less gain and drive, more sweetness from a tube screamer

[gutsofgold]

I have one of those Tubulator pedals... basically a TS9 in a crappy plastic box. I always wanted to mod a 'screamer into a nice clean boost/mild overdrive. Where I can really dial in a slight/subtle overdrive and only get a mild drive even at max "drive" settings. I would like the drive knob to very sensitive and subtle and mostly use the volume knob like a boost.

-First thing I am going to do is replace the input and output buffer transistors with something nicer (MPSA18?)
-Then socket the chip... it's going to be low gain so noise characteristics of the chip aren't too significant... just want something "sweet" sounding
-Replace the diodes with ge 1N34a

I would like to lower the available gain without replacing the drive pot so I figured increasing the 4.7k resistor in the clipping amp to say 10k? This would give me a gain range of 5.1 - 55.1 as opposed to the original 12-117. Would this effect the hi-pass filter the .047uf/4.7k combination forms here? Is there a better way to do what I want? 

[BAARON]

1. Changing the buffer transistors won't make a difference.  They're a transparent buffer.
2. Chip: do try something nicer than a 4558.
3. Diodes: replacing your diodes with germanium ones will make it clip MORE, not less.  They have a forward voltage of ~0.3v, whereas the stock silicons are about ~0.6v.  Using germanium = more clipping, less output volume.
4. Replacing the 4k7 with a 10k will significantly reduce the gain boost in the upper mids/treble frequencies you usually get with a Tube screamer.  It will flatten the sound out considerably.  You may or may might not like it.

Yes, there is a better way to do what you want.  Use LEDs as clipping diodes.  They don't clip until ~1.6-2.1v (depending on the type of LED) so even without changing the amount of drive on your control you'll have a cleaner sound with way more output volume (3x cleaner, 3x louder, as far as voltage gain is concerned).  Plus, they clip "smoother."  I think it would do 90% of what you are trying to achieve.

[gutsofgold]

I never liked the sound of LED clipping but I only tried it in a Big Muff. I guess I could give them a go, I'll test a few different colors and see what has the highest forward voltage.

[roseblood11]

Led´s don´t really clip "smoother".

Have a look at this site:
http://diy.musikding.de/content/view/12/13/

The last picture shows LED-clipping...


In the tubescreamer most of the distortion is produced by the opamp, but of course changing the diodes makes a difference...

[gutsofgold]

I thought that was the case, I remember putting LED's in a Big Muff which is a good indication of diode clipping, and they had a lot of presense and high end bite. They were a lot clearer and took more to clip but the resulting distortion wasn't pleasant to my ears. Maybe for a fuzz, not a distortion/overdrive though.

I would like to use germanium diodes but decrease the gain.

[zyxwyvu]

I would like to lower the available gain without replacing the drive pot so I figured increasing the 4.7k resistor in the clipping amp to say 10k? This would give me a gain range of 5.1 - 55.1 as opposed to the original 12-117. Would this effect the hi-pass filter the .047uf/4.7k combination forms here? Is there a better way to do what I want? 

If you keep the product of the resistor and capacitor the same, you will get the same frequency response. So if you double the resistor, halve the capacitor to get the same sound.

I thought that was the case, I remember putting LED's in a Big Muff which is a good indication of diode clipping, and they had a lot of presense and high end bite. They were a lot clearer and took more to clip but the resulting distortion wasn't pleasant to my ears. Maybe for a fuzz, not a distortion/overdrive though.

I would like to use germanium diodes but decrease the gain.

I think germanium diodes would definitely work better for you. You may need to decrease the gain even more though, since they clip earlier. You might also want to try putting a small resistor (~1k maybe) in series with the diodes. This will make the clipping even softer.

[aron]

Yes +1 on the resistor in series. It might be enough with the stock diodes. I have that same pedal and to me it already clips less than my other tube screamers. Interesting.

[BAARON]

In all fairness, a Big Muff does not use clipping diodes the same way as a Tube Screamer does.  At all.  What works in one will not necessarily work in another, and vice versa.

I hadn't seen a scope readout of LEDs as clippers before, but those sine waves are being hard-clipped to ground, which is not how the Tube Screamer works.  Clipping diodes work differently when they're in the feedback loop of a non-inverting op-amp.  Specifically, they work because when the voltage of the signal rises above the diode's forward voltage, it starts conducting with an impedance of a few ohms, with the caveat that the signal is now boosted equal to the voltage drop of the diode in order to make the op-amp's negative input match the positive input.  This means that everything in the amplified signal above the forward voltage of the diode gets amplified with a gain of 1 plus the diode's forward voltage.

If the forward voltage is higher, there is less clipping.  It stays cleaner.  It is more touch-responsive.  And it is LOUDER.  LEDs have a high forward voltage, and in the context of a Tube Screamer, they are certainly not a hard-clipping device; hence why I suggested them.  I generally find they have a nice transition between clean and dirty tones, hence why I said they sound smooth.

In your first post you said you wanted to use the pedal as a volume boost with very mild overdrive.  The biggest volume boost you can hope to achieve with two Ge diodes back to back is about +0.5-0.6 volts on top of your original signal.  That's barely anything.  Yet another reason to try LEDs.  Don't dismiss them for a Tube Screamer until you try them!


Regarding putting a 1k resistor in series with the Ge diodes: doing this means that when the diodes begin clipping, they have an impedance of 1k instead of just a few ohms.  This does not raise the voltage level that they clip at: rather, it means that when they begin "clipping" and would usually change to a gain of 1x (+0.3v), it's actually very slightly more than 1x.  However, because the bottom half of the voltage divider in the feedback loop is 4k7+0.047µF (and the reactance of a 0.047µF cap is pretty high at the bottom of the guitar's range... 42k, to be precise), the difference is negligable.


I'd try LEDs and try Ge diodes, and see what each kind sounds like in a Tube Screamer so that you know from first-hand experience.  You'll get very little volume boost from the Ge, and even an unamplified guitar signal can push Ge diodes into clipping, so it's unlikely that you're going to get a particularly clean/low gain sound out of Ge diodes, but try them anyway.  The LEDs will be loud and clear, and I've never had a client tell me that they sounded harsh or gross in the modded TS style pedals I've sold.

And of course, if you like the volume of the LEDs with the rounded clipping of the Ge diodes, you could always try them in series, so that the soft peaks of the Ge diodes combine with the high forward voltage of the LEDs...

[petemoore]

  What voltages are the diodes presented with and what voltage do the diodes begin to clip.
   AFAs the BMP, it's got 2 clipping sections in it, not sure which if all were changed to LED clippers, and what voltage they're actually hit with or whether or not...etc.
 AFAs the TS...or what-every circuit like that or not, fiddle-diddlin' with diodes 'n such...with GE in the FB loop, or even to ground, signal output may be weaker than desired, the sound may be compressed and grindy to a greater degree.
  And could also depend on whether you have a boost to bring the level back up when Ge's knock it down, or whether ge's w/resistor to raise output is a tried option.
 All that said maybe you'd like an fet or CMOS type whizbanger, or even a ROG Reamer, can be made to fascilitate easy componential swap-try's or even BB'd quickly.
 Hopefully this started to sound like rambling based on insufficient input and wide variability of what we're talking about exactly..Suggest to do some fiddling until you feel satisfied because it's relatively inexpensive and more informative than trying to get a solid out of the vague, ''moving cloud'' descriptions, or to look outside that box at some more expensive components as the box contributes to the way they work.
  @@Rate, sometimes it's the thing right after the box in Q which effect the sound as expected or not, even a little boost or compressor even after the box can make a big, unexpected result, often favorable...at least in the sense ''now...that's been tried out''.
 

[George Giblet]

My preference is 4x1N914 (2 each side).   Tune in the 47nF bass cap to suit you taste.

[Ripdivot]

You could use several germaniums in series to get the level you desire but still maintain the germanium tone.

[Brymus]

I never liked the sound of LED clipping but I only tried it in a Big Muff. I guess I could give them a go, I'll test a few different colors and see what has the highest forward voltage.

I did like the sound of LEDs once but not anymore.So I agree most times they sound nasty.
In my TS types I like a 1n4001 with 2x 5711s in the FBL and add 2x 1n34 to ground at output on switch.
The 4001 with the 2x 5711s has a nice smooth asymetrical distortion,much better than 914s or 4148s

[arawn]

There is a different chip ou can try also The rc4560. It is a TI chip, intended as an upgrade from the 4558. It has a lower noise floor and better gain characteristics. The best part is you can get samples from TI for Free! 

[Mark Hammer]

1)  Any and all suggestions for different op-amps are usually predicated on inserting them into the stock circuit.  It is the interaction between the op-amp's internal structure and the diodes in the feedback loop that makes the difference.  Given that the OP wants to make other changes to the circuit, chip changes may represent they predicted "improvement"....or they may not.

2) Although it is a trivial matter to simply swap out the 500k gain pot for a lower value, that will reduce the overall max output.  Consequently, the optimal strategy is to try and aim for less clipping with the same, or perhaps more, output level.  I would suggest use of a 2+2 diode configuration.  That is, 2 diodes in series replacing each single diode presently in the circuit.  You can do this on the sameboard simply by unsoldering one end of the existing diodes, installing one end of an identical diode in the empty spot, and twisting together the free ends of the two series diodes.

3) A slightly warmer and less "honky" tone could be achieved by replacing the .047uf cap in the first op-amp stage (which starts tapering off bass around 720hz) with a .1uf or perhaps .15uf cap.  If you replace the 51pf cap in the feedback loop of the first op-amp stage with an 82-100pf cap, that will roll off more top end at max gain while not doing anything particular noticeable at gains less than 3/4 or so.

4) Take a look at Jack Ormans excellent document on the TS-style tone control and ways that can be modded to achieve different tonal shifts.  Well worth a read.