Valve Distortion

Written by steve Print Email

Next step is to cascade several valve stages together and see what happens.

Using same single valve stage as previously now was time to see what happens when multiple stages are cascaded together, each stage being capable of overloading the next as in a real amplifier. Several parts of an original circuit are fairly straightforward to simulate with simple lowpass filters, in particular CR ( Capacitor-Resistor ) filters are commonly used between stages to block DC and to tailor the low frequency response of the circuit. In a real amp too much low frequency content can cause muddying of the sound as the low frequencies are of a higher energy than the high frequencies and distort much sooner. Looking at any circuit diagram of a Marshall amplifier reveals that the low frequencies are heavily reduced in the early stages to enable more distortion in the later stages yet retain a reasonable sound at the output, this is achieved using a bypass capacitor on the cathode resistor.

    Experiments with Guitarix Components

    Series of experiments with aim of using the "tube" components
    to build amplifiers as close as possible in topology as real valve amplifiers

    Steve Poskitt Copyright 2013

    Blog 2 - cascade valves to see effects of overload

// Input gain so can test overload
gain =  vslider("Gain[alias][style:knob]",0,-20,20,0.1):db2linear: smoothi(0.999);
// A valve is represented by the tubestage function
// By time gain is at +6dB there are numerous harmonics above 22k with 1kHz input sine
// so there is a need for a lowpass filter to calm this down.
// Similarly you can also see harmonics at the very low end which could also cause problems
// In a guitar amp this is no problem as likely to roll off above 6500Hz anyhow

stage2 =  tubestage(TB_12AX7_250k,10.0,1500.0,1.204285):highpass( 2,40):lowpass( 2, 18000 ) ;
stage1 = tubestage(TB_12AX7_68k,10.0,1500.0,1.204541) ;

// cascade produces more harmonics and a thicker distortion
process = stage1:*(gain):stage2;

Again results very promising as frequency plots show as gain is increased the number of harmonics and their levels increase accordingly. As in a real circuit the distortion was heard to gradually increase until a certain level and then become unlistenable. Listening tests again confirm that the sound was very similar to that of a real amplifier.

Next step NFB( Negative Feedback Loops ).