The Beatles Pedal

[pilau]

This is my first time posting here... I've been trying, for the better part of a year, to obtain this pedal. It beautifully replicates the sound of the Beatles - and much better than premium and expensive alternatives like Aclam's Dr. Robert or Jext Telez's White Pedal (no dis). Hear for yourself! Unfortunately the maker has gone completely AWOL... I was able to get a few replies from him last year but he disappeared. I think he's had enough with making pedals. His website and official social media pages have seen zero activity over the last year except for comments like "it's been months and I haven't received my order yet"...

So after months of holding out & being hopeful, I felt that enough is enough, and decided to take a crack at tracing this seemingly simple pedal using whatever little evidence there is to scrape from the maker's social media. I annotated everything I could figure out on my own. I also chose to hide the logos and maker information because I feel uncomfortable, considering the maker's behavior, to hurt his business, should he decide to come back some day.

First of all, the website provides the following technical information:
This is a completely original circuit (like most if not all of his excellent stuff)
The drive potentiometer controls the amount of distortion that is applied to the signal.
The level potentiometer controls the overall output level of the stompbox.
Runs on standard Boss power supply.

And now here is what I have so far: (link to imgur album)
1. First thing that stands out to me is how he was able to recreate the same effect with completely different resistors and capacitors
2. I guess the biggest challenges are the transistors, capacitors, potentiometers & diode - very little of those is visible due to camera angles - but my hope is that perhaps there are some common truths (it is a sort of overdrive/light distortion pedal after all) and you lot who breath this stuff on the daily could perhaps fill in these blanks based on common practices etc. - fingers crossed!
3. I think it's obvious there's 2 different kinds of transistors in use. However, it's only possible to see the labels of 1 of them because of the camera angles - and even then, I can't make out what the whole thing, but I'm pretty sure it ends with 56A. Rings any bells?
4. I included extra crops of the visible transistor in different brightness & contrast presets in case it helps anyone make out which one it is
5. I have good reason to believe the second transistor is a 2N2222A. The reasons are: the maker is using it in other pedals (I saw it in 2 fuzz models of his), and in those photos the packages looks exactly the same: the shiny texture, the dimple on the rounded side and even that extra plastic sticking out from the top.



The question marks mean I wasn't sure I was able to discern the exact colors of the rings



This point to point version is missing 2 capacitors, 1 resistor & 1 diode, but reportedly sounds exactly the same!



Zoom in to help figure out this mess


Obviously there are a few pieces missing here, but this is what I could trace so far (disclaimer: I have no idea if anything here actually makes sense. Take this with a grain of salt):


Needless to say any help is greatly appreciated!

[grrrunge]

Second transistor looks like an old Philips made BC556A

[grrrunge]

Here's what I got from the photos of the one with flying leads. Just another common emitter amplifier
The PCB version looks like he has added reverse polarity protection, power supply decoupling and a small ceramic capacitor - probably for HF noise suppression.

[pilau]

Here's what I got from the photos of the one with flying leads. Just another common emitter amplifier

Cheers grrrunge, for taking up the challenge! And also thank you so much for drawing such a beautiful circuit diagram

The PCB version looks like he has added reverse polarity protection, power supply decoupling and a small ceramic capacitor - probably for HF noise suppression.
IMG_20230131_065538.jpg

That's exactly the kind of information I was hoping to learn.
1. It looks to me that the reverse-polarity protection diode sits on the V+ rail, instead of across the V+ and GND right?
2. What do you reckon the 100ohm resistor is for?
3. What is the purpose of the 2 decoupling capacitors in the signal path? (right after the input & right before the level pot)

[Frank_NH]

The irony of the various "Beatles pedals," which are marketed to give you that authentic Beatles tone, is that the Beatles themselves didn't use too many pedals in their recordings and concerts. Up through 1968, their pedals were mainly fuzzes and volume controls. Hence the guitar tones you hear on their classic albums came mainly from their amplifiers (usually cranked), but also from studio equipment like Fairchild compressors, studio echo/reverb, ADT, etc. Hence, pedals like the Dr. Robert, Jext Telez's White Pedal, and numerous others are attempting to simulate the amplifiers and in some cases the studio effects. Note that the various Vox amplifiers they used in Revolver and Sgt. Pepper sessions had on-board reverb, tremolo, and fuzz/distortion.

Simple transistor gain stages can give you a "Beatles" sound if paired with the right guitar and amp. If you want to build a simple effect for Beatles overdrive/distortion, the Lumpy's Tone Shop - Lemon Drop is a good one.

viewtopic.php?f=7&t=22365&p=259106#p25910

[pilau]

Frank, thanks for your reply. I'm aware of and familiar with everything you wrote, and you are absolutely right with all of it. By the way, I looked through your post history, and you seem to be quite the expert on the topic. I must say it's pretty cool that you chimed in

I am also familiar with Lumpy's Lemon Drop, aka Eletric Majik 7 Series Overdrive. It's a great pedal for 70s tone. I also agree that a coupling with the right guitar and amp is essential. Have you gave a listen to the sound sample I shared? I think the pedal in question nails the Beatles tone much better than the Lemon Drop.

Either way I'm still hell bent on cracking this one!

[pilau]

I've kept messing with this circuit in my head, trying to come up with more ways to uncover the missing puzzle pieces:
1. Are there common potentiometers that pedal builders frequently use in similar circuits/configurations? That refers both to resistance and taper too
2. Same question about the decoupling capacitors in the power supply path & in the signal path

a small ceramic capacitor - probably for HF noise suppression.

3. Finally, what capacitor value would normally be used for the high frequency noise filtering?

[Nocentelli]

1. Are there common potentiometers that pedal builders frequently use in similar circuits/configurations? That refers both to resistance and taper too

The maximum drive available is set by the 200r resistor: The drive pot wil only decrease the gain as the resistance between pins is increased from zero. Crudely, the ratio of the collector resistor value to the emitter-resistance-to-ground value sets the gain. A10k would probably give a wide range and suitable "feel" to the control.

The output cap value and volume pot value together set the amount of low end in the output signal: 100n and A100k is very common.

[grrrunge]

Agreed - 10K log taper for the gain control sounds like a sensible value.
Volume pots should be log taper as well. 500K log would be a good starting point. Lower values than that will start to impact the maximum available gain from the common emitter amplifier severely.

[pilau]

Thanks Grrrunge and Nocentelli.

It looks to me that the reverse-polarity protection diode sits on the V+ rail, instead of across the V+ and GND. What do you think?

[mediy]

Thanks Grrrunge and Nocentelli.

It looks to me that the reverse-polarity protection diode sits on the V+ rail, instead of across the V+ and GND. What do you think?

To my mind that is the better way of doing it. You get some voltage drop over the diode, but in return you get much better protection.

[pilau]

Thanks Grrrunge and Nocentelli.

It looks to me that the reverse-polarity protection diode sits on the V+ rail, instead of across the V+ and GND. What do you think?

To my mind that is the better way of doing it. You get some voltage drop over the diode, but in return you get much better protection.

Yes I agree, thank you for the added confidence.


What do you reckon this 100ohm resistor is for? (marked in orange)

[grrrunge]

This is probably a good starting point for experimentation.

There are too many uncontrolled variables to my liking in this design.
Output impedance is high, and input impedance is low.
Input impedance varies with the gain setting, and output impedance varies with the output volume setting.
No pulldown resistor on the input.
The tiny mlcc looks like it sits between output and gnd. However it doesn't make sense that way.

The reverse polarity protection is how I would have done it though. I give it 1 point for that

[Manfred]

Except for a noise level reduction, a parallel connection of the transistors does not change anything.
Each silicon small signal transistor can be replaced by any other in the low frequency range.
The invidual internal capacitances of these only have an effect in the high frequency range.
I ran some circuit simulations under different combination with small signal transistors with low and high Hfe.
The gain of the stage changed only slightly in each case, the frequency response remained the same, a change in the shape of the distorted signal did not change anything either.
The 10kOhm Potentiometer worked fine.

[grrrunge]

Except for a noise level reduction, a parallel connection of the transistors does not change anything.
Each silicon small signal transistor can be replaced by any other in the low frequency range.
The invidual internal capacitances of these only have an effect in the high frequency range.
I ran some circuit simulations under different combination with small signal transistors with low and high Hfe.
The gain of the stage changed only slightly in each case, the frequency response remained the same, a change in the shape of the distorted signal did not change anything either.
The 10kOhm Potentiometer worked fine.

+1
HFE will mainly affect input impedance in this application.

Current noise from the internal emitter resistance willl drop by the square root of 2 whenever you double the amount of transistors. However you need better layout skills than the ones on display here, if this is where you are hunting for noise improvements.

The effect of parasitic capacitance will vary with gain (Miller effect) along with the output impedance of the signal source driving the common emitter amplifier.
IE: For high gain settings and high impedance pickups, parasitics will be more significant.

[Manfred]

HFE will mainly affect input impedance in this application.

Not easy to say so since there are two mixed negative feedback types in this circuit, a voltage negative feedback and a current negative feedback.
The voltage negative feedback is determined by the resistance between collector and base and the internal resistance of the guitar electrics.
For the principle measurement I ignored the guitar electrics.
It showed that there were differences of the input impedance with the different parallel circuits of the transistors but these were in a similar range, so that the influence of Hfe was not so much.

Current noise from the internal emitter resistance willl drop by the square root of 2 whenever you double the amount of transistors.
However you need better layout skills than the ones on display here, if this is where you are hunting for noise improvements.

That is so far clear to me that this noise figure improvement is not essential, there are the "moving coil" pickup preamplification, there were several BJTs connected in parallel.

The effect of parasitic capacitance will vary with gain (Miller effect) along with the output impedance of the signal source driving the common emitter amplifier.
IE: For high gain settings and high impedance pickups, parasitics will be more significant.

The miller capacitance (C_CB * Stagegain) is transformed between base and emitter and is added to C-BE there.
This capacitance is then connected in parallel to the R-BE whose value is somewhere between 20 to 40 kiloohms.
With the 2N2222 transistor C_CB is maximum 8pF, with the BC548 6pF, this results in maximum millers capacitances of 1000pF to 1280pF, where C-BE is already negligible.
That means these capacitances of the transistors in the parallel circuit variants are approximately also in a range.
In how far this capacity results in a tone change in the interaction with the pick-up, one must try out in practice.
The measurements of the frequency response showed a flat course between 13Hz to well over 100kHz.
Edit: Chart with corrected Values.

[grrrunge]

Manfred, I respectfully disagree on your comments on the significance of miller capacitance, when taking pickup inductance and winding resistance into account.
Below is a simulation of the circuit where the gain setting is stepped. The top plot shows input impedance while the bottom plot shows overall frequency response.
For a worst case analysis I use a model of a high output humbucker to feed the circuit. Using single coil pickups will yield less grim results, however.

[Manfred]

Thanks Grrrrunge,
very interesting, I was not shure regarding the pickup modelling circuit.
I made a fault, I used 470n insteed 33n Capacitors, so I have to correct the chart above soon.

[Manfred]

Now I took a closer look at your circuit simulation. The equivalent circuit of the pickup is not complete, the capacitor which replaces the capacitance of the winding is still missing.
The capacitor in parallel to the output which replaces the capacitance of the instrument cable must also be added as well as the tone control circuit.

I rummaged in my documents from the beginning of the 1980s, at that time I began to be interested in the topic and found an article by Helmuth Lemme from March 1981 and my own notes.
I soon gave up my own measurements because the necessary measuring instruments were not affordable at that time.
Helmuth Lemme is a graduate physicist and is a pioneer in this field.
https://www.buildyourguitar.com/resourc ... /index.htm
Later, after studying electronics, I had the mathematical skills to make calculations.
But then I gave that also up, because there was no data for most of the pickups and it also made no sense in general.

[grrrunge]

Agreed, interwinding capacitance and cable capacitance won't improve the matter.
However, interwinding capacitance is negligible as it is mostly series coupled.
Cable capacitance on the other hand could very well be a contributing factor. At 150pF/m cables can't be longer than a few meters before having just as much influence as the Miller capacitance we discussed above.