I replaced the op amp buffer + inverting drive stage with a MOSFET booster, and replaced the tube stages with with FETs in the normal manner. Note that the design inverts phase, which the original did, too.
I moved the tone controls to put it before the last gain stage because otherwise it was just insanity on the gain levels. It's already pretty nuts even with all the cuts in front of each stage, but this gives a lot more control over the overall clipping.
The tone control and switch are really what make this cool... Really I'm just re-purposing once again the tone control from my Sakura amp, but everything worked out just right for me to get a third mode out of this one and to have a usable bass control.
1) The bass control just shaves off some low frequencies below about 400Hz. It's is a shelf filter, so the bass isn't gone, just de-emphasized, and at 5:00 the cutoff is at 18Hz.
2) Treble/Tone control: This is based on a version of the Big Muff tone control, with the resistor to ground for the lowpass removed. A lot can be done to radically alter the behavior of the tone control just by varying C9, so that's what the switch does. R8 is a small resistor (I picked 2.2K) to limit the ultimate range of the tone control so that it doesn't turn into a full low-pass filter, but it can be omitted for some very very dark settings in the scoop mode. I just don't like tone controls that bottom out.
Three modes:
a) Mode 1 flipped to one side: Flat treble at ~3:00 on the knob, at 5:00 a VERY tiny boost in the treble above 700Hz, and CCW cuts the treble at that same frequency, like a see-saw. With both tone controls down, it'll form a mid hump right around the middle of a Fender amp. (If I were to leave off the switch, this is what I would use as the stock mode.)
b) Mode 2 in the center: At ~3:00, the mids are boosted by about 3dB centered ~1KHz (bass and treble are flat). There's again a slight treble boost at full. CCW, the treble gets cut at 2KHz, so it's milder than mode 1, for just shaving off the harsher harmonics created by the distortion. This mode was a freebee on the layout by adding one capacitor, so I saw no reason to omit it. It's not as dramatic as the other modes, but it will usually sound flatter when there's a lot of distortion present.
c) Mode 3 (flipped the other way) has a mid scoop at ~250Hz for added clarity (this is the "mud" frequency for guitars, so it's a good frequency to cut). CCW, the treble is cut very deeply, which allows for bass boost with the bass at full and the tone turned down, or a treble boost with the tone up and the bass down. But with the treble and bass at about 9:00, it's back to a nearly flat response but with the gain cut a lot, for some "mostly clean" boost settings.
Jon used an on board LT1054 charge pump to run the circuit on +18v, but I'll leave the method/chip selection of doubling voltage up to you. The 2k and 10k trimmers are for biasing the JFETs, and the 100k trimmer is for biasing the MOSFET. Here's the schematic for reference.
Looks like the schematic has a mistake in the chargepump. Pin 8 is V+ not pin 1. Pin 1 can be connected to V+ though.
ReplyDeleteVerified, using the 18v setting from the Road Rage board in my test rig (LT1054 chip).
ReplyDeleteTube-y sounding with good touch-sensitivity and flexible tone controls.
I'm not sure how to set the bias, but other tube-to-FET designs from Run Off Groove have between 50% and 70% voltage on the JFET drains. On my build, the first trimmer actually measured 9.1k and the lowest Q2 drain voltage I could get was 11.4v. You may want to measure your trimmer first or use a bigger one for a lower Q2 drain voltage. For the MOSFET, I found the two points on the 100k trimmer where the sound cut out and then set it roughly in the middle.