Tube Screamer Clone Mod

[duhvoodooman***]

Between the Build Your Own Clone effects pedal website forum and several other sources, I've been reading and thinking about several potential modifications to the BYOC Tube Screamer TS-808 clone I recently built. My aim is to make it into a "Screamer+", i.e. I want to preserve the pedal's present tonal capabilities, while adding some incremental mojo! Here's what I've come up with as my "mod plan":

1. The stock configuration has the characteristic "midrange hump" that the TS-808 is well known for. I tried an optional higher rated capacitor included in the kit to flatten the response some and extend the bass rolloff frequency from the stock 720Hz down to about 160Hz; my Strat sounded pretty good with it, but my humbucker guitars got muddy at the low end. Consequently, a single "hard-wired" resistor seems to me to be a significant limitation. I need choices!
   
   At the same time, I came across a mod referred to as the "more/less" or "Keeley Baked" mod, named after a TS-9 mod offered by Robert Keeley, who sells a variety of aftermarket modified effects. This mod involves changing a couple of resistors on the PCB. One allows you to get a cleaner tone at the bottom end of the pedal's gain range (the "less"), while the other significantly extends the gain range, giving a much higher attainable degree of distortion at high drive settings (the "more"). However, this resistor mod interacts with the capacitor mentioned previously, so that increasing the gain range requires going to a higher cap value, to avoid moving the bass rolloff frequency even higher than 720Hz.
   
   Combining these two concepts, I'm going to install a rotary switch that will allow me to select either stock or high gain modes, and give me three different bass rolloff settings for each of them--a total of six settings, one of which is the stock TS-808 configuration. This will add a fourth knob to the top of the box.
   
   
2. The stock tone control on the TS clone is a linear 25K pot, to mimic how the original TS-808 was configured. Unfortunately, it doesn't work very well. The middle of the pot's range does absolutely nothing; adjusting between about 9 o'clock and 3 o'clock has no discernible tonal effect. Below 9 cuts treble and above 3 boosts it, but the whole middle is a big "dead band". Going to a linear 2K pot apparently solves this problem and gives a nice smooth range of control through the pot's full wiper arc.
   
   
3. The IC chip, or "opamp", used in the TS-808 is always a hot topic. Some claim that the original early 80's JRC4558 chips are the secret to the 808's revered tone, while others claim that's pure hogwash and that current production samples of that same chip type sound just as good or better. Several of the "boutique" overdrive pedals use the newer RC4558P chip, which has improved fidelity characteristics and gives a somewhat smoother tone. The BYOC TS clone kit comes with one of each, and uses an 8-pin socket connector on the pedal's PCB to allow easy switching between the two. But several other opamps are also compatible with this design, so I've ordered a couple that have been reported to sound good--a TI TLC2272ACD and an STM LM833N. Both are newer chip designs with lower noise and some other improved performance characteristics, and at less than $2 each, why not give 'em a try? The lower noise should be a definite plus at the higher gain settings.
   
   
4. It's kind of amusing that, with all the flap over which IC chip is best, this is not what determines how the signal clipping is done, which to me is the key function that transpires within an overdrive pedal! That is determined by a combination of diodes that feed the signal to the opamp. The TS-808 used a pair of 1N914 silicon diodes in a symmetrical configuration--connected in parallel and oriented in opposite directions. Asymmetric arrangements--either unequal numbers of diodes in the two directions, or different types of diodes, or both--are very popular and are said to give a more musical & tube-like character. Several different diode types beside the stock silicon can be used, including germanium, LED's and others.
   
   To give more flexibility in the character of the signal clipping, I'm going to install an on-on-on DPDT switch wired to three different diode configurations. One will be the asymmetric Si diode arrangement I'm currently using, one will be a combination of Ge diodes, and one will be a pair of LED's. I won't decide exactly how to configure the latter two until I hook them up to a strip of perf board and test them with the unit, and see what sounds best to me.
   
   Incidentally, this is essentially the same approach as the "Landgraff" boutique overdrive pedal, which is basically a modified TS-808 circuit with switchable diode modes, one of them being LED's. They sell for $390 apiece!!!
   
   
5. As you can see from the above plans, I'm adding more hardware to what is already a cramped arrangement in the stock casing for the BYOC kit. So I've also ordered a slightly larger box. I found a source that sells them powder coat pre-painted, and they had a bright yellow one that jumped up and yelled "Buy me!" It even inspired me with a name for the pedal, when I get it finished. Remember that bright yellow sweetened popcorn snack when you were a kid? Well, turn the name around a bit, and you have the name I plan to use--the ZONKIN' YELLOW SCREAMER!!

The parts are all on order, and it will probably take a couple of weeks to receive them, wire 'em up and debug the mods. I'll plan to post some pix and sound clips when they're available....