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 the order of 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 …
Average characteristics in the AC125 family
Maximum collector-emitter voltage: 10 to 20 V.
Maximum dissipable power: 0.1 to 0.2 W, rarely more.
Maximum collector current: 0.1 to 0.2A, rarely more.
Gain: 50 to 150 (type AC125) or 100 to 250 (type AC126).
Maximum frequency: 0,7 to 2 MHz (not more).
If they are well sorted (because often leaky) and well biased, transistors of this family let one build Fuzz and boosters of quality. They then bring a typically clear and nervous tone.
From left to right : Philips OC75 ; Mullard AC125 ; authentic NKT275 ; german AC122 ; russian MP41 and GT1322.
Underlined items have been used in historic pedals.
items followed by an asterisk * are npn models.
European ACnnn et ACYnnn in TO1 case
They could be found in almost all consumer electronics of the 60s, but little in guitar material (except Vox amps)
AC125 type (maximum gain 150)
AC125, TO1 Philips case… Clear and nervous tone, in Fuzzboxes as in boosters.
AC125, long TO1 case by Tungsram… Rather less leakage. Same sound character.
AC132, 134, 151… Often less bright than AC125; which is not necessarily a defect.
Professional model in Black Glass case. Low noise and very musical ; rare and pricey.
AC126 type (maximum gain 250)
AC126, TO1 Philips case… An AC125 with more gain. Same clear and nervous tone.
AC116, AC121, 122, 123… High gain, but leakage current can be too …
AC160, 163… Common in German quality material. A more veiled tonality than AC126.
ACY11, 14, ACY23 à 32… Industrial models worth to be tested; generally higher reliability.
Former European models in Black Glass case
Components of previous generation, which had a strong character …
They have an old school midrange tonality that can be very typical.
For these reasons, some have remained unmatched sound references.
One of the best in Fuzzboxes (Marshall Supa Fuzz, Vox Tone Bender Mk 1.5 and 2).
Not easy to bias, but the texture and the high-medium timbre are then superb.
Very sought after, therefore rare and expensive.
Used in the Vox Tone Bender Mk 1.5 and 2 in 1966-68.
OC81D has been over-researched and hoarded, so now untraceable.
OC79, 83, 84…
Rediscovered during the 2000s by Tone Bender Mk 2 professional clone makers.
They replace fairly well the original OC75 or OC81. To be searched for.
OC70, 71, 72, 74…
The oldest black glass : Rustic and groovy … Sometimes excessive background noise.
Not colourless. Can give beautiful vintage fuzz, well textured and not sour.
Various quality or reputable european models
NKT275 … A low-end AC125, cheap at the time, but that sounded good.
It is the original transistor of the 1966 Arbiter Fuzz Face, before the success of the AC128.
Not necessarily the best, but the real …
Originals can’t be found no more since a long time. Contemporary copies or fakes are numerous, and even exploited by major brands … You should better look elsewhere …
SFT 337… Another cheap AC125-type .
Used 1968-75 in Italian Tone Bender 1.5 and 2, with SFT363 or OC76 as output transistor.
Good preamplifier transistors, AC125 and AC126 type respectively : A good deal !
On the other hand, SFT322 lacks gain.
Models from Eastern Europe
Many Russian transistors sound well, and are generally more reliable than their Western counterparts :
Their moderate leakage make them easy to bias.
Not always at the top in fuzzboxes, but reliable, and advantageous in preamps and boosters.
Quite easy to find on the net, and at Eastern Europe prices.
MП16Б, MП25A, MП26A, MП37A*, MП39Б, MП40A…
Low gain (<80) but a nice dense texture in the last two of the list.
Suitable for small boosters, or low-gain Fuzz Face used as overdrive.
MП20A et Б, MП21A et Д, MП41A, MП42Б…
60-120 gain , perfect for classic fuzzboxes.
Each has specific colour and texture, to be discovered and exploited for classic fuzzboxes.
MП42Б is at the same time plump in the lows and clear in the highs.
MП20A et Б, MП21A et Д, MП41A are more in medium tonality, more or less brilliant.
ГT2307… Military quality, but with moderate gain (about 80)
Very good AC125 type for preamps and small boosters … Much less leaky.
ГT108Г, ГT109Г… Basic quality. More reliable and less leaky than AC126.
ГT1322, ГT1323… Military quality, and moderate leakage.
Very good AC126 types for bally preamps and boosters.
Czech production by Tesla …
Welcome as npn, but poor musicality in my humble opinion …
Various American and Japanese, for the record
Just an overview by the way, far from exhaustive …
2N104, 215, 237, 283, 368, 369, 437, 438, 487…
2SB459… Very good sounding preamp transistor, as well in booster and Fuzz Faces !