"Hotrodding" the ROG Supreaux Deux

[MarkL]

So I've just discovered this neat little circuit and have seen John Patton's excellent demo of it on youtube.  Very nice and a unique tone.  What I'd like to do, though, is beef up the gain/drive a bit on this thing and maybe filter out just a bit of the mids.  So I have a few thoughts as to how this might work:

1. Replace both 2N5457's with J201's....and this will require changing the respective trimpots to 50K trimmers to compensate, correct?  I am guessing that this might also involve changing the source resistor on Q1 -- but I think perhaps socketing it and trying a few options would be the best way to go here.

2.  Decrease the 100K input resistor and the 270p cap after it just a little.  So, maybe an 82k/220p combination there?

3.  Possibly change the source resistor on Q3 as well, maybe to something a little higher as a hotter signal would be coming in.

4.  Increase the 1n5 tone resistor to a 1n8 or even 2n (so, two 1n's in parallel).

5.  Include a small bypass cap to ground on the source of Q1 for a little added presence. 

6.  I am guessing that the 68n cap right before the volume pot could be tweaked a a bit as well...?

I'd welcome any thoughts and suggestions.

[stecykmi]

1. not sure, but you might as well if you think there will be an issue. it's still possible to bias the 2n5457's with 50k trimmers, after all. then end goal is to bias the drain 2/3 of the supply voltage.

2. probably won't make much of a difference. this network acts as a low pass filter so in general, decreasing the value of the resistor or increasing the value of the capacitor lowers the corner frequency, aka the frequency before treble starts being cut. the stock values of 100k & 270p are already very high, creating a corner freq pretty much above or near the top of a guitar's range, so any higher and this won't have very much effect on the signal at all. the 100k resistor does act as an attenuator with the 1M resistor to ground, but the ratio between the two is so high that very little signal is actually bled off. the cap also helps subdue RF noise, so it's important in that sense as well.

3. this will affect the output volume only, i would leave it unless the Q3 is adding too much clipping

4. the ROG website mentions that increasing this cap will result in a darker tone, so this one is to taste

5. this will work, it might also be nice on Q2. I would start at 1uF and work your way down. bypass caps tend to need to be quite large. as the value decreases, fewer treble frequencies will see the boost in gain.

6. increase this cap for more bass (or decrease for less).

[midwayfair]

I'll go through your suggestions one at a time and then suggest a few things.

1. I would not change Q1. You will increase the gain with a J201 in that position, but you will also lose some benefits of the 2N5457 in input stages, which could result in some bad behavior with larger signals or loss of treble.

2. 33K/220pF is fine for that resistor and more in line with present FETzer valve design. However, recognize that decreasing that will increase the cutoff frequency of the input treble cut. The frequency cutoff is pretty far up there already (over 7K). With a 33k, you'd increase it to ~22K Hz. There's nothing coming out of a guitar's strings up that high. Heck, you can't even hear that high. There's some extra capacitance hidden in FETs that will add to that, but it's still going to be way up there. It's mostly to avoid noise. But that input cap combination is part of the sound of the amplifier.

3. Don't rebias the FETs. At least, not without realizing what you're doing. Read this: http://runoffgroove.com/fetzervalve.html. The FET biasing introduces harmonics at the proper ratio to make it sound more like tubes. Just underbiasing the transistors -- and a certain booteek company went this route -- might produce more gain, but it won't sound the same either. If you REALLY want to increase the gain, put 22uF-47uF caps from the source to ground on Q1 and Q2. This will max out the gain for all frequencies on the guitar without upsetting the bias. Again, part of the modern FETzer valve design. However, recognize too that you can send the FETs into cutoff, which could result in bad behavior. The circuit already has what I think is a lot of gain; it's enough to "sag" the FETs, which indicates that they are already quite saturated.

4. That's a low-pass filter in conjuction with the 100K before it. What are you trying to achieve by increasing the capacitor?

5. See above. You could add presence, or increase the total gain. You can't do both. This is a bright circuit already, and I would have to cut the treble on my amp if I had the tone control all the way up. What specifically about the sound are you trying to change about the pedal in making this change? The better place to make this change would be Q3; if you add presence early in the circuit, you'll increase the treble and introduce more intermodulation (by boosting the harmonics, which would sum poorly), but most of the increase will be lost by the multiple low-pass filters in the circuit and by the FETs themselves (which do not perfectly reproduce high treble frequencies).

6. The 68nF cap is the output capacitor. It forms a high-pass filter with the volume control. The cutoff frequency is ... 23.4 Hz. That's at the basement of human hearing and certainly passes all frequencies on a guitar; the lowest frequency experiencing any attenuation is ~80Hz, with the low E (72Hz) experiencing perhaps one or two decibels of attenuation. Changing this cap will do little, if anything. In fact, it's probably a fuller response curve than your amplifier unless you run your bass control at max.

You mentioned that you wanted to scoop the mids. That's a tough one. Almost anything you do for that will cut the output or the gain. However, you could steal the last 6 components in the Azabache to introduce a 400Hz notch. You could also split the Q3 biasing resistor (the trimpot) into two resistors as a divider. One goes to the +9V, one goes to the Q3 drain, and the output is taken from their junction. You only need about 3.3K total. For an example, look at the Fuzz Face. I don't have a good explanation, but this produces a midrange notch and it's one reason a lot of vintage fuzzes have scooped tones.

Of course, adding a big muff tone stack is the other way to go about it. This loses a lot of output and would especially lose a lot of treble, so I'd suggest sticking a buffer on the end of the circuit, then putting in the tone stack and tweaking, THEN buffering the output after the tone stack once more to prevent whatever follows from being loaded down by the tone control.

Also, recognize that it will get a lot more distorted simply by including the "Bottom" switch. I left it out in mine because it was more gainy than I needed.

I want to say too that you might consider making another pedal based on a different amp if you want a different sound. ROG tailored this circuit to sound like the Supro 16T, and it's as close to dead on as you can get. There are other designs on ROG based around different amps that might be more in line with the sound you want. But few FET designs sound particularly good for high gain. If you want that, I suggest just building a fuzz or distortion and using it with the overdrive.

[MarkL]

Thanks for the comments and explanations.  This really helps, and clarifies a lot of what I had trouble digesting in the ROG article on the Fetzer Valve.  I'll noodle with the circuit a bit but as you've implied, it seems to already be suited for a certain application very well, and there's perhaps a better platform for getting a gainier device. 

[Thomas_H]

More gain?  Go Thor!

[midwayfair]

More gain?  Go Thor!

+1, that thing's insane. Not scooped, though. Maybe the Umble instead? I remember that having so much gain it didn't make it off the breadboard.

[alanp]

The English Channel has insane gain.

[jubal81]

Try adding a SHO or some other boost in front of it. FET drives respond extremely well to boosted input and you can have an OOMPH button.