P1 AX84.com Amplifier Distortion Viewed On An Oscilloscope
- Nov 13, 2010
- in Guitar Kit Builder - Kits 101
Guitar amplifiers today are prized for the tonal characteristics of their distortion - the way they change the pure signal from the guitar pickup. The primary method of creating that distortion is to increase the levels of the signal to a point that exceeds the amplifier's ability to reproduce cleanly. This results in a clipping of the signal such that the top and bottom portions of the wave are leveled off, instead of smoothly following the input signal. Levelling may also be described as clipping or squaring.
A waveform that is clipped contains a great many harmonic overtones of the basic input frequency. The frequency and amplitude of these overtones are what give different types of distortion their tonal qualities.
We've got a nice video here where you can see what a distorted guitar signal looks like on an oscilloscope while you're hearing what is being played. We'll discuss more about what you're seeing below, but the amplifier circuit is also worth some discussion. It's a P1 Amplifier from the AX84.com Cooperative Tube Amp Guitar Project. AX84 is a great resource for the guitar amplifier builder. Their purpose is to provide:
- A great environment to learn about music-related electronics in general. AX84 is a place where those with experience are interested in helping diligent beginners who are willing to do their homework.
- A place where those with more experience can field their ideas to peers and get useful feedback.
- An environment that respects the past, but is not enslaved by it. Their belief is that innovation is good and it didn't die in the 1950s.
The P1 is a simple tube amp modeled after the classic small guitar amps of the 50's and 60's. It's designed to be a managable project for the first time builder while still offering a satisfying result. It features a 12AX7 preamp tube that provides two stages of preamp gain, an EL84/SV83 power tube in a Class A single-ended configuration to provide a very loud 5 watts or so of output power and bass, mid and treble tone controls with a single gain control. It doesn't get a pristine clean tone or a raging death metal tone, but occupies that warm territory in between.
In the video you'll see this amp wired up with alligator clips, in just a temporary test configuration. This can be a very dangerous way to operate any circuit with high voltages and we don't recommend you try it this way. When you watch the video you'll see the oscilloscope - that piece of gear with a TV-like screen - that is displaying the shape of the output signal from the amplifier. As each note is picked, you can first see that the amplifier is overdriven and that the output signal is squared off. As the picked note decays in volume you can see the squared wave briefly return to a smooth sinusoidal wave. This is the point where the input signal has decayed enough that the amplifier is no longer being overdriven.
By the way, when the player is bending notes you'll see the horizontal width of the waveforms shrink and expand, indicating the frequency is increasing and decreasing.
OK let's take a look at the video.