A Guide To Transistors For Guitar Pedal Building


Transistors are active components for our circuits and we use them to amplify or switch our signal. These semiconductors generally contain three pins and for building guitar effects pedals, we’ll generally be using bipolar junction transistors (BJT’s). However, some lower gain pedals may make use of field-effect transistors (FETs)


When a transistor is being used as an amplifier, it takes a small electric current at one end produces a much larger electrical current at the other. In a guitar circuit, this is essentially boosting the signal to produce a much larger output volume.

When being used as a switch, a transistor can allow a small current to pass through one part and turn it into a much larger current. Essentially, the smalled current turns on the larger one.


Bipolar junction transistors (BJT’s) are components made of three layers and can either be a thin layer of P-type semiconductor sandwiched between two layers of N-type (NPN transistors) or a thin N-layer sandwiched between two P-Layers (PNP Transistors). This combination of two junction diode construction produces two P N junctions. A Base (B) and Emitter (E) junction and a Base (B) and Collector (C) junction.

Most transistors that you’ll use will be NPN but if you need to use a PNP transistor, you will need to reverse the polarity of the voltage as the structure of the transistor is reversed.

To figure out what type of transistor you have, you should always find the corresponding datasheet online. This will not only tell you what type of transistor it is, but also the assignment of the pins.

When it comes to building guitar effects pedals, there are two main types of transistors that we use. Silicon transistors and germanium transistors.


Germanium transistors have a relatively low gain and make for a smoother, vintage style when compared to a silicon alternative.

The most common germanium transistors that you’ll find are:

  • AC127, AC128
  • OC45, OC75, OC81
  • NKT275
  • MP41
  • GT1322

They all have different gain ranges and sounds but it really comes down to personal preference as to what works for you.


Silicon transistors are much higher gain and make for a much more modern-sounding pedal when compared to a germanium alternative.

The most common silicon transistors that you’ll find in pedals are:

  • BC108
  • BC109
  • BC209

While these have much higher gain ranges, their tolerances vary quite drastically. For example, the BC108 has a gain range of anything between 110 and 800 making it difficult for find a matched pair.

However, you can get a similar amount of gain from something like a BC549C has a typical gain range of 270 to 500 making it much easier to find a matched pair for your circuit.


It’s important to note that transistors don’t have a sound. They have characteristics. A silicon BC109 will have higher gain than a germanium AC128 but both are fairly high gain making them perfect fuzz guitar pedal transistors. It’s up to you to figure out what it is you want your pedal to do and sound like.

The best advice to have on this topic is to buy a load of different transistors and test them out to see what you prefer.


Transistors are mainly measured using gain (hFE). Depending on the type of transistor, the amount of gain available can vary substantially. Some transistors have huge tolerances where others are much tighter.

For example, the BC108 transistor which is commonly found silicon fuzz face pedals has a gain range from 110 to 800. Whereas an OC71 (also found in some more vintage Fuzz Face pedals) has a gain range of 30 to 110.

As some pedals, like the classic Fuzz Face circuit, require two or more transistors, understanding these gain ranges becomes incredibly important. Traditionally, the classic fuzz face has a gain range of 70-85 for the first transistor and 120-140 for the second transistor.

Due to these tolerances in gain varying so wildly, it’s very rare that you’ll ever find two transistor fuzz pedals that sound identical unless the creator has painstakingly used the exact same values for their transistors across all their pedals. Which is incredibly unlikely.


Transistors are a little more complicated to test than a passive component like a resistor or capacitor. However, using a digital multimeter, we can test them through a few simple steps.

First of all, you need to locate the pins and label what each of them is. If you’re measuring a bipolar junction transistor, the pins are Base (B), Collector (C) & Emitter (E). However, if you’re measuring a FET, the pins are Gate (G), Drain (D) & Source (S).

You can find this pin information for any transistor just by searching for it in Google. You should be able to find the corresponding datasheets fairly easily that will provide all of the necessary information.


Before adding a transistor to your build, it’s worth checking to ensure it’s not shorted or open. If it fails any of the below steps, you can assume it’s a bad transistor and you can safely move on to testing a different transistor.

If you’ve bought a specially selected match pair of transistors, these checks should already have been carried out. However, it never hurts to double-check.


Connect the red probe from your multimeter to the Base (B) of the transistor and the black probe to the Emitter (E).

If the transistor is an NPN type, you should see a voltage drop of anything between 0.45 and 0.9 volts. If you see this, the transistor is good to move onto the next step.

If the transistor is a PNP type, your multimeter will read OL or Over Limit.


Leave the red probe on the Base (B) of the transistor and more the black probe over to the Collector (C).

As with the previous step, if this is an NPN type transistor you should see a voltage drop of anything between 0.45 and 0.9 volts.

If this is a PNP type transistor, your multimeter will read OL of Over Load.


Connect the red lead from your multimeter to the Emitter (E) and the black lead to the Base (B) of the transistor.

For a good NPN transistor, your multimeter should read OL or Over Load.

However, if you’re testing a PNP transistor, it should read a voltage drop between 0.45 and 0.9 volts.


Connect the red lead of your multimeter to the Collector (C) of the transistor and leave the black lead on the Base (B).

For a good NPN transistor, your multimeter should read OL or Over Load.

However, if you’re testing a PNP transistor, it should read a voltage drop between 0.45 and 0.9 volts.


Connect the one lead of your multimeter to the Collector (C) of the transistor and the other lead to the Emitter (E). This should read OL or Overload.

Swap the leads over and recheck. You should also see OL or Overload.

If you get anything other than these readings, you can assume it’s a bad transistor and not include it in your build. Unfortunately, not all transistors will work.


Some multimeters may have an hFE setting and punchouts to insert your transistor. This is a very crude way of figuring out what the gain levels of the emitter are.

By lining up the pins of the transistor with the connectors on the multimeter and setting the dial on the multimeter to hFE, you should be able to start measuring the gain levels of the transistor.

If you don’t have a multimeter with a built-in transistor checker, you can build one with a breadboard, a batter and a 9v battery.

Set your multimeter to measure low current (mA) and set up your breadboard to resemble something like the below:

The next step is to connect a 1M Ohm resistor between the positive side of the battery to the base of the transistor.

Your multimeter will then show a current flow between the Collector (C) and the Emitter (E).

You can then divide this figure by the voltage of the battery (9v) to estimate the gain (hFE). The equation for this is:
Milliamps reading (mA) / Battery voltage (V) = Gain (hFE)

Why not check out my other posts about the other common components when building DIY guitar effects pedals here:

A Guide To Capacitors For DIY Guitar Pedal Builders

A Guide To Resistors For DIY Guitar Pedal Builders

If you’re new to this and would like to know where to buy transistors for your DIY effects pedal projects, check out my guide here:

Where to buy DIY guitar effects pedal components

Hi, I'm Pete!

I have been a guitar and effects pedal enthusiast since 2005 and electronics tinkerer since 2017.

I’m here to help you begin your journey with building DIY guitar effects pedals. Get in touch with me if you have any questions.

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