What Are Transistors?
Transistors are small electronic devices that can act either as switches or amplifiers. In the world of guitar audio, they were at one point a new invention intended to replace old tube technology. Just as everything electronic gets replaced, op amps were a newer technology to replace transistors in many applications. And digital technologies replace op amps in some applications. Of course, tubes, transistors, and op amps are all still in use in guitar effects.
What Are Transistors Used for In Effects?
Transistors are used in numerous ways in guitar effects. Sometimes they are used as buffers. Sometime they are used as amplifiers. Sometimes they are used with other components to remove or boost various frequencies. We aren't going into the details of how transistors work or what they are doing in particular circuits. That's far too much to cover, interesting as it might be. Our purpose here is to provide some information so you can select customized transistors for some of our modules.
There are a number of different ways to classify transistors - by the technology used in their design, their construction, their purpose, and so on. We're going to classify them in a rather unconventional manner. We'll divide them into groups based roughly on their interchangeability in effects circuits. "Roughly" because some are simple substitutions, some may require some simple tweaks, but nearly all could be made to work in most circuits with a bit of work. We'll focus on "interchangeable" that is a simple swap, or a simple swap with some simple adjustments. We don't provide transistor options that require a lot of changes to the circuit. That level of modification could easily result in what would effectively be a new circuit, requiring more options to be built into our boards than is feasible. Even simple swaps and swaps with simple tweaks can provide substantially different tones from the same circuit. However, unlike diodes that are often used in circuits, it is considerably more complicated to support "switchable" transistor options. Once you select your transistor, it will be soldered in place and that aspect of your sound will become a permanent choice.
There are many beliefs and opinions, mixed with facts and partial truths regarding specific transistors in specific circuits. While we aren't going to weigh in on all those debates, we are going to present you with some facts to help guide you through the process of choosing different transistors, should you choose to do so. As we discuss the facts, you will see why there is so much debate about which transistors should be used in which effects.
There are so many inter-related issues that we could discuss that it is difficult to find a good starting point. So let's start with some information about germanium transistors, since that gives flavor to many other topics. The first widely available transistors were built using germanium, decades ago. In a sense, it was the availability of transistors that made small effect circuits possible. Before transistors, it was a world of tubes. A tube guitar amplifier is much like a simple guitar effect circuit with a power amplifier added to it to drive speakers. Imagine guitar pedals being about half the size and weight of a guitar amplifier head. That's a no-go.
Germanium transistors were widely manufactured in the 1950's, and continued to be manufactured in quantity through the 1970's and even 1980's in some parts of the world. Guitar effects in pedal form came out in the 1960's, so the only transistor choices they had were germanium. At the time, semi-conductor manufacturing was very crude. Parts had very wide spec ranges, and even then many parts didn't meet specs. The specs were often so loose that even if a part was in spec, it may not be usable for particular purposes. Yields were often low, as was the manufacturing capacity, so prices were relatively high and there were frequently shortages of transistors. Designs often had to accommodate extremely poor parts performance so that pedal manufacturers could use even "reject" transistors in their designs. This of course resulted in wide variations in sound from one pedal to the next of the exact same model from the exact same manufacturing line. Players would often try many identical pedals to get one that sounded right to them. Even then, there was no guarantee it would sound the same any two times you played it. Germanium transistor performance varied wildly with temperature, even just variations in what would normally be considered "room temperature", or the build-up of heat in the circuit while it was powered up.
In addition to this temperature-induced variation in performance, germanium transistors had another general issue. The electricity that went through them didn't always go where it was supposed to go. Electricity "leaked" from one part of the transistor to another, degrading the performance. There were also two types of construction, "NPN" and "PNP". How they differ isn't important. The important thing to know is that they aren't interchangeable without making other changes to the circuit. PNP was far more common, but NPN was sometimes used. Part of the reason for that must have been the absolutely abysmal quality of the NPN versions. Many of them were truly awful. It is difficult to find NPN germanium transistors that are even usable in guitar effects. In retrospect, it is surprising that anyone could build guitar effects with these awful components and make them sound good. But they did. It seems sort of a quirk of fate that fuzz pedals were among the first popular pedals. With some clever circuit design, that much distortion can cover up a lot of the problems with those junky transistors. Many of those pedals became classics, a cornerstone in the world of guitar effects, and are still popular and manufactured today. The Fuzz Face is a good example. But just as famous as their sound, is their notorious inconsistency. It seems no two of them sound alike, and only a fraction of them sound "good", or the way people think a "good" Fuzz Face should sound.
As you can see in the picture above, many of the old Ge transistors had metal "can" covers. A few had a sort of glass cover. You might sometimes see reference to "black glass" transistors. Those are typically Ge transistors from a couple of manufacturers that used a black glass cover. It's doubtful that the glass itself makes those transistors sound better. But those transistors do enjoy a reputation for great tone, most likely because they happened to be used in some good effects, but perhaps also because they were made a bit better than some of their contemporary competitors. Even so, many of them have performance characteristics that make them unsuitable for use in many effects circuits, so it is not true that "black glass" is always a good thing.
Thankfully, silicon transistors followed soon after germanium transistors. Multiple problems were solved with the introduction of silicon transistors. First, they don't leak and second, their performance is nearly unaffected by temperature. In addition to these two major improvements, manufacturing methods and capacities brought down costs and greatly improved availability. There were still two types at first, NPN and PNP, which still weren't interchangeable. And some key performance specs still had wide ranges of acceptable values. Those wide ranges of specs still meant that some transistors weren't good for some purposes. But they were now cheap enough and plentiful enough that they could be sorted into groups with much smaller spec ranges, making it much easier to design circuits that performed consistently and correctly. Additional design and manufacturing developments led to additional types of silicon transistors, JFET and MOSFET. Sometimes JFET and MOSFET can be substituted for each other or for NPN or PNP (either germanium or silicon), and sometimes they couldn't be substituted or substitutions required changes to the circuit. But the number of different types of silicon NPN, silicon PNP, MOSFET, and JFET transistors grew to quickly provide many options in each category.
You may see references to some transistors with unusual descriptions. In the picture above, the third one from the left is a "glob top" transistor. That just means they are covered with a button-shaped blob of some type of plastic. Some of the glob tops have a reputation for great tone and are highly sought-after, such as the infamous 2N5133. It is much debated that the model of the transistor, much less the shape of its plastic cover, actually matter that much. Most are "nothing special" transistors that were used because they were cheap and available and had the right range of gain. Many others are virtually indistinguishable from them in the same circuit. But they do look cool, and score a few mojo points for that alone. The fourth from the left and the rightmost are examples of silicon "metal can" transistors. These are typically early silicon transistors that were given metal covers like their germanium predecessors. Some models of silicon transistors, such as the BC108C, BC109C, and others do have a tone preferred by many, particularly in fuzz circuits. While it is possible that the metal caps contribute to that tone, perhaps by blocking a bit of EMF noise, it is more likely that the specs of that model of transistor are more important. Many of those metal can transistors also come in a standard plastic package, which sound pretty much identical.
Types of Transistors
Now we've introduced the major types of transistors that are used in effects circuits:
- germanium PNP
- germanium NPN
- silicon PNP
- silicon NPN
For the most part, circuits will still sound OK if you substitute one part for another of the same type. Some circuits are design to allow simple adjustments to make those same-type substitutions work easily. Some circuits exist in different versions that work for different types of transistors. The Fuzz Face is again a good example. You are probably aware that there are both "germanium" and "silicon" Fuzz Faces, as well as a version that uses both germanium and silicon transistors together.
What else matters besides the transistor type?
We have multiple transistor types we can choose, but we are still stuck with wide variations in some key specs for most transistor models. Depending on exactly which transistor is selected out of a groups of same-brand, same-model transistors, an effect may sound or perform differently. Just having the right type of transistor isn't always enough. So what else matters? This is where the discussion gets a bit vague. The answer is "it depends". It depends on the design of the specific circuit and what function the transistor is performing. That's generally straying far enough into detailed technical issues that giving a correct answer becomes difficult. The detailed technical analysis of even simple circuits can be extremely complex requiring something like an electrical engineering degree, complete with a robust background in calculus. Most of us just want something that sounds good, not a degree program.
So let's approach this from a different angle. All we're trying to do is figure out which transistor substitutions might sound better than the default transistor(s) in some effect. There are some specs for transistors that can help us, but their ranges are either unknown or too wide for general use, especially for germanium transistors. When we build a circuit, we actually have to test individual transistors to get ones in a usable spec range, then maybe tweak some other components to get that specific transistor to sound good. Trust us, that's not something you want to help with.
It is also true that over the years, for the well-known circuits, people have already tried most of the different transistors available and found a small group of them to be "best" for a particular circuit. We have our experience and that of others to help narrow the field. It's not just type of transistors we know, we know general spec ranges, too.
How to select a transistor for your GT rack effect module
So we can divide the problem into different parts to solve the problem more easily and conveniently. First, there are recommendations for specific transistor types and models that result in "good" sounds. You may already have some favorite silicon transistors you like in fuzzes, for instance. Or you've tried different ones, or read reviews, or otherwise gained some insight into choices you may prefer. We can provide some recommendations, too. Then we will figure out the specific specs needed for that model of transistor in that circuit. We may ask you some questions in order to optimize that, but you won't have to worry about the details that much. You pick the transistor type, and we'll pick the specific transistors that work.
We have many different transistors in stock. The list below reflects what we had on hand at the time of writing. The actual list will shorten as we run out of certain rare transistors and lengthen as we find new ones that work well for particular functions in particular circuits. We've included a few comments about suitability of some transistors for specific purposes or types of effects. Our module descriptions contain additional detail specific to that module's circuit.
Note that we have a number of Russian transistors. Russia kept on making germanium transistors for a while longer, and they got pretty good at it. Many of them have very low leakage and consistent performance. They are not well known, but they can be pretty good.
You can look up details about all these transistors on the internet. You can often find opinions about their use in particular effects circuits if you want to search a bit and wade through a lot of "stuff" to find it.
Germanium PNP Transistors
Most circuits that use germanium transistors use PNP. Many models are famous because of the pedals they appeared in. But few, if any, of these famous models are special in any way other than scarcity and high price. There are other choices that will sound as good or better, maybe slightly different, maybe not...
- 2N1305 - good medium gain transistor for a wide range of uses
- 2N1307 - good medium to high gain transistor for a wide range of uses
- 2N1309 - good medium to high gain transistor for a wide range of uses
- 2N2613 - generally bad transistor made famous for being used in some of the first fuzz pedals, hard to find, expensive
- 2N2614 - generally bad transistor made famous for being used in some of the first fuzz pedals, hard to find, expensive
- 2N270 - generally bad transistor made famous for being used in some of the first fuzz pedals, hard to find, expensive
- 2N527, 2N527A - nice medium gain transistor
- AC125 - pretty good fuzz transistor, wide range of gains, relatively available and generally a good choice at reasonable cost
- AC128 - OK, but others are usually better, some not usable due to high leakage or low gain which contributes to slightly higher cost (many scrap in a typical lot)
- AF138 - lesser known model that works very nicely in fuzzes, wide range of good gains, generally lower leakage, good option
- ASX12D - somewhat uncommon, often used as an alternate for NKT275, OK if you can find ones with correct gain and leakage
- CV7003 - military version of the OC44 transistor, among the best for boosts and fuzzes, getting expensive
- CV7005 - military version of the OC71, good if you can find ones with the proper specs, getting expensive
- CV7355 - military version of the 2N1309, but generally with higher gain
- GT108G, 1T108G - Russian, higher gain transistors that make good fuzzes and boosts
- GT109G, 1T109G - Russian, another high gain transistor that should make good fuzzes and boosts
- GT115D, 1T115D - Russian, high gain transistor that should make good fuzzes and boosts where a bit of leakage is good
- GT308B, 1T308B - Russian transistor that sounds good in boost and fuzz circuits
- GT308V, 1T308V - Russian, very similar to GT308B but with specs that work in a wider range of circuits. 1T is the military grade, virtually identical
- GT320V, 1T320V - Russian, medium to high gain transistor that should make good fuzzes and boosts
- GT321E - Russian, medium gain transistor but far too much leakage for most uses
- GT321V - Russian, medium gain transistor but far too much leakage for most uses
- MP16B - Russian, lower gain, better for boost than fuzz
- NKT275 - Famous British fuzz transistor, difficult to find good vintage ones, widely faked, newer reissue of the same name not the same
- OC42 - British transistor that can be outstanding in boosts and some fuzzes, reasonably affordable
- OC44 - British transistor used in boosts and fuzzes, often preferred when they are available, getting expensive
- OC71 - British transistor good for fuzzes if you can find ones with the proper specs, getting expensive
- OC72 - British transistor good for fuzzes if you can find ones with the proper specs, getting expensive
- OC75 - British transistor good for fuzzes if you can find ones with the proper specs, getting expensive
- OC76 - British transistor good for fuzzes if you can find ones with the proper specs, getting expensive
- OC77 - British transistor good for fuzzes if you can find ones with the proper specs, getting expensive
- OC81, OC81D - British transistor good for fuzzes if you can find ones with the proper specs, getting expensive
- P416B - Russian, medium to high gain, good for a variety of purposes
- SK3004 - good for certain fuzz circuits if you can find ones with the proper specs, difficult to get usable ones
Note: The American and British germanium transistors are often given high marks and command high prices as they get more scarce. They were used in many of the classic effects, particularly fuzz effects, of the 60's and 70's, which certainly contributes to their mojo values. But keep in mind that most Ge transistors were pretty junky and many didn't even meet their own very wide "acceptable" spec values. Pedal designers had to design around some of their shortcomings, notably high leakage and/or low gain. The Russians continued to make and improve on their Ge transistors for years longer than the American and British manufacturers. Their later Ge transistors are of much better quality than their western counterparts. With one notable exception, no popular pedals were made in Russia using Russian transistors. And, of course, during the Cold War years there was little interest in using "junky Russian technology". But the fact is that some of those Russian transistors work and sound great, and are available at a fraction of the cost of the popular American and British transistors. For a while, the famous Big Muff pedals actually were made in Russia, using Russian transistors - silicon ones, as they were made in the post-germanium '90s. But those are some of the best-sounding Big Muffs ever. Just sayin'...
Germanium NPN Transistors
Few circuits use germanium NPN. That's good because most available germanium NPN are junk.
- 2N1306 - among the best of the Ge NPN transistors we've found, medium to high gain, reasonably available
- 2N1308 - among the best of the Ge NPN transistors we've found, medium to high gain, slightly higher on average than 2N1306, reasonably available
- 2SD352 - gains OK, but too leaky for most uses, reasonably available
- AC127 - can be OK for some fuzzes, but most are unusable due to high leakage
- AC130 - can be OK for some fuzzes, but most are unusable due to high leakage
- AC176 - can be OK for some fuzzes, but most are unusable due to high leakage
- AC187 - can be OK for some fuzzes, but most are unusable due to high leakage
- CV7351 - good medium to high gain transistor
- GS507 - good medium gain transistor, very uncommon
- MP11A - pretty good quality Russian transistor, medium gain
- MP38A - almost identical to MP11A which apparently is its replacement
- NTE103A - can be great, but ones with usable specs are nearly impossible to find, usually not available
Silicon PNP Transistors
A few circuits use silicon PNP transistors, particularly those adapted from PNP germanium circuits. These transistors tend to land in the same gain ranges and aren't particularly distinctive in their sound. Because of that, there's usually not much to gain by picking one over another. If the original circuit transistor is available, it's probably just as well to use it. You aren't likely to stumble across some toneful substitution that makes the circuit sound much better or different. Other than circuits with some vintage PNP transistor, the 2N3906 is pretty much The PNP silicon transistor- highly available, cheap, and solid performance.
- 2N3906 - this is the workhorse of silicon PNP, works well for everything
- 2N4061 - used in the Zonk Box
- 2N4125 - used in some older flangers
- 2N4126 - used in some older flangers
- 2N5087 - like the 2N3906, another good all-around performer
- 2SA970 - used in some Boss effects, such as the HM-2
- 2SA1048 - common, inexpensive used in some pedals built in Japan (eg early Boss, Ibanez)
- BC177- perhaps used in some older Electric Mistress builds
- BC309 - perhaps used in some older Electric Mistress builds
- BC327-40 - slightly higher gain than most other PNPs
- KT3107E - decent low gain Russian transistor
- KT3107L - decent high gain Russian transistor
- MPS6523 - used in some Octavias
- RN2206 - used in Expandora
Silicon NPN Transistors
This is the most popular type of transistors. There are many, many models. We stock a huge variety for those that want to use the same transistors used in the original circuits. We also carry a number of good substitutes that offer good tonal varieties. Many of these transistors have been hyped up to legendary status because of some popular pedal they were used in. Truth of the matter is that the model of the transistor doesn't necessarily have much to do with the resulting tone, as long as you know the specs that need to be met for a particular circuit to sound good. The hyped transistors were usually just the cheapest and most available at the time the pedals were made, which had "close enough" specs. We're not sure there is a case where we can't rival the original pedal's tone with new production transistors.
Our overall favorite transistor is the 2N5089. It is a quiet, high gain transistor that works in most circuits for most purposes. It is our "go to" transistor in most cases, except for when a lower gain is specifically needed. Most of these transistors are "unremarkable". It's mostly a matter of getting the right gain level. That'll get you a decent tone almost every time. Switching between models for most purposes won't have much impact. Fuzzes are an exception. You can change up fuzz tones by selecting different transistors and different gain levels.
- 2N2222 - medium gain
- 2N2222A - medium gain
- 2N2369 - low gain
- 2N2924 - somewhat difficult to find, used in Vox Treble Booster
- 2N3391 - used in the Fender Blender
- 2N3391A- used in the Fender Blender
- 2N3565 "glob top" - used in some early fuzz pedals
- 2N3566 "work alike" - lower gain
- 2N3904 - good workhorse transistor suitable for most purposes except high gain, medium to medium-high gain
- 2N4123 - low gain
- 2N4401 medium-high gain
- 2N5088 - used to be our favorite all-around transistor until we discovered the 2N5089 which is a bit quieter, still an excellent transistor for nearly any purpose, mostly medium-high to high gain
- 2N5089 - our current all-around favorite that works great for nearly everything, usually high gain
- 2N5133 - nothing special transistor hyped to legendary status. Nearly impossible to find real ones with usable specs, very expensive. 2N5089 is a great replacement for fuzzes, 2N5088 or 2N3904 are good replacements for lower gain
- 2N5172 - medium gain, used in some Big Muffs or Big Muff clones
- 2N5828 - high gain, used in some Big Muffs
- 2SC1815 GR/BL/Y - medium gain, used in several vintage effects from Japan (Boss, Ibanez, etc)
- 2SC1849R - medium gain
- 2SC2362G / KG - medium gain
- 2SC2458Y/GR - low to medium-high gain
- 2SC644 - low to medium gain
- 2SC732-GR - medium gain, used in several vintage effects from Japan (Boss, Ibanez, etc)
- 2SC828A/Q/R/S/Y - low to medium-high gain, used in several vintage effects
- 2SC945P - medium gain, used in several vintage effects
- BC108 - low to very high gain, can be good for fuzzes
- BC108C - high gain BC108, good for fuzzes
- BC109 - low to very high gain, can be good for fuzzes
- BC109C - high gain BC109, good for fuzzes
- BC169B - medium gain
- BC169C - medium-high gain
- BC182L - medium gain
- BC183 - low to high gain
- BC183C - high gain BC183, can be good for fuzzes
- BC184C - medium gain
- BC184L - high gain, good for fuzzes, boosts, overdrives
- BC239C - high gain, good for fuzzes
- BC546B - medium gain
- BC547B - medium gain
- BC547C - medium-high gain
- BC549C/CTA - medium to high gain
- BC550B - medium gain
- BC550C - high gain, good for fuzzes
- KT3102BM - Russian medium gain
- KT3102DM - Russian medium-high gain
- KT3102JM - Russian low gain
- KT3102VM - Russian high gain, good for fuzzes
- KT3012E/EM - Russian very high gain, good transistor, good for fuzzes
- MPS6521 - medium-high gain, used in some Octavias
- MPSA13 - very high gain
- MPSA18 - very high gain, good transistor with some popularity
- NTE123 - medium-gain
- NTE123A/AP - medium-gain replacements for NTE123
- SE4010 - low to high gain
It's often best to stick with the original circuit's JFET, as you need to adjust the circuit to the specific JFET installed. Then again, that adjustability may provide just the range needed to use a different JFET. If a circuit uses a JFET, it probably has design features or sound crafted specifically for JFET transistors. Switching to another type may not sound quite as good.
- 2N3819 - great, not easy to find
- 2N5457 - great, getting hard to find real ones, prices rising
- 2N5458 - classic, getting hard to find real ones, prices rising
- 2N5459 - possible sub for 2N5457 and 2N5458
- 2N5485 - another possible sub for 2N5457 and 2N5458
- 2N5640 - sub for MPF4393 or MPF4392
- 2N5952 - classic phaser transistor, getting hard to find enough real ones to build matched sets, prices rising
- 2SK170 - used in a few older effects, getting expensive, difficult to find real ones
- 2SK246GR - older model, good for clean boosts, getting hard to find
- 2SK30A-GR - common in old Boss pedals, getting difficult to find real ones, usually not used in signal path anyway
- BC264D - hard to find, used in some European designs
- BF244, BF244A - used in some European designs
- BF245C - sub for BC264D, used in some European designs
- BF256B - sub for BC264D, used in some European designs
- J106 - used more for switching than signal processing
- J111 - used more for switching than signal processing
- J112 - used more for switching than signal processing
- J113 - used more for switching than signal processing
- J201 - most popular JFET, getting difficult to find real ones, price is rising, available in surface mount form
- J202 - used more for switching than signal processing
- J310 - sometimes used as a clipping diode
- MPF102 - clean sound
- MPF105 - not commonly used
- MPF4393 - hard to find, not commonly used
Not many circuits use MOSFETs. Most that do use BS170. A few use 2N7000
- 2N7000 - most popular for use as MOSFET clipping diode, still reasonably available
- BS170 - most popular MOSFET, getting hard to find real ones, price rising
- BS250 - sometimes used in place of BS170, but also hard to find