AC Line Voltage and Guitar Tone Mania

Transcription

1 AC Line Voltage and Guitar Tone Mania Hans Michel UIUC Physics 199 POM Physics of Music Fall Semester, 2004 Ever since Eddie Van Halen plugged his amplifier into one in the mid seventies, the variac has been a mystical presence in the guitar world. Credited for a lot of his infamous sound, many people started to use them, soon to discover they were a good way to destroy an amp. Variacs are variable transformers that take the electricity from the wall outlet, and turn the voltage either up or down. Reckless use of variacs soon led the guitar world to shy away from them. But since the advent of rock and roll as we know it, line voltages from outlets in America have gone up ~ 10%. Amps from the vintage era behave slightly differently now than they once did. I was interested in this phenomenon and used a variac on an amp of mine to find that differences in line voltages had a definite effect on the overall sound of the amp. I then decided to try to quantitatively capture what it is that is affected by these differences in line voltages. A vintage designed tube amplifier has many points in the circuit that can be affected by these changes in voltage. The amplifier I used is a copy of the legendary Marshall 18 Watt combo. It uses EL84 output tubes, 12AX7 preamp tubes, and an EZ81 tube rectifier. The most obvious places the difference in voltage would affect the signal are the tubes plate voltages, which greatly affect the overall sound of any amplifier. Also a difference in the tubes filament voltage directly affects the way the tubes function. The biggest reason for damage to tubes due to variac use is dangerously low filament voltage, which strips the filament s chemical plating, and causes tube failure. Pretty much 1

2 anywhere else in amp is also affected by these differences. How all of these combined differences affect the overall sound of the amp is what I set out to find. The first and most obvious place to start for a basic input signal is a sine wave. I ran a sine wave generator into my amp to find the harmonics that it added to the original signal, and how those harmonics were affected by differences in line voltage. I tested five voltages: 130 volts, 120 volts (current U.S. standard), 110 volts (former U.S. standard), 100 volts, and 90 volts. We ll take 120 volts to be the standard line voltage to compare the differences of the other line voltages to. For this particular amp, the second and third harmonics are relatively strong, and there is a drop off at the fourth harmonic. Starting at ninety volts, the harmonics overall are at there weakest values. The second and fourth harmonics are improved, however, and the third harmonic is at its lowest. At 100 volts the harmonics are at their overall highest the second and fourth harmonics are emphasized, and the second is at the highest value for all voltages. The third harmonic is lessened. At 110 volts, the original voltage this amp was designed for, the second harmonic is greater, while the third and fourth harmonics are low. At 130 volts the second and fourth are yet again higher, with a very low third harmonic. The even order harmonics are what give a guitar amp a sweeter tone, and odd order harmonics give a guitar amp a harsher tone. While these are just the first harmonics in a wide spectrum, they give a good general idea as to what these differences in frequency response are. Another parameter of guitar tone that is affected by the use of a variac is the sustain of the instrument. To test this I picked a note on my guitar in the same place on the string with the same strength standing in the same place in the room, and changed only the voltages running to the amp. Recordings of these notes at different voltages were analyzed using a computer program. After finding an average slope for the decay of 2

3 amplitude on each graph of amplitude vs. time, I used the formula for the time constant T=(t)/(ln(initial amplitude/amplitude at time t)) to find the amount of sustain of the fundamental frequencies amplitude for each voltage. At 90 volts, T=0.83 At 100 volts, T=

4 At 110 volts, T=0.68 At 120 volts, T=0.65 4

5 At 130 volts, T=0.70 Sustain is improved as the line voltage gets farther away from the standard line voltage. Inconsistency in my playing is certainly a factor to deal with. I did my best, but I am sure I was not perfect. Even taking that into consideration there is still an undeniable correlation that the sustain is improved as the line voltage drops. The difference in sustain between 120 volts and 130 volts could easily be from different picking styles, but from what I heard while playing around the sustain was improved. Obviously this experiment was only on one specific amplifier, so I can not say that for every guitar amplifier the same things are true. The circuitry of the amp used is similar to many, many guitar amplifiers, so it can be concluded that differences in line voltages would most likely have a similar effect on other tube amplifiers. Subjectively tone is improved as line voltage is lowered. The tone is smoother and the sustain is very noticeably improved. There is something about the 130 volt line voltage that sounds grittier. The lower you go with line voltage, the more complex the sound is. Differences in higher harmonics must be what make the 130 volt setting less 5

6 pleasing than the lower ones, because from the harmonix I measured here it should sound more like the lower settings. Since the glory days of Eddie and his infamous sound a lot of things in the guitar sphere have changed. Hopefully enough people will learn how to use variacs safely so that they can again become a staple with rock and roll. It s impossible for an amp designed for a different line voltage used today to have the original sound, but with a variac that sound is attainable. The differences in the way the amp behaves are slight, but very much present. With careful variac use, modern players can discover new timbres for their amplifiers, and vintage amps can have their day in the sun again. Rock and roll will sound like it should again. 6

7 Raw Data: X Y1 Y2 Y3 Y4 Y5 Y6 Bin Center FG Output 18W 90 18W W W W 130 Frequency Amplitude^2 Amplitude^2 Amplitude^2 Amplitude^2 Amplitude^2 Amplitude^2 Harmonic # (Hz) (volts^2) (volts^2) (volts^2) (volts^2) (volts^2) (volts^2) E E E E E E E E E E E E E E E E E E E E E E E E-12 Individual Harmonic Ratio Data: X2 Y1 Y2 Y3 Y4 Y5 Y6 Bin Center FG Output 18W 90 18W W W W 130 Frequency Amplitude^2 Amplitude^2 Amplitude^2 Amplitude^2 Amplitude^2 Amplitude^2 Harmonic # (Hz) (volts^2) (volts^2) (volts^2) (volts^2) (volts^2) (volts^2) E E E E E E E E E E E E E E E E E E E E E E E E-08 7

8 Individual Harmonic Ratio Analysis 1.00E E E-02 R a t i o ( H a r m o n i c N / F u n d a m e n t a l ) 1.00E E E E E E E E Harmonic # FG Output 18W 90 18W W W W 130 8

9 Amplitude^2 vs Frequency 1.E-02 1.E-03 1.E-04 A m p l i t u d e ^ 2 ( d B - r m s V ^ 2 ) 1.E-05 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 1.E-11 1.E Frequency (Hz) FG Output 18W 90 18W W W W 130 9

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