This is an eurorack adaptation of the sunn((o beta lead amplifier circuit, I used to play a rhodes through this amp, and i really liked the tone. Many guitar amps are quite limited in the frequency spectrum, because they’re thought for guitars, this one is not, at least to me, i loved it and I thought, why don’t you make it for eurorack? And so I did, this thanks to fender ( who bought sunn((o in the 60/70ies) that released all the service manuals on their website, go and have a look because its a goldmine. The amp runs on +-15v and uses CMOS hex inverters in the distortion section. Apparently hex inverters where used a lot in audio circuits because the had a very similar “structure” as opamps, but that’s all I know, there are various distortion circuits which uses them though.
I followed the schematic as it is except for a couple things:
-The drive pot: i used a dual gang 100k instead of 1M, way easier to find, had to tailor a bit the values around the pot and scale them down following a 1:10 ratio;
-I omitted the send and spring reverb section, which uses a very particular 30k s-taper pot. I could have worked it around by using a linear pot and putting resistors in parallel in order to obtain that curve, but at the same time I wasn’t interested in having a spring reverb ( as i have already some nice digital reverbs in my effects chain );
Since I wanted it to be a bit more versatile I added a few things:
I felt it was essential to have a VCA at the out, as they are extremely useful and lovely.
Since I used 2 sections of a 2164 for the VCA and I still had 2 available, I decided to add a second vca on a feedback loop, this to test and see how much more tonal variation i could get. The nice thing about FB loops is that they create an oscillation, which can then be shaped on the EQ section of the circuit, and at certain settings when increasing the feedback amount you can clearly hear a sequence of tones harmonically related to each other.
I spent some time as well trying to find a way to get CV control over the EQ section, but without success.
For the panel I went for a classic chinese pattern.
YOD((O , filters and tetragrid are best pals
how to change pot tapers
Sunn((o fender schems