Transistors

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.

Classifying Transistors

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.

Germanium Transistors

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.

Silicon Transistors

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
JFET
MOSFET

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. Many of the popular circuits have variants that use known replacement transistors, which can be helpful in understanding what the substitution will sound like. However, in many cases you'll be taking a bit of a leap into the dark, with only some rules of thumb or life experiences to help you land where you intended. There are so many circuit designs which impact the sound as much or more than the transistor selection that it is difficult to predict the results at any level of detail. Yes, it will almost always work. Yes, you will almost always get a good result. But is it exactly that specific tone you're hearing in your head...? For that you basically have to try it to find out.

A few notes:

When it comes to NPN vs PNP, you need to use the same type as the circuit requires. They are not interchangeable, unless a circuit has been specifically adapted to use them both, as some of the older fuzz circuits have been. You may know that some of the older fuzzes have positive ground circuits, instead of the normal negative ground. They cannot be powered in the normal way with negative ground circuits. Often they only allow battery power to prevent power problems. The type of ground in those old circuits is usually related to whether they use NPN or PNP transistors. The good news is that in GT effects we have separate power boards for all of our effects. They all take the same type of power so you don't have to worry about grounding issues. The power boards convert the incoming power to whatever voltage and grounding is needed for the effect circuit internally.

In some circuits, you can use either germanium or silicon transistors. Germanium transistors were the only ones available in the early days of effects. And in the early days, many effects were boosts or fuzzes. As a result, you would probably only be thinking of germanium vs silicon in an older boost or fuzz circuit. An example is the Fuzz Face, which was made with both types over the years. In general, the silicon transistors will have higher gain than the germanium ones, although you can certainly choose a low gain silicon transistor, or at least one of similar gain range. It is difficult to find words that accurately describe detailed differences in tone, but silicon is often described as more aggressive sounding than germanium. But with the higher gain of silicon, you can reach gain levels that "smooth out" getting somewhere close to the sound of a nylon zipper, if that makes sense. Low gain germanium can be very smooth and glassy, or with their higher leakage can be rude and splattery at higher gain levels, getting you to that famous in-your-face, "stick it to the man" sound of the opening riff of "Satisfaction". So you can get a big change in tone by swapping silicon and germanium. However, ... Those changes can be so different that it doesn't even sound like the same effect. If that's what you want, go for it. But it is likely that you should actually choose another effect that already has that base tone instead of trying to coax it out of some other circuit designed for a different sound. Again, we're primarily talking about fuzzes, here. There are *many* fuzzes to choose from, so experiment a bit before you go for the radical tone change.

Remember, if you choose a circuit with germanium transistors, you are generally opting for a less predictable tone. Temperature variations alone can have a major impact on how those old transistors sound. But, when you have one of them dialed in, they can be really good. To deal with their finicky nature, we generally advise that you consider a circuit that has an external bias control or add on the option to get one. That bias control allows you to compensate for much of the variability in performance. There's still no guarantee you can dial in that exact tone you like every time, but it greatly improves the odds. By twiddling the gain/fuzz and tone controls, you can probably get close enough most of the time. But you must always understand the nature of those transistors and their occasional refusal to behave.

Germanium transistors were made in the Soviet bloc countries for decades after the western countries stopped making them. Some of those newer production Soviet ones were built to good specs, with lower leakage and less temperature sensitivity, giving them somewhat more consistent performance. Experience with those Soviet transistors in effects remains somewhat thin, so it is more difficult to get suggestions about which ones to try in different circuits. While their performance can be more consistent, and they can sound great, they may not sound like the original (probably junky) transistors in those circuits giving us tones our ears have come to expect. Better? Worse? Or just different? You decide. If you go adventuring, you may want to let gain be your guide in this type of substitution. Go for a similar gain as the original might seem like a good idea, but given the wide range of gains in the originals it may be difficult to guess what the gain should be. But after a while of listening to various germanium fuzzes, you probably have an idea of what higher or lower gain might sound like. Then you can choose low, medium, or high gain replacements depending on the general tone you are chasing. Some effect circuits are so popular that there are various strategies of selecting transistor gain levels. Fuzz Face and Tonebender circuits and their derivatives are good examples. By poking around a bit, you might find one of those strategies sounds good to you can request those gain ranges to get you in the ballpark.

While not always true, it is often the case that germanium transistor effects will clean up better than silicon ones when you lower your guitar volume. Or maybe they just respond in a more noticeable way.

The majority of effects that use transistors use silicon transistors. But there are many that use JFET or MOSFET transistors. Often JFET and MOSFET transistors are used only in the on/off switching of the effect. Boss pedals are famous for that. Those switching transistors have no real impact on the audio tone, as they are not in the audio path. But some effects do use JFET and MOSFET transistors in the audio path. Many find that the FET transistors have a more "tube-like" tone than plain silicon transistors. Some circuits can use either type, so you can find some nice variations with substitutions. And since we're on this topic, let's digress for just a moment. FET transistors often have FET diodes in them. This allows you to use part of a FET transistor as a FET diode. This is done primarily in hard or soft clipping sections of a circuit. Some feel you get that more tube-like sound when you use FET diodes instead of silicon diodes for clipping. There are also many, many types of opamps. Opamps are made of lots of transistors internally. As you have probably already guessed, some opamps may use FET transistors or silicon transistors, with the FET-based opamps getting that more tube-like sound than the regular silicon-based opamps.

Transistors are often used only for input or output buffers in a circuit. Those can have some impact on the tone, but often it is more a matter that there is or isn't a buffer than it is a matter of which transistor is used for the buffer. You won't likely notice much difference by changing buffer transistors, although changing the type between silicon and FET give you a little something-something.

You may get some significant tonal differences between different levels of transistor gain. Germanium generally has much lower gain levels than silicon, particularly if you leave out the higher gain levels that come with very large levels of leakage that often become unusable. For germanium, "low" might be somewhere between 30 - 50 hfe, "medium" 50 - 90 hfe, and "high" 90 - 150 hfe. Those are just ballpark numbers and sometimes "low", "medium", and "high" have different meanings in some of the more popular circuits. On the other hand, for silicon "low" might be 30 - 150 hfe, "medium" 150 - 400 hfe, and "high" 400 to 800 or even several thousand hfe. Those different gain ranges are part of what contribute to the overall difference in sound between silicon and germanium. Substituting a "medium" silicon for a "medium" germanium may not be what you intend. You may want a "low" silicon vs a "medium" or even "high" germanium. If, for example, you are picking a silicon transistor to replace a germanium transistor because you want more predictable performance, you may want to keep the actual gain level similar if you are otherwise happy with the germanium sound. Or maybe you also want a more "silicon" sound and you'll opt for a higher gain with the silicon transistor.

Expectations. This is where things get tough. The guidelines above may or may not lead to the results you expect. There are a lot of other things besides transistor type and gain level that impact the results, which could impact whether you get a result that meets your expectations. Here are some of the reasons:

Transistors are general purpose electronics components. They aren't just for audio use. Even when they are in audio products, like effects circuits, they may not be in the audio path or having any impact on the tone. Transistor specs rarely include information about their audio properties. Occasionally you'll find some vague information about noise levels, which can help you pick less noisy models. Sometimes you'll find tech information about bandwidth and frequencies, if you can figure out exactly what it means and how to make use of it. The older the transistor model, the less likely you'll find a detailed spec sheet at all.

It is highly unlikely that any mass-produced effect was made with transistors that were individually tested for even basic performance, like gain level. They were bought in bulk by model, maybe with some expectation about the range of gain. Performance could vary widely. Or the circuit may be designed to accommodate a wider range of specs to get a reasonably consistent sound. Boutique builders on the other hand may well test every single transistor for more reliable results. In the case of wide variation in performance, you may be basing your expectations on the one or two of those pedals you've heard, which may sound very different from others. Or in the case of more consistent circuit performance, changing transistors may not have as much effect as you would expect.

Inside circuits, transistors perform some basic functions. But all those other components are doing their thing, too. The audio signal can sound very, very different at different points in the circuit as the audio is being manipulated to achieve designed results. Those internal shifts in the tone will often swamp the changes you can achieve with just a transistor swap. While transistor swaps are a common difference between different variations of a pedal circuit, they are often not the only change, or even the biggest change, between those variants. This happens frequently with boutique pedals that are derivatives of well-known pedals (as most are). Knowing that your favorite boutique Fuzz Face uses an XYZ transistor may only be part of the story. There may also be, say, capacitor changes that are changing the frequency response before or after transistor gain stages and that capacitor change accounts for most of the difference you hear. Of course, those other tone-shaping components may be in other pedals in your chain. So while it may be true that David Gilmour uses a specific fuzz pedal with a specific transistor for the tone of a specific song or solo, know also that he is carefully shaping what goes into and comes out of that fuzz pedal with other pedals, amps, pickups, etc to get his overall sound. You'll find many good Pink Floyd tribute bands that have great David Gilmour tones and don't use any of the same equipment, let alone same transistors.

Should you try a transistor swap? Only you can say for sure. If you are looking for a very specific change in tone, a transistor swap alone may or may not get you there. If you are very knowledgeable about the circuit and everything that is happening to your tone in that circuit, then perhaps you can make a more informed decision, but that's out of the range of expertise for most of us. Sometimes you just have to try it and find out. It's like Forrest Gump and his box of chocolates in some ways. Generally, it is probably a good idea to try different effect circuits before trying more subtle mods to a specific circuit. Maybe it isn't a modified Fuzz Face you need. Maybe it is a Big Muff, Tonebender, or one of the other many fuzzes that are available. Or maybe one of those others gets you close enough to what you want that a bit of tweaking can get you there. Or maybe it isn't a transistor change you need in the first place. Check out other available mods to see if they may be what you need instead of or in addition to a transistor swap. Just know that no one else can hear what's inside your head, and our English language is pretty sloppy for describing detailed sounds, so it is easy to miss the mark of hitting an exact tone without some experimentation in some form.

The Transistors

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.

2N2907
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
2N6519
2SA733P
2SA970 - used in some Boss effects, such as the HM-2
2SA1015GR
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
2SC373
2SC1815 GR/BL/Y - medium gain, used in several vintage effects from Japan (Boss, Ibanez, etc)
2SC1849R - medium gain
2SC2240-GR
2SC2362G / KG - medium gain
2SC2458Y/GR - low to medium-high gain
2SC536
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
ATP2222A
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
KT3102E/EM - Russian very high gain, good transistor, good for fuzzes
MPS5172
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

JFET Transistors

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

MOSFET Transistors

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