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.
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.
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.