anyone know of a simple transparent boost that I can add on to Tubescreamer?

Started by Hav, September 26, 2014, 11:16:15 AM

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I know biasing BJTs can be a bit trickier, what if the OP replaced the transistor with a JFET booster? Just add an equal drain resistor so the gain is set close to unity, then add a switch that adds a bypass capacitor + a series pot on the source?


Two volume knobs on a switch perhaps?  Seems pretty simple and won't be messing with the tone control stage, extra transistors, or extra op amps. 

I'd try it first as it takes few parts and keeps the tone the same.  Might accomplish exactly what you said you wanted.


Thank you everyone for the replies! I am getting a better understanding...

It makes total sense now that it goes TS > Boost > Amp compared to what I was saying before... (Boost > TS > Amp).

Sorry if i can't reply to everything said, a lot of it is just past my current knowledge... and I haven't quite worked out how to go from a schematic > pcb layout yet..

Quote from: aab0mb on September 27, 2014, 12:07:45 AM
Two volume knobs on a switch perhaps?  Seems pretty simple and won't be messing with the tone control stage, extra transistors, or extra op amps. 

I'd try it first as it takes few parts and keeps the tone the same.  Might accomplish exactly what you said you wanted.

This looks very interesting, and very simple which I like.. however, if this is the one footswtich for the pedal, how do turn it on and off? This looks like it will just switch between the two different volume pots. However, this would be unbelievably awesome as it will get into a 1590B.

Quote from: David on September 26, 2014, 05:30:38 PM
What about an Adjusticator?  Pretty simple, runs on 9V, huge gain AND even cut if you need it.  You could crank the crap out of the Screamer's output but still keep the overall level manageable by cutting the level on the Adjusticator.

This also looks like a great idea. As I said can't read schematics too well but would this work?:

Quote from: PBE6 on September 26, 2014, 04:00:13 PM
Some simple ideas:

2. Alternately, wire a DPDT switch (again a 3PDT switch will be fine) that bypasses the feedback resistors R7 and R19 and replaces it with a lower value resistor, or better yet a pot wired as a variable resistor.

Again, this sounds like an awesome idea, but slightly unsure how this translates into a layout... If anyone could do a really quick sketch and explain why its been drawn like that it would probably help me and a million other noobs understand a bit more? I know i'm cheeky to ask!  :icon_wink:

Quote from: PBE6 on September 26, 2014, 04:29:40 PM
YouAre, this used to bother the bejesus out of me because if you put a Tubescreamer on a scope you see something quite different than what you expect.

In an ideal world, if you place diodes in the feedback loop of a non-inverting gain stage (just like the Tubescreamer), then once the boosted signal reaches their forward voltage threshold the diodes start conducting like crazy. However, this does NOT turn the gain stage into a unity gain buffer (this is the counter intuitive part - you would think the diodes turn into essentially straight wires at this point, but they don't). The reason is that the forward voltage drop across the diode affects the opamp performance, as becomes clear if you look at the governing equations:

Vout = Av(v+ - v-)

In a normal gain stage, v- will be some proportion of Vout based on the voltage divider formed by Rf and Rg. The trick here is to see that when the diode is conducting, v- is not simply equal to Vout as in a buffer, but:

V- = Vout - Vdiode

Subbing this back into the first equation we get:

Vout = Av(v+ - v-) = Av(v+ - (Vout - Vdiode))

Dividing by Av (a huge number), we get approximately:

0 = v+ - (Vout - Vdiode)

Solving for Vout:

Vout = v+ + Vdiode

This means that the output is a combination of the original signal (v+) and the clipped signal (Vdiode). This is responsible for the relative transparency of a Tubescreamer-style overdrive (if you listen closely, the original signal can be heard alongside the hash), as opposed to a MXR Dist+ style distortion where any signal above Vdiode is simply chopped off. This also explains the shape of the waveform of a Tubescreamer being fed a sine wave, which looks like a normal sine wave but pinched on the sides and stretched upwards in the middle.

In answer to your other question, the output from a 2 V input would be close to 2.7 V and not 0.7 V as you might otherwise expect.


Quote from: karbomusic on September 26, 2014, 02:15:47 PM

Based on his last reply, I don't think that would really matter if all they want is what they already have a little louder. Maybe we are all are overthinking something simple. :) I might recommend the "variable gain" booster at the bottom of this article:

Yep, again another great idea, i'm actually going to try draw this out, will post back here to see if im right or wrong with what I have done..

Thank you all for replies again so far!


Just to let you guys know I actually started building this pedal before I started this thread, but I used a Brian May Booster > TS > amp and you know what it actually sounds quite awesome! Major sound alteration of the TS which I would like to get rid of in the next build (hence this thread). I shall put some pics and sound clips up soon incase anyone wants to know the turnout of this build I've just done. Turns out to be a very vey versatile pedal from some crazy high gain distortion all the way through the tubescreamer sound to a very small break up boost!



I have finished a project I started before the thread which was a Brian May > Tubescreamer and personally I think it sounds quite awesome (but not what I was looking to accomplish from this thread).

I will get some soundclips of this up as soon as I have a sec, and also when I finish this new build which will probably be Tubescreamer > AMZ Booster > Amp. Thinking of doing something slightly different on this one though, thinking of putting in the Ins and Out Jacks for each effect (4 in total), so I can wire it up whichever way I want, should I change my mind in the future, and link through other effects etc etc.

Will need help on layout out the offboard components though as a quick test, could quite get it all to fit in a 1590BB.. will start another thread for this when the time comes..

Updates as soon as!