UTILITY: Test Box

When you're diving into breadboarding or building circuits, one thing becomes clear—you need a test box.  It can be used with a breadboard or a completed circuit, before you spend time boxing it up in a finished enclosure.

Testing using an enclosure that is known to work eliminates one area of trouble shooting, which often stumps new builders - and that is of course off-board wiring.  If you don't have a firm understanding of off-board wiring, you really need to spend some time on this, as it's crucial to building a working pedal.  

It's also handy that the test box can plug into a regular pedal power supply, battery or bench top power supply - basically treat it like a regular pedal, with a few extra benefits.   Before I had a full workshop set-up, my test box was mounted on a piece of plywood with a breadboard next to it, so I could carry it from the dinning room table to my amp to test (it didn't take long for the wife to suggest an alternative workspace).

I’m still using the very first one I ever built, which looks a right mess, but it works. 

TEST BOX WIRING DIAGRAM

It's fairly simple - it's just normal off-board wiring.  Use whatever switch wiring you prefer; the wiring below is probably the most common and easiest to understand, which is why I used it in the diagram.  It's not what I use in my pedals these days, but is it what's in my test box.  I prefer a wiring scheme with the circuit input grounded when bypassed.

Here's how to build your own:

REPURPOSE

Find a failed project or an old pedal casing you no longer need.  Remove the original circuit, but leave the rest of the components intact.  This gives you a good starting point.

MAKE SURE IT WORKS

Before anything else, test the existing stomp switch and make sure it works.  Reliability is key for your test box, so it's important to verify this first.

TERMINALS
On the top of the enclosure, mount the test sockets.  Personally I like the banana binding sockets as they're really designed for this kind of thing.  You can plug into the top, and/or secure wires with the screw down connection at the bottom.  

Some people use speaker terminal connections - while they work, you have less options for connections.

You will need the following connections:

• Circuit input
• Circuit output
• 9V power
• Ground

SWITCH 
Solder the input and output terminals to the corresponding contacts on the switch (where the old circuit was attached).   Stomp switches are easy to damage with a hot iron, when recycling an old enclosure, do as little as possible and do it fast.

JACKS 

I moved the input and output jacks to the top of the enclosure, as I found it easier to work with on a bench.  

POWER & GROUND

Solder the 9V and ground connections from the old, removed circuit to the newly mounted terminals.

DC / BATTERY CONNECTION

There's no battery connection, as I prefer to test circuits using a generic pedal power supply.  Why?  The cheap power supply should highlight any issues you have with circuit noise - if you plan on selling the pedal, it's good to know that it sounds fine on a terrible power supply.   

I do attach a battery to the terminals or breadboard at times to test vintage style circuits, as I don't always include the option for a DC connection on the vintage style pedals that I make.  I think a battery is the best possible supply of DC power, as it's isolated and ripple free.  That being said, I've never had an issue with my Voodoo Labs power supply using vintage style positive ground circuits with next to no power filtering or protection.    

R2R ELECTRIC: Germanium Pre-amp

It's no secret as to what the R2R Electric germanium preamp is based upon, which is an old and very rare Hofner preamp.  The Hofner is a full-range germanium booster, which was once used as a way to plug guitars into old console radios way back in the day.

As this is a simple Rangemaster style circuit, any layout used for a Rangemaster could be used instead of the one below.  

R2R ELECTRIC GERMANIUM PREAMP LAYOUT

R2R spends a bit of time testing every component used, to get the sound that they're chasing. Since not all components are created equal, it’s worth dedicating time to testing and selecting components to ensure you get the most out of this simple circuit.

One notable choice is the AC107, which R2R recommends as a low-noise, higher-quality alternative to the OC44 or OC71 commonly used in Rangemasters. This decision is practical: while OC44s are highly sought after due to their use in Rangemasters, they’ve become increasingly expensive and are often prone to noise issues.

Side note:  An excellent podcast featuring Chris from R2R can be found here on the Fretboard Journal.   Chris talks about his journey and process. 

And thanks to the person who very kindly helped out by sharing some photos of their pedal, confirming that the boost pot is wired the same way as a Rangemaster. 

Boost / Volume pot:  R2R implemented the volume/boost control in the same way as the Rangemaster. The stock Hofner circuit uses a 10k resistor to the collector and has no volume control - R2R replaced the 10k resistor with a 15k pot. If a 15k pot is hard to source (as it's an uncommon value), a 20k pot is a suitable alternative. You could also experiment with a 10k pot to see how it performs.

Input resistor:  There's no need to worry about using the exact value for the input resistor. The stock Höfner schematic specifies 37k, which is an unusual value. R2R opted for 36k. As long as your choice is close, it should work fine.

It’s worth noting that the input resistor forms a voltage divider with the 10k resistor connected from the transistor base to ground. Together, these resistors reduce the input signal by about 80% before it’s amplified by the transistor.

Emitter resistor: The stock Hofner circuit uses a 1k5 emitter resistor, but there's room for variation, which can be tuned to match your chosen transistor. The R2R layout uses a 3k9 resistor, like the Rangemaster. Smaller values are more likely to produce some dirt, while larger values will result in a cleaner sound. This is a great spot to use a trimmer pot if you want adjustable control.

Capacitors and Bias Resistors:  The capacitor values and the 100k/10k bias resistors remain consistent across designs. However, there's flexibility with the capacitor values:

• The input capacitor can be reduced significantly without noticeable changes in response. For instance, 1u is nearly indistinguishable from 4u7.
• The 100u emitter bypass capacitor can be reduced to 47u with minimal impact.
• Similarly, reducing the output capacitor slightly will have negligible effects on the sound.

Of course, collectively changing a lot of component values at once will increase differences, but ultimately, it is just a fairly generic single transistor booster in therms of the circuit.

Drop down resistors: R2R included 1M input and output drop-down resistors wired to the stomp switch, which you can see in the photos below. These are not present in the Höfner circuit but are a common addition to prevent switch pops. Including them is recommended.

Board:  I’ve not listed the size of the board or any other info on the point-to-point layout - I’m sure you can work that out yourself. 

Images below from a prototype - the 1k5 resistor may not have made it into production models.

BREADBOARD: How To

Breadboards are one of the most effective learning tools when it comes to electronics.  Not only are they great for prototyping and testing circuits, but they also offer a hands-on way to fine-tune and troubleshoot designs—perfect for testing guitar pedals.

In this post, I’ll walk you through how I use a breadboard.  While this approach may not be for everyone, it’s a method that’s worked well for me, and it could help you get more comfortable with circuit building. 

Testing transistors for the Colorsound hybrid fuzz box.  This cheap breadboard doesn't have the power rails connected as per normal - I had to jumper them.

BREADBOARDS - HOW TO

So, where to begin?  You will need:

• a breadboard
• power supply - bench top, battery or pedal power supply will do
• components to play with
• jumpers, which can be solid core wire, or you may have a bunch supplied with the breadboard
• input / output jacks, ideally with an on/off switch, or better yet a test box

You also need to be able to read a schematic and have some basic understanding of how the circuit works.

PLANNING

I don’t typically pre-plan my breadboard layouts.  I’ll take a quick look at the schematic and then dive right in.  Early on, I tried using DIYLC (software for creating vero layouts) to map out my builds, but I found that breadboarding is a hands-on, three-dimensional process.  Once I started placing components on the board, my plans usually went out the window—so I’ve learned to just dive in and adjust as needed.

SO HOW DO THEY WORK?

Breadboards are relatively simple. They consist of rows and columns of holes that are electrically connected in a grid pattern. Along the edges of the board, there are vertical power rails—one for positive voltage and one for ground. You can buy fancier breadboards with additional features, but they all work in roughly the same way: you place your components and make connections using jumpers.

PLACEMENT

When I build a circuit on a breadboard, I aim for a logical, linear layout.  I don’t try to make the circuit as compact as possible.  Instead, I leave a little bit of space between components to allow for easy adjustments and troubleshooting.  Generally if you're breadboarding, you will be swapping components in and out and this means leaving room for clumsy fingers.

For simple circuits, the main goal is making sure everything fits on the board and is properly connected. Once you get into more complex builds, you might need to think about space a bit more, but for testing pedals, spaciousness is often your friend.

For some circuits, I'll build most of it on vero, and then test some components on the breadboard.  There might be a particular section that I want to play with and test, without having to build the entire circuit on the breadboard.  I often use little clamps for this.

TESTING

Usually I feed the circuit a sine wave from my signal generator and test that it's working as expected on one of my scopes.  You could easily use an audio probe to do this as well.

For larger or more complicated circuits, I test as I build, making sure that each stage works.

After it's confirmed to behave as expected, I usually plug in a guitar and run it to my amplifier, as real world testing is the always the best.  Just need to be a little cautious here - I have an amp that I'm not attached to for this purpose.  I've never killed an amp in testing, but there's always a chance right?   There is often a bit of popping and unsettling noises when touching parts on the board - just be aware of this.  

When testing, I usually have a selection of parts to swap in and out to see how they effect the sound - in some cases I'll use a pot instead of a fixed resistor to sweep values.  When I find a setting that I like, I'll take the pot out of the circuit to measure the value and replace it with the nearest value fixed resistor.

For transistors and caps, it's just a case of swap them and see what happens.  

I usually have a few pots sitting in the parts draw with solid core wires soldered on for breadboarding.  It's handy to be able to swap a few around to see what works better for you in the circuit.

TROUBLESHOOTING

OK - you've put all your parts on the breadboard and it looks fine, but it doesn't work.  What next?  Same as testing a regular circuit, check your voltages and probe the circuit to see where you went wrong.  

It's often a component leg in the wrong hole, or a possibly a poor connection somewhere.  Poor connections can be a thing with cheap or dusty boards, quite often on the power or ground rails.  I normally tap and wobble parts to see if they make noises, maybe reseat transistors.  In short, if it doesn't feel secure, it probably isn't a good connection.  Although this could also be a sign that I need to spend more on my breadboards instead of getting cheap ones for a few dollars from China...

PRE-BUILD TESTING

In some cases I breadboard the entire circuit that I plan on making on a breadboard first, using the components that will be used in the final build.  This is usually only for circuits that I know will be tricky to tune.  Example: a MKI Tone Bender.   In some cases I’ll just use the breadboard to test the transistors and not worry about the rest of the circuit.  

OPTIONAL:  COMPONENT LABELS

If you're like me and don't have the best eyesight, small masking tape labels on the legs of components is sometimes helpful.  Especially when you start pulling out small metal film resistors and leaving them all over the bench.  

I can read 4 band, 1w carbon films easy enough, 1/4w metal film is a tad tricky for me.  Sometimes I avoid using 1/4w metal film for this reason.  Same can be said for some caps - greenies are sometimes hard to read.  I sometimes write the value on with a marking pen.

D*A*M: Drag n Fly DF-05

The D*A*M Drag n Fly DF-05 is a hybrid fuzz pedal that combines silicon and germanium transistors, drawing inspiration from the classic Fuzz Face design. 

What sets it apart is its unique filter control, located at the input of the circuit.  This control blends the input signal between a 10uF and a 4n7 capacitor, offering a wider tonal range compared to the traditional 2u2 found in standard Fuzz Faces.   Its brethren the Dragonfly DF-06 has the tone control at the end of the circuit, using something similar to a MKII.

While the overall circuit is straightforward, it incorporates some non-standard component values to fine-tune the sound, and there's a touch of the Vox Tone Bender in some circuit variations - and there were many variations on this circuit.

D*A*M DRAG N FLY DF-05   VERO LAYOUT

This is one of the layouts used by D*A*M - the 0 ohm resistors are often replaced by jumpers under the board.

Looking at the image below, it appears that they used velcro to mount the circuit to the back of the pots.  

SCHEMATIC / LTSPICE

Component values have differed across various production runs over the years. The schematic serves primarily as a guide, illustrating the circuit's topology rather than providing definitive values.  

Anyone familiar with D*A*M circuits will have seen the input capacitor blend on a few other designs.  

This is what the signal looks like at the base of Q1.  Basically it sweeps the bass response from thin to fat.  The small bump in the middle is a result of the 47pf cap across the collector and base of Q1.

VARIATIONS

There are so many…  check the D*A*M forum post here  These are just some that I've seen; there are probably more.  So if you don't have the "right" transistors, maybe don't stress about selection too much, as a lot were used over time.

• 10k instead of 18k resistor on Q1 collector (likely to be transistor selection related)

• 2 x trimmers for biasing on some layouts

• 56k resistor instead of 120k between the base of Q1 and the emitter of Q2

• 10uf and 3n3 on the input blend

• R1 / 1 meg resistor on the input, not always used

• Q1: BC108, BC107B and plenty more unidentified silicon transistors

• Q2: AC176, AC187, OC76, OC82, AC128, CV7112, OC140 (note some are PNP)

note:  this is a revised post from April 2022 with updated information, schematic and layout.

D*A*M: Dragonfly DF-06

This the D*A*M Dragonfly DF-06, which is somewhat confusingly not the same circuit as the D*A*M Drag n Fly.  They're both in the Fuzz Face family, but they each approach the tone control differently.  The Drag n Fly does the tone shaping up front, while the Dragonfly does it at the end of the circuit, using a method similar to a MKIII.

Not having one available to trace, I’ve relied on photos available online.  Unfortunately a few capacitor values can't be seen, however, on the upside, it's not hard to guess what they might be to get something very close to the mark.

For me the key value is the capacitor forming a high-pass filter in the tone section.  Breadboard it and see what you like, or be prepared for a bit of soldering on the vero board.  The other caps certainly have a role to play, my feeling is that while they contribute to the overall bass response, the filter section is where the magic happens. 

Possible high-pass filter values:

24k    4n7    1.3khz

24k    3n3    2khz

24k    2n2    3khz

D*A*M DRAGONFLY DF-06    0.15" VERO LAYOUT 

TRANSISTORS

It's a hybrid silicon / germanium circuit.  The DF-06 below uses a BC337 & an OC141.  The BC337 is very similar to a 2N2222A which us a regular low gain NPN transistor.  Just watch the transistor pinouts, as the BC will be the reverse of the 2N.  i.e.  Flipped 180 degrees.

If you don't have any decent NPN germanium transistors, drop in a silicon and I'm sure it will still work - but you may need to tweak some resistor values to get it to work well.   This may even be the case with germanium transistors, as I’m sure Dave Main will have biased this to get what he wanted out of these particular transistors.  

STYRO CAPS

As you can see, the styro cap (the silver one) on the input to ground is larger than the one in the high pass filter section, which leads me to believe that it's probably a 10n, similar to what might be found on a MKII Tone Bender.

D*A*M DRAGONFLY SCHEMATIC

As a few values are not known, this is really just a guide to what the circuit looks like ands it's handy for me to test possible values in LTspice.  Where values are unknown, I think a safe bet is running with component values used in fuzz faces and tone benders.  Seems like an obvious choice.  An AC127 was used in LTspice, as I didn't have access to the model for an OC141.

Like most things pedal related, and I've mentioned this before - nothing beats building it and testing values on a bread board.  

If you want to play with the LTspice file, you can find it here.  If you don't have the right transistors in your library, just change them to a generic low-gain silicon transistor.

INPUT CAP C2

I've shown a few values below, which as mentioned above, are probably common fuzz face or tone bender values.

TONE CONTROL - HPF CAP C4

C4 forms a high pass filter with R6, the 24k resistor to ground.  The high pass filter is mixed in with a direct signal from Q2 via C3 (100n cap).  This is quite similar to a MKIII Tone Bender arrangement.

Lower value = more mids.  Higher values have a bit more sizzle.

Not much around in terms of video - this is all I could find

NOTE:  I had posted a Dragonfly layout back in early 2022.  I've reposted with additional information, instead of updating the old post.