Switching transistors
ex : 2N404 (the american classic)

The first transistorized circuits for automation and computing required specific components that were straightforward in their response but without extreme bandwidth. Rather close to radio frequency transistors, they are distinguished by their reliability and deliberately moderate maximum frequency. Some models may produce a vintage sound grain, suitable for Fuzz Face. They could make as much reference as the ubiquitous small power audio transistors : American do-it-yourselfers have not been mistaken, and have since a long time embraced the classic 2N404 …

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High Frequency Transistors
ex : OC44 (Rangemaster, 1966)

High frequency germanium transistors have appeared as a second generation. Their elaborate technology made it possible to obtain a consequent gain even at radio frequencies : 30MHz, then 100MHz or more.
More expensive, and fragile at the beginnings, they have not been much used by pedal manufacturers, before all concerned by economy and robustness.

The OC44 was however exploited by Arbiter in the first Treble Booster, the Rangemaster, where its gain and bandwidth yet modest worked perfectly.
With the contemporary recoil, we rediscover the quality of these transistors, the fineness of their sonic grain, and their real interest in designing modern boosters and fuzz …

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Audio preamplification transistors
ex : NKT275 (Fuzz Face, 1966-67)

These are the most common germanium transistors. They were designed for audio pre-amplification circuits in consumer and mid-range equipment, where they quickly replaced the tubes. They are distinguished by a gain which can reach 200, and a maximum frequency of about 1MHz.

Pedals made in 1966-68 first operated with black glass models like OC75 and then encapsulated glass like OC76 and OC81 … As the AC125 and AC126 by Philips / Mullard were a bit expensive, cheap versions like the NKT275, and some SFTxxx had their hour of glory before being forgotten, then rediscovered around 1990 …

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Output audio transistors
ex : AC128 (Fuzz Face, 1966-69)

These transistors were at first designed to drive the loudspeakers of radios and consumer audio products. They are distinct from other families by their dissipable power and their maximum collector current.
In 1967, some models have been known as bringing a so called “creamy” sound texture to Fuzzboxes. Therefore, they gained a reputation that made this family of components enter the guitar effects culture.

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The TO1 standard was the first all-metal case used for germanium transistors when the black glass models were abandoned around 1964. This concerns European series like ACs, industrial series ACYs, ASYs, ASZs, the british NKTs and the japanese 2SAs…
In the USA, there are larger JEDEC standard cases (TO5 box) or smaller (TO18 box) as the european TO1.
Transistors manufactured in Eastern Europe used an elongated version of the TO1 case : some ACs, SFTs manufactured under license, and the national productions in the East.

The TO1 dimensions were not strictly followed by all manufacturers, which can seem surprising nowadays. From another point of view, this can help identify the origin and sometimes the quality of the transistors. This article is here to share some pointers and background information.

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