I think Brain Wampler doesn’t need any introduction in the DIY community. He wrote several books and is still active on many DIY forums. He even helps out for tracing his own company’s (Wampler Pedals) products. This entry is about one of his former designs that is not produced anymore: the Cranked AC. As the name suggests this pedal tries to emulate the sound of a hard pushed Vox AC30 amp. Brian made some newer designs to achieve this sound (Ace Thirty, called Thirty Something lately) and this design was taken out from production. I don’t know the real reason, but I assume he wanted to achieve a closer match of the sound, add some more features (additional switchable boost, better tone control, additional AC30/AC15 switch), and I can very well imagine that he wanted to change to opamp desing instead of using the already obsolete JFETs. The tone is really rather bright as in case of the Vox amps, but still is balanced and one can achieve quite high gain. The tone control works in a wide range and with my mod this range got even wider. Here is a little demo of the copy I have built, played by the guitarist of the band Steroid:
The design is a three-stage JFET design. The idea is to use the good old tube simulation made with JFETs. The curiosity is that the individual stages are not constructed around a single JFET (as usual), but around paralel JFET pairs. To be honest I don’t know the reason. I have googled around a bit and on freestompboxes.org the only explanation is that the sound is meatier, fuller and beefier this way, but I have not found any deeper technical explanations. The schematic follows the quite standard first stage -> gain control (a simple voltage divider that shunts some of the signal to ground) -> second and then third stages -> tone control -> volume control scheme. In the original Cranked AC the two lowpass filters at the end (R2 + C7 and R3 + C8) are fixed, my mod is to make the caps switchable with a dpdt switch and so these filters can be bypassed. With this mod you can achieve an even brighter sound with bypassing those filters. The original is built with J201 transistors, I have tried 2N5457 and J202 types as well, but unfortunately none of them gave a good result to my ears. Of course this is a question of taste too, so socket the trannies and try them for yourself. Even mixed pairs could be tried, hiwever I’m noit sure if the biasing would then work correctly.
As usual you can find my PCB layout and the corresponding docu under the Projects menu.