Tonebender MKI brighter

[mattmayhem]

Hey folks,

I have a tonebender mki that was built for me. I read that it was based on a Maestro fuzz tone, but it sounds significantly darker and more muffled than than my fuzz tone clones. Would it be possible to swap a cap in the circuit to brighten this pedal up or is it more involved than that? Thanks!

[Fenton Bresler]

The ridiculous 2.2M resistor on the end is going to make the brightness very dependent on what you're plugging into. Any capacitance to ground in the following circuit is going to make one hell of a LPF along with that 2.2M of resistance. Even the capacitance present in 6ft of a typical instrument lead will roll off the highs.

[mattmayhem]

The ridiculous 2.2M resistor on the end is going to make the brightness very dependent on what you're plugging into. Any capacitance to ground in the following circuit is going to make one hell of a LPF along with that 2.2M of resistance. Even the capacitance present in 6ft of a typical instrument lead will roll off the highs.

Interesting. So would you recommend changing the value of that resistor or placing a buffer after the tone bender?

[Fenton Bresler]

I would replace the 2.2M with something around 1000x smaller and then experiment with a cap to ground across the volume pot to get the desired top end roll off.

[Rob Strand]

The points Fenton Bresler brought up are valid but it's not as bad as 2M2 implies.
The output impedance is set by the pot more than the 2M2 itself.  If you set the pot to half-way the impedance is just under 250k.

The thing about some of those old pedals is they need the cable capacitance the knock down the nasty frequencies above 2 to 3kHz.    Variations in cables means this isn't consistent but it's often better than nothing, except for extreme cable lengths.


I think your issue has probably got more to do with the differences in the output circuits  between the Maestro and the Sola Sound.   On the Maestro the last cap is 3n3 whereas the Sola Sound is 100n;  both feed a 47k load more or less.  That's going to make the Sola Sound a bit more bottom heavy/muddy and perhaps that's what is missing.  So perhaps play around with the 100nF cap on output to see if  pushes the sound closer to where you want it to be.

[Fenton Bresler]

The points Fenton Bresler brought up are valid but it's not as bad as 2M2 implies.
The output impedance is set by the pot more than the 2M2 itself.  If you set the pot to half-way the impedance is just under 250k.

Interesting! I ran a few simulations and the 2M2 resistor isn't nearly as influential as I thought. How does one calculate how much it's really "worth"? I'm only seeing the frequency of the LPF formed with any following capacitance to ground roughly double when replacing 2M2 with 2K2.

[Rob Strand]

Interesting! I ran a few simulations and the 2M2 resistor isn't nearly as influential as I thought. How does one calculate how much it's really "worth"? I'm only seeing the frequency of the LPF formed with any following capacitance to ground roughly double when replacing 2M2 with 2K2.

The general idea is the capacitance 'sees' the impedance looking *into* the output terminals.   So if the pot is set half-way you see 250k to ground.   Then along the other path you see the other 250k of the pot in series with 2M2  then it hits the transistor stage.   As a first approximation you can treat the transistor stage as zero impedance.  So looking into the output terminal you see the 250k to ground in parallel with 250k in series with 2M2 ~ 250k // (2M2 + 250k) = 227k which is just a bit under 227k.

More detailed analysis can include other stuff:
1) The 1M input impedance of the amplifier will appear in parallel with the 250k to ground, which brings the 250k down to 200k.
2)  Going to finer details: You could add the output impedance of the last transistor stage, 15k in parallel with 47k, to the 2M2 + 250k but it's not going to make a difference.
3) Going to still finer details, If the output cap is small then it will disconnect the 15k at low frequencies leaving only the 47k.    This means the output impedance is different at low and high frequencies ie. the response isn't flat.

Point (3) represents a very small effect. 
Point (2) represents a small effect.  If you changed to 2M2 to 2k2 you should realize that the 15k // 47k is now bigger than the 2k2, but it's still small compared to the 250k which is in series.  (With 2k2 the capacitance sees  just under 250k/2 = 125k.)

The general concept here is the Thevenin equivalent circuit.  The cable capacitance is driven by a circuit with an impedance equal to the Thevenin equivalent impedance.  (You can read-up about it on the Web).

[mattmayhem]

The points Fenton Bresler brought up are valid but it's not as bad as 2M2 implies.
The output impedance is set by the pot more than the 2M2 itself.  If you set the pot to half-way the impedance is just under 250k.

The thing about some of those old pedals is they need the cable capacitance the knock down the nasty frequencies above 2 to 3kHz.    Variations in cables means this isn't consistent but it's often better than nothing, except for extreme cable lengths.


I think your issue has probably got more to do with the differences in the output circuits  between the Maestro and the Sola Sound.   On the Maestro the last cap is 3n3 whereas the Sola Sound is 100n;  both feed a 47k load more or less.  That's going to make the Sola Sound a bit more bottom heavy/muddy and perhaps that's what is missing.  So perhaps play around with the 100nF cap on output to see if  pushes the sound closer to where you want it to be.

Thanks for the info! This did the trick. Luckily had a 3n3 and now it sounds a lot more in line with the sound I was after.