Prentiss Effects

Let's build the Madbean Turnabout (a.k.a. JHS Andy Timmons clone). I got the chance to play the JHS @ (Andy Timmons) at the Chicago Music Exchange a while back and it sounded pretty freaking good. It had a nice, thick overdrive/distortion and the clipping options made pretty drastic changes which was cool. So when a vero layout based on the pedal showed up, I knew I had to make it. I'll go through the fairly simple circuit theory that this pedal uses and then quickly go over my process for building this pedal. Lastly, I'll say a couple words about how knowing the theory behind this pedal helped me to troubleshoot an issue I had.

Please note: the circuit used here is from the Madbean Turnabout which is similar to, but not exactly the same as the JHS @. There are only two significant differences which I'll point out as they come up.

The Theory 

Something I like about this circuit is that it's pretty straightforward from a theory standpoint. Nothing crazy, just basic building blocks that result in great tone. We'll be using the schematic from the Madbean website for this but taking it one block at a time. I've gone ahead and marked up a copy of the schematic based one what the blocks are for our analysis here:

Power Section

This is a pretty standard power section. The 100uF and 10uF caps do some DC filtering to get rid of residual ripple from the power supply. A pair of 22k resistors forms a voltage divider to bias the op-amps for the signal path.

1st Gain Stage

The first gain stage is very simple. It's a standard non-inverting op-amp configuration like you might see in a Tubescreamer. The difference here is that it doesn't include diodes in the feedback look for clipping. A low pass filter, centered at 723.8Hz is made with R5 (1k) and C4 (220nF) which could be fun to play with in order to change up how the mid frequencies are affected.

If you look at the gain range for this section, then using the equation for a non-inverting op-amp we get 2 at the low end and 101 with the drive knob all the way up.

2nd Gain Stage

The second gain stage is another basic building block. It's set up in an inverting op-amp configuration. In this case R6 and R7 set the gain for this stage at 100. Nothing super fancy is happening here, you could tweak those resistors to mess with the gain but it's probably not worth doing since you already have the gain knob in the first stage.

Note: The coupling capacitor, C7, is different between the Madbean and JHS pedals. In the JHS version this is a 2.2uF cap as opposed to the 1uF in this circuit.

Clipping Options

The clipping options on this pedal are pretty cool. The pedal uses an on-off-on SPST toggle switch. In the center position the contacts of the switch don't connect. Since the LEDs are outside of the switch, they are able to conduct and clip the signal. When the switch goes to either of the other two positions, the diodes and/or MOSFETs have a lower clipping threshold than the LEDs so they conduct and clip the signal instead of the diodes.

Note: This is just how the Madbean Turnabout configures it. Changing clipping diodes is a great way to achieve new sounds from a circuit. The JHS Andy Timmons has a similar 3 way toggle, but they use LEDs for their 50W mode, BAT41 diodes for the 25W mode, and no diodes for their 100W mode. You can try all sorts of combinations with your own setup. Just make sure they have lower forward voltages than whatever the combination in your middle position is or the middle position will start clipping first.

Tone Controls & Output Stage

Summary

References

The Mod

Parts:

• Joyo Ultimate Overdrive
• 500K Log Potentiometer (link has a drop in replacement for the board mounted one)
• 1K5 Resistor
• 470K Resistor
• 100nF Capacitor

Schematic

Here is a schematic of the changes we'll be making. We'll then go step by step through how to perform the mod.

Steps 

When desoldering, it will probably be easiest to do it from the underside of the board. I recommend marking which components to remove on the underside. Then you can put the board back into the enclosure and use the enclosure to stabilize the board while you desolder.

1.  Desolder the drive pot and replace it with the 500K Log pot. This will give you a smoother  transition through the gain range of the pedal. Alternatively, you can solder a 330K resistor across the outside lugs of the gain pot per the original mod.
2. Solder the 1K5 resistor to the outside lugs of the Tone pot. (You may find it easier to do this on the under side of the board)
3. Solder the 470K resistor to the outside lugs of the Volume pot. (See note in step 2)
4. Remove C9, a 1uF capacitor, and replace it with the 100nF capacitor.
   Note: If you have a board version prior to "2010.11.24" then you'll need to replace C7, which is a 10uF, instead. 
5. If you're board has a version date of "2010.11.24" (which all the recent ones do) then you'll need to remove the germanium diode and turn it around. The silkscreening is wrong and means that only one half of your signal will be clipped if you don't change this. I put sockets on the underside so I can easily swap out different diodes to taste. 
6. Optional: The wires from the footswitch to the main board are cheap and prone to breaking. I recommend replacing all of them with some good wire so it doesn't die on you when you're playing live. 

What it does

The mod is fairly simple as far as theory goes. The original mod by Freekish Blues uses 3 resistors in parallel with the pots to lower their values and thus limit their range. By using a 330K resistor, the gain pot goes from 1M down to 250K. I prefer to just swap it for a 500K Log pot since this is the preferred value on the OCD. Also, the log taper makes it a smoother transition across the gain range so it doesn't need to be limited to just 250K. The volume pot gets reduced in the same way from 500K to 250K.

The tone pot is a bit more interesting. It forms a simple low pass filter with the 100nF capacitor. With the pot turned all the way up the cutoff frequency is originally about 16k. By reducing the value of the pot with the parallel resistor, we get a different cutoff frequency. The 1K5 resistor takes our tone control down to a value of 1.3K which is pretty cool since we can't just go buy a potentiometer of this value. By doing this the cutoff frequency becomes roughly 122K. Now, to be fair, you can't hear about 80% of those frequencies...but it is brighter. Also, since the tone pot is a log taper, you're going to have a really wide range of usable settings across the whole knob.

Lastly, the capacitor change just limits how much bass is allowed to pass through the circuit before hitting the tone control.

I hope you found this a helpful bit of info. If you did then leave a comment telling me what you found most useful so I can focus on those things in later posts. Also feel free to leave a link to any pics of your mods.