speakers.
