Distortion on the O-scope

Here are a couple of circuits on the Oscilloscope I've been messing around with. The circuits are going into my Tube Hottie Stompbox in the next few days. While testing each individual gain stage, I snapped some photos. First, let's look at the FET ( Field Effect Transistor ) This type of transistor is super-easy to bias, hard to screw up, and offers a HUGH input impedance, for acoustic instruments, or electric. Additionally, alot of folks think they can build a FET circuit to sound just like a tube, but here I'll show you the difference.

















Above is my simple single stage FET preamp, with a .5V peak-to-peak signal in, and about 2.3V p-p out, showing a gain factor of about 4.6. The supply voltage for the preamp was 9 volts, a common value in the stomp-box world.
















Same Preamp, same applied input voltage, but with a 12V power source. Note the increased gain created by a slightly higher bias current. The additional headroom doesn't hurt either.
















Here's the preamp back at 9V power, with 1V p-p on the input. Notice the bottom of the wave form is not exactly the same shape as the top. This is the beginning of distortion. Chances are you'd never hear this, but it's there all the same.

















Here's a 5V p-p input, and the output is clearly distorted. The flat spots on top show the transistor is cut-off, meaning the input signal can not drive the current any lower than it can during a negative voltage input swing. The flat spots on the bottom show the transistor current is saturated, meaning it can't carry any more current: more positive voltage input will not result in an increased signal output level.
















Here I'm just applying a 15v p-p signal, which is just plain stupid. Note that it does not increase the output voltage, but our square waves are about as square as we can make them. This is how early function generators made square waves for test equipment.

















Here's a tube circuit: a 12AX7 running on about 150V. The input signal here is about .2V p-p, the output signal is around 8V p-p, a gain factor of 40.

















Here's the tube with an input signal of .5 volts, we could also call it 500 millivolts, hotter than any guitar signal by itself. Notice the distortion waveform. We've never really reached saturation for the tube ( I hope the lower plate current will make it last longer ) but we do get a good solid cut-off at the top of the wave. Look at the left side of the square sections. The gentle rounding of the square section ( like a bar of soap ) is why Tubes sound better, less harsh than a transistorized distortion. Yes I know this is asymmetrical distortion, but I want to try it.
















Here's the two wave forms side by side, so you can see the difference between FET distortion and tube distortion. The one on the right will be more pleasant, and more musical.

I will use both sections in my stomp box pedal. The FET section will be an easy way to get some increased voltage signal ( while staying clean ) to make sure I get into the overdrive section of the tube, which is why I'm building the pedal in the first place. - Jim Cox