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....

[tot_Ou_tard]

You are a freak Vood.

I mean that in a good way.

I look forward to getting Zonked by your clips.

[M29]

A total freak man.......yeah like tot said

Ya got me wantin to do one!!

Thank you for all the work you are putting into this.

M29

[sunvalleylaw]

A total freak man.......yeah like tot said!

Ya got me wantin to do one!!

Thank you for all the work you are putting into this.

M29

Yep, more motivation to do my own rather than find a used TS-9 to have modded. Maybe need to start with that larger box though!

[Hogfullofblues]

Great post DVM. I was pretty much able to follow that even though I am electronically challenged. Looking forward to hearing that bad boy too.
But I think DUH Zonkin Yellow Screamer might be more appropriate.

[duhvoodooman***]

I've got the Zonkin' Yellow Screamer (TM) all wired together. All that's left to do is to drill the holes in the new casing, label it and reassemble. Here's a photo of the current state of the project. The "guts" of the unit are mounted back in the TS clone stock enclosure (I did this so that I could test it), but the two main "design enhancements" are hanging out either side. I also include the new casing in the photo, to show the size & color.

On the left you can see a small piece of perfboard with some components soldered to it, wired to a DPDT toggle switch. This is the 3-way clipping mode selector--"stock", "hot" and "cool".

One the right side is a 6-position rotary switch that allows you to select normal or high gain modes, each with three bass distortion levels--"normal" (the stock mid-range hump), "rumble" (more bass), and "thunder" (still more). The perfboard contains the resistors & caps to do that.

Everything works according to plan, and it sounds pretty amazing to me! Of course, I may not be completely impartial!! I'll record a clip or two in a couple of days and post it here.

[sunvalleylaw]

Dang, more drool. Can't wait to hear the sound!

[F_BSurfer]

DVM how did the 2k pot work out for you have been thinking about this additional mod myself

[duhvoodooman***]

BIG improvement. I now have a tone control that actually makes an audible difference across its entire sweep range, not just the first and last 20%! Highly recommended....

[duhvoodooman***]

Here's the current state of the project. One of our crack machinists here at work came in early this morning and drilled all the holes in the new enclosure, and I assembled it to make sure the fit-up was good. It'll look better when it has all the graphics applied, but I like how she's lookin' so far!!

Attachment 853

[ted s]

One of our crack machinists

What kind of crack ? a$$ or sidewalk ?

Looks good DVM !

[Tone2TheBone]

Tremendous! Now some vicious sound clips of what this puppy can do would be very nice. Looks great so far.

[duhvoodooman***]

Now some vicious sound clips of what this puppy can do would be very nice....

Planning exactly that for this weekend!

[Tone2TheBone]

Planning exactly that for this weekend!

Some "before and after" turns of the knob would be useful that way we can hear what your modifications do versus the stock options etc. You're living the dream man!

[duhvoodooman***]

OK, here are some clips I recorded using my "Zonkin' Yellow Screamer" modified TS-808 clone pedal. To summarize the mods I made, the main changes were threefold:

• Three switchable clipping diode arrays to change the character of the overdrive. In addition to the stock diode type (silicon), I also added germanium diodes (an early, soft-clipping type that gives a more subtle, smooth tone reminiscent of when a tube amplifier just starts "breaking up") and yellow LED's (a late, hard-clipping type that gives a more aggressive, chunkier distortion, often referred to as "Marshall-like" in its quality).
  
• Normal and high gain modes. Using two switchable resistors, the stock gain range or a "TS-on-steroids" range can be chosen. The latter has a maximum gain of almost five times the stock maximum. But it's also noticeably noisier, as you might expect.
  
• Three switchable bass rolloff values. The stock TS-808 was built such that the distortion rolled off sharply below 720Hz, and was a key reason why the Tube Screamer had a characteristic mid-range "hump" in its output. By using different capacitors, I've added the ability to extend the bass distortion range down to either ~340Hz or ~160Hz. These lower ranges, particularly the 160Hz, can get rather muddy sounding, especially with humbuckers, so you have to be careful with their use.

Below are posted three audio clips. All are recorded through the clean channel of my Peavey Delta Blues 30W tube amp, mic'ed into my PC's sound card and captured/edited with the Audacity program. Here's a brief description of each:

1. The first clip is a demonstration of the gain and bass rolloff selectable settings. The stock silicon clipping diode type is used throughout, so these clips should all have the characteristic Tube Screamer distortion character. Recorded using my Strat on the bridge/middle series pickup combo. Two riffs are played consecutively a total of seven times. Here are the ZYS settings for each:
   
   
   
   For the final two pairs of riffs, recorded at the higher gain range setting, I intentionally strummed and held a chord at the end to demonstrate the pedal's sustain.
   
   
2. The second clip shows the difference between the three clipping modes. Recorded using the same Strat setup as the first clip. Here, a single riff is played a total of seven times, with the following ZYS settings:
   
   
   
   The last set of three riffs at the higher gain range don't convey the gain impact as much as I would have liked, since I had to dial down the volume some to maintain domestic tranquility. With more volume, these really scream--trust me!
   
   
3. The last clip is another comparison of the three different clipping modes, but this time with a humbucker guitar, my Epi Elitist LP Standard, using the SD "Pearly Gates" bridge p'up. A familiar Who chord riff is played four times, with the following ZYS settings:
   
   
   
   The last of the four riffs, using the yellow LED diode array, captures Townshend's Marshall crunch pretty well, I think!

Here are the links to the three clips:

Clip #1

Clip #2

Clip #3

[Tone2TheBone]

Very interesting and so far they sound great. I've only listened to the first one so far. Question though...what was the tone knob set at on the pedal and what were the tone knobs set at on your Peavey?

[duhvoodooman***]

Tone settings:

Pedal: flat (12 noon)
DB: B-7, M-10, T-8
Strat: 7
LP: 5

Keep in mind that the Strat p'up combo I used was the bridge and middle in series, which is not the stock setting, where the two p'ups are in parallel. This is one of the new p'up combos I got from the "Strat Lovers" wiring mod I did a few months back. This gives a real nice pronounced "quack", but is fuller and has less top-end emphasis than the usual parallel combo. You can hear it for yourself the first cycle through the riffs in clips 1 & 2.

[tot_Ou_tard]

Very nice Vood, I prefer both the soft clipping & the yellow LED to the stock for the single coils and that LED for the buckers is nice n' crunchy.

Your dry sound with the singles is very nice too. I would "select" that pretty often.

[duhvoodooman***]

For you hardware geeks like myself, I thought I'd provide some additional details on the clipping diode setups. To me, this is the most important single feature within an overdrive pedal, because its primary function is to "clip" the input signal, thus distorting its sound. The debates about using the "true TS-808 chip" in a Screamer clone seem pretty funny to me, since the difference between a NOS JRC4558D, current production JRC4558D, or an RC4558P opamp are very subtle, compared to what you get by changing diode types.

As I understand it, diodes are essentially a one-way electrical gate, allowing current to pass through them in only one direction. Diodes apparently require a threshold voltage to start working, and different types "turn on" faster than others. So when the input signal exceeds the threshold voltage, the diode turns on and restricts the current flow, i.e. "clips" the signal. The turn-on time determines how "hard" or "soft" the clipping is, with slower turn-on giving a softer clip.

For these clipping applications, diodes are typically used in pairs, wired together in parallel but opposite current flow orientations. The opposite polarities allow one diode to clip the positive signal (i.e. top half of the sine wave) while the other clips the negative signal (bottom half). Use of such a matched pair of diodes is referred to as "symmetrical" clipping, since both signal polarities are clipped the same way. However, you can use two different types of diodes or place additional diodes in series on one side vs. the other to give "asymmetrical" clipping, where the two signal polarities are clipped differently. This is reputed to give a more natural or tube-like tone. Such arrangements are very popular in modified TS-type pedals offered by many of the "boutique" pedal makers.

The original TS-808 used two 1N914 silicon diodes for a symmetrical clip. Silicon diodes have a fairly high threshold voltage, and turn on quickly, i.e. "hard". However, I opted to use an asymmetrical arrangement of one 1N914 and two 1N4001 diodes for a somewhat smoother character. This was included as an option in the BYOC Overdrive pedal kit I started with on this project. So, my "stock" TS-808 tone isn't 100% absolutely stock, but it's still very close. The difference is pretty subtle.

For the second clipping mode, I chose 1N34A germanium diodes in an asymmetrical 2-and-1 arrangement. Germanium diodes have a lower turn-on voltage than silicon, but also turn on more slowly, giving a softer clip. The resulting tone is significantly smoother than the stock TS-808, and it reminds me a lot of the overdrive tone I get from my Delta Blues tube amp when I push the clean channel volume way up to saturate the power tubes and run the output through an attenuator to knock the volume back down. I assume it relates to the lower voltage threshold, but running through the germanium clipping diodes really reduces the overall pedal output volume significantly, so you have to turn up the Level knob to maintain overall volume. But it's a really nice, restrained, bluesy overdrive!

The last diode combo I used was two 3mm yellow LED's in a straight symmetrical arrangement. Use of LED's for signal clipping is at the heart of the very expensive ($379!!) Landgraff Dynamic Overdrive pedal, which is little more than a TS-808 with switchable silicon diodes and LED's for the clipping section. People rave about this pedal and some other similar LED-based overdrives because of their aggressive "Marshall Plexi" sound. LED's have a higher voltage threshold than silicon, and they turn on "hard". They cut the signal least overall, so produce the highest (loudest) output of these three combos. I have to admit, I like the sound of the LED clipping mode, especially with humbuckers.