Pedals galore.
Having been satisfied with the previous experiments with a simple 2 stage valve preamp wrapped around the tilttone control and resonance/presence feedback loops I thought it would be interesting to hear what it had to do with several other effects. Using the models already available in the Guitarix source I went into mass production!
One Tone Control
While building a small practice amp I did some research into variations on a single tone control as I did not want the complexity of a full guitar tone stack.
After much experimentation I settled on a very simple circuit used in the famous Big Muff pedal from ElectroHarmonix as it provides a wide range of usable guitar tones and works extremely well in the amplifier which is loosely based on the Fender Champ topology. It comprises of 2 filters, one low pass the other high pass, blended together with a potentiometer.
First Plugin
While I was busy experimenting with the dsp valve code the Guitarix team began releasing LV2 plugin versions their amps, at first these were hard wired configurations as the standalone amp has so many options it was not possible to make the plugin particularly usable unless it was simplified ( this is now not the case as a fully configurable plugin has now been produced ).
With the aim of becoming familiar with the LV2 architecture I decided to code a plugin based on my experiments and hence was born my first guitarix preamplifier.
This consist of a 2 stage valve preamplifier with a feedback loop and presence and resonance controls. Distortion/saturation is controlled by the level of the input gain.
Feedback tone controls.
In many guitar amps a presence control is inserted in the feedback loop. In effect this is high pass filter to ground, gradually bleeding more high frequencies to ground and thus decreasing the treble content of the feedback signal, which in turn means that the treble is accentuated in the final output.
To simulate the presence control a simple adjustable low pass filter was inserted into the feedback loop.
Negative Feedback
In most guitar amplifiers a negative feedback loop is incorporated to flatten the frequency response and reduce noise, often taken from one of the speaker taps on the output transformer back to the cathode of one of the preamp valves. As this is not possible with the current valve models I decided to wrap the loop around various combinations of valves to see the results.
It is important that the feedback signal is 180 degrees out of phase with the original signal to ensure that he feedback is negative otherwise we will end up with howling feedback.
The valve models already invert the signal so if feedback is implemented around an odd number of valves no extra processing is necessary, if an even number the signal must be inverted.
Generally the level of feedback in guitar amps results in an overall gain reduction of between 6-10dB.
Valve Distortion
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.