Birth of a Chump

Written by steve Print Email

Having been pleased with the results so far it was time to try and build a complete guitar amplifier. To keep it as simple as possible the design is again loosely based on the Fender Champ, very simple in design, 2 stages of preamp valves and one power valve.

12AX7 --> Volume --> 12AX7 --> 6V6 --> 1x 8 speaker

The valve models already exist in the Guitarix code but no attempt had been made to model the effects of output transformers or NFB ( Negative Feedback ) so I decided to try to add something to the circuit to include these. NFB had already been tried in an earlier experiment and only changes here were to make it switchable with the hope of giving the amp a wider range of tones.

After some research it seems simulating an output transformer is really not trivial and so I settled on a very simple model of highpass and lowpass filter until I can get a more sophisticated model.

In addition I felt that it would useful to have both a master volume and a tone control of some sort so revisited the tilt tone control and added a master volume, making the simulation topology :

12AX7 -> tilt tone -> gain -> 12AX 7 --> 6V6 --.>  transformer --> speaker

                                                       |                                                       |

                                                       feedback loop -----------------------


After some playing with parameters results started to sound pretty good, very much a clean and slight crunch amp. Feedback thins sound slightly  and alters the flavour of the distortion so a useful feature but need to work on eliminating the squealing at higher volumes.

In respect to the original I called this "The Chump"!

The Chump

The Chump