Hotrodding the Tube Screamer:

Strangely, the Tube Screamer has an unusual frequency response and a limited range of gain. Luckily, it can easily be tuned to your personal needs.

One capacitor controls treble, one capacitor controls bass, one resistor controls gain. Excellent.

There is room around the knobs for a miniature toggle switch or two. A high gain/low gain switch is probably useful. Possibly a switch to change clipping diode configuration too. Or a bass boost. Or just make permanent internal changes. Or use internal dip switches. Or leave it the way it is.

Does "Mod X" really make a difference?:

If you want to know whether changing 2 resistors makes a real difference to the sound, use a DPDT or rotary switch to switch the changes in and out, assuming it is safe to to so. Listen to the difference (or the lack of it). No need to take someone else's word for it.

Many effects pedals contain cheap, inconstent components, with performance variation among "identical" pedals, so modification results may vary.

You can put the circuit in a larger case with all sorts of knobs and switches. You will lose the easy battery change, and the result my be less sturdy than the original. I wonder if you could put a stomp switch on the striped part beside the battery compartment of the original casing? It looks almost big enough.

I changed the 4k7/0.047uF attached to pin 2 of the opamp to 2k4/0.1uF, and changed the 51k resistor attached to the drive control to 22k. This gaves me less drive at the lowest setting, and (almost) double the drive at the highest setting, without changing the frequency response.

1k/0.22uF multiplies drive by 4.7. I've tried that with the 51k resistor changed to 10k, and the drive control is sensitive, but useable. Noisy at the maximum position (gain = 500, like the SD-9), but fun. You can also add extra clipping diodes for that

"SD-9 Sonic Distortion" sound which has a fuzz flavour when you turn the treble up. See Screamtendo 64. You could also add a switch to put a 500k resistor in series with the 500k "Drive" pot. This doubles the drive at the highest setting, without changing the frequency response. See "Boost Mode" below.

There is increased hiss and squealing with increased gain, so you might want to increase the 51p capacitor across the clipping diodes to about 100p if this is a problem.

You could add a clipping diode pair between ground and the output of the first opamp. The Ibanez Sonic distortion and the above GM ARTS distortion schematic do this. You will need a limiting resistor (about 1k) in series with the diodes to prevent one of them switching on all the time. My Fuzz Face '69 clone has a switch for a pair of Ge diodes in series with a 10k resistor across the output, which is useful too.

You could replace the original diode pair with a 1N914 on one side, and two (or more) 1N4148 in series on the other. LED's are worth a try, let your ear be the judge. I have used one silicon diode on one side and 2 on the other (mixture of types). This produces a more musical distortion, depending on your taste.

You could use the Craig Anderton Tube Sound Fuzz "Rhythm/Lead" concept to add a footswitchable "Boost Mode" with its own separate Distortion control. A 1M pot with extra 100- 200pF cap across it, in series with the 500k drive pot will do.

Update: DMZ now have a better schematic for this concept on the "Son-of-Screamer".

You could reduce the gain and use a low noise preamp for a clean "leave it on all the time" boost. Havling the value of the 51k resistor attached to the drive control halves the minimum gain without changing the frequency response. It reduces the maximum gain by 5%. You could combine this with the "More Bass" for a more hi-fi frequency response. This sounds especially good to me.

To improve audio quality, replace the tone filter tantalum capacitors and 1.0u non-polarized with film types. The replacements are larger, and more expensive. The difference may be subtle, but electrolytic capacitors do cause more audio distortion and noise than film types. You could remove the clipping diodes completely to give a less compressed distortion with more volume. I did this when I had a DOD Overdrive Preamp 250 (I can't believe they are reissuing that) to make it sound less unnatural. As a compromise you could replace each clipping diode with two silicon diodes in series.

This produces less diode clipping (which I think sounds better) and less distortion with more volume at low drive settings. Look at the 4 diode group in this schematic:   GM ARTS generic distortion schematic 

Conversely, you could take a SD-9 Sonic Distortion and add a switch to reduce gain and/or bass response to make it sound more like a Tube Screamer.

Gain-with-bass-cut knob.Replace the 4k7 gain resistor with a 5k pot in series with a 2.2k resistor. As you increase the gain the bass response is reduced, which makes chords less muddy (by reducing intermodulation distortions created when a low frequency modulates the gain of a high frequency - the human ear is very sensitive to this). The MXR Distortion+ did this. At lowgain you get more bass for a more natural "clean boost" sound. The ability to select this mode using a footswitch would be useful.

Change the 4k7/0.047uF attached to pin 2 of the opamp to 4k7/0.1uF or 4k7/0.2uF. Going as high as 4k7/0.47uF is worthwhile, but most film capacitors start getting physically big at this point. My favourite Tube Screamer mod. This does not boost bass compared to the clean signal, it just reduces bass loss. You could use an electrolytic or tantalum 1.0uF if you were really stuck, the sound quality wouldn't be so good. I tried a tantalum 2.2u, you do actually get a bass boost with that.

Following the clipping stage there is a 1K resistor leading to a 0.22uF capacitor to ground. Reduce the value of this capacitor. By half is as far as I would go, it increases hiss. You can have less treble by increasing the capacitor value. Reducing the value of the 1k resistor also gives a brighter sound.

Add a DPDT switch for both More Bass and More Treble at the same time. The mid range boost is popular for single coil pickups, the opposite effect might work well with humbuckers. Or electric bass. Scooped + More Distortion = Heavy Metal, apparently.

Put in a DPDT footswitch to remove all circuitry from the signal path when "off". Sometimes you end up with switching noise, or no sound if you hold the switch down. It will avoid the "switch squawking" problem, though.

The little black switch inside the battery compartment gets dirty eventually, making its operation embarrassingly unreliable. Open up the battery compartment and blow compressed air into the switch. If using lungs, don't inhale. Some people squirt cleaning/lubricant materials in and report good results, but some chemicals may dissolve or gunk up your switch irreversibly. I used air, then WD40, and my switch works very well now. You can take the switch apart, but it's risky. See "Big Box" General Tube Screamer Hotrodding: for a more permanent solution.

Update: I took the TS-10 switch apart with my fingernails. It contains a spring and a V-shaped tinny springy bit of moving metal contact, which explains the unreliability. It wasn't dirty so much as cheap. I suspect it didn't work too well when it was new. Anyway, it's worth opening up and checking for filth if you are having problems. Open it halfway to see the orientation of parts before the spring flies across the room. The TS-9 switch is different, I hear. So is the TS-5.

I drilled a hole in my TS5, put in a DPDT stomp switch for true bypass, and used one of the now- redundant switching FET's for a GEO - style Millenium Bypass. You can remove about 40% of the components on the PCB. Wierd.

A rotary switch changing the values of the 4k7/0.047uF would be useful, as would a clipping diode selector.

Have two, they're small:

Use two Tube screamers onstage, one with "Less Distortion", one with "More Distortion". This is about the same price as one Screamer in a Big Box with footswitches, and just as versatile.