Minimal and Scaled Tube Screamer Version

Let’s continue covering arguably the most famous overdrive pedal out there – Tube Screamer. This post continues building on previous posts: breadboarding the pedal and how it shapes the tone.

In this post I’ll try getting Tube Screamer sound with completely different set of components. Well, different values of the components – it is still exactly the same topology of the circuit.

I’ll cover quickly the methodology, then propose the alternative circuit, and then do a quick analysis of it. I’ll breadboard it (there’s also table with bill of material) and then compare the sound of it to my TS9 pedal.

If all goes well, we get a very close sounding circuit. While looking like a possibly pointless exercise, it is important to try things out 😊. This will either confirm or disprove the points I made in the previous post. Fingers crossed I don’t embarrass myself 😁.

If you have no patience to read all of this – just watch the video straight away.

Methodology

How to approach this? Let’s say I want to use 10K pots everywhere for some reason. How do I preserve all the filter cut-off frequencies for example?

Let’s do this exercise on the overdrive pot:

First of all – I want to preserve maximum gain of 41dB. If I replace 500K pot with 10K pot, I need to scale R4 to preserve the gain. The new pot has 50 times lower resistance, so I need to use about 50 times smaller R4 resistor.

If I use 100 ohm resistor, and I change R6 from 51K to 1K I get the gain range from 20 to 41dB. That’s what I want. So I effectively scaled down all resistors 50 times. (I don’t need to touch any other resistors mind you, since they are not affected by this).

Now, in order to preserve 720ish Hz cut-off frequency, I need to do something with C3. The theory is – if I scale down my resistor, I need to scale up the capacitor. If I do this 50 times, I get – 47nF*50 is about: 2.35uF, let’s call it 2.2uF (for a standard value). Will this work?

Here’s what the calculator says:

Note the scaled version:

That is pretty close, so at least in theory, this should work! I need to actually scale C4 as well, since the high pass filter using it is now affected.

After that, I just need to go and scale all the other potentiometers in a similar fashion. This will mean that I’ll probably need to adjust lots of components, but I’ll be just following this methodology and I should be more or less fine.

Alternative Circuit

I had to scale down Tone pot 2 times and Level pot 10 times. This is the alternative circuit I came up with:

Note on the designator values. I tried keeping the component designators in such a way that I can relate them between the “original” and my alternative version. So if you see component R101 in the diagram – the component is equivalent of R1 in the original schematic, R116 is equivalent to R16 etc.

Just so I keep the track what I’ve changed and what were the original values compared to the new – alternative values. And when I start writing about different pedals using the same circuit topology, I’ll do similar thing, I’ll just identify them as R201, R301 etc.

Analysis

How close did I get the scaling?

Well, if I add all the relevant filter cut-off frequencies here’s what I get:

Just a quick glance and all looks pretty good. Let’s have a closer look.

Clipping Section

Not much to say here for the clipping section:

Minimal differences, and very unlikely that they’ll cause any noticeable difference in sound.

Tone Section

Let’s have a look at the tone section:

The same story here – pretty much everything the same. I did put a warning there at the filter formed by output coupling capacitor – C107 and the Level pot (RV103). I also decided to use linear pot instead of audio pot. This means that for only 5-10% of the rotation there will be some effect on the audible spectrum. Just something to test out before committing.

I’m in two minds about Level pot being linear or audio. Boss pedals, for example, use linear pots for Level control. Also – I do have to note that I’m using linear pot for Tone – the taper in Tube Screamer is G (or 4B). I’ll have slightly bit less of a control, but should not affect the sound itself.

Component Selection

When I said in clipping section that everything looks pretty much the same, that isn’t really true. What is not normally seen on any of the schematics is the component selection.

I’m not sure what exact capacitor is used in Tube Screamer for C103, but I expect it does not have tighter than ±10% tolerance. I’m using 1% tolerance resistors, but I expect that original TS808 used something like 5% tolerance. I’m just speculating, the tolerances could be even worse.

The whole point I’m getting to is that I’m using electrolytic capacitor which has ±20% tolerance. So 720Hz cut-off really translates to 600-900Hz range for ±20% tolerance if I’m not mistaken. ±10% cap tolerance gives 650-800Hz. This might actually make some audible difference if the difference is at the extreme ends. Just saying.

Frequency Response Diagrams

Let’s see how the frequency response looks like for the two versions of the circuit. It’s dangerous to infer anything about sound from this, but if they at least look alike I’m on the right track.

Here’s how frequency response looks like while sweeping Overdrive knob:

The above diagrams look very much alike. Very little difference – which would indicate that the math and logic behind is correct.

If I look at 3 different Overdrive levels and do the Tone sweep, let’s how does this looks like:

Again, looking pretty similar, I’ll need to test the circuits, but looks like I’m not far off at any rate.

Spice

In case you want to check this out in SPICE, here’s the simulation diagrams:

Breadboarding It

Let’s do the breadboarding of this, here’s a diagram I came up with:

I used pots with PCB pins so I just plugged them into the breadboard.

Here’s the DIY file:

Bill of Material

Here’s the list of components I used for breadboarding:

Designators	Component	Notes
	Breadboard	Any breadboard will do, I used full size breadboard. You probably don’t want anything smaller than that.
	Jumpers and wires	As many as you need. I got some online, but 24 AWG solid core wire will do just fine.
	9V battery	I used 9V battery snap with Dupont wire.
	DPDT push button	I used ALPS SPPH410100 latching push button for bypassing the effect – totally optional.
IN	Switchcraft 12B	In – stereo jack (mono will do too)
OUT	Switchcraft J111	Out mono Jack
RV101, RV102, RV103	Alpha 16mm 10K linear pot	Any 10K linear pot will do, I used 16mm with PCB pins – for Overdrive, Tone and Level control
C101	10n (0.01uF)	Metal film PET cap
C103	2.2uF	Electrolytic cap 25V
C104	2.2n (2200pF)	Metal film PET cap
C105, C106	470n (0.47uF)	Metal film PET cap
C107	1uF	Electrolytic cap 25V
C110	100uF	Electrolytic cap 25V
C111	47uF	Electrolytic cap 25V
C112	100n (0.1uF)	Ceramic X7R
D101, D102	1N4148	Small signal diode for clipping
R101, R106	1K	1% 250mW metal film
R104, R108, R111	100 ohm
R116, R117	10K
R105, R110, R118	2.2M
R107, R109	470 ohm
U101	TL072	op-amp

Trying it Out and Final Thoughts

Here’s the video of me trying it out:

I’m sure some keen ear has heard a difference in sound. The question is how much in a band situation you could hear it, and if you’re not just imagining it in the first place 😋. At any rate – I unilaterally declare it a Tube Screamer.

There seems to be some faint, high pitched oscillation noise heard around 16:15 mark in the video. I only noticed it when editing, I did not notice it at all while recording. I’ll investigate and report back at some stage.