the AC128 transistor

31 January 2019 / by Jacques Charbit

INTRODUCTION: The nanotube

Silicium might be an endangered species. Progress around carbon electronics (nanotube and tripods) and nanotech are going so fast that a well-known Japanese TV brand is issuing next year a flat TV screen using carbon nanotube. Much more than this, in our days of digital technology, it is quite uneasy to find the simplest Silicium transistors essential for fuzz box craft.
In this context, it may seem ridiculous to focus on the old germanium electronic science, which main actors are just fond memories of the past.
Nevertheless, and I am not the only one to think this way, nothing sounds half as good as a well-tuned germanium distortion device for electric guitar.

This is why today, I would like to introduce you to my old buddy, the AC128 germanium transistor ,
exclusively used in my handmade Mercer Box and KATAPULT pedals .

Besides being in the famous original Fuzz Face, the AC128 was used in numerous domestic electronic appliance, and certainly one of the most common European transistor. This explains why it is still possible to find nice new-old-stock of these jewels.

They usually come in 3 different types :

The real thing

This is both the oldest and arguably the best sounding AC128. It is very similar if identical to the infamous NKT275 which equipped the original Jimi Hendrix grey fuzz face .
But this wonderful piece of crap has a major drawback : completely random technical data in a VERY wide range. This means that any given theorical design has something like 30% odds to function right away. This fact has 2 consequences:

1- Each distortion device using this same transistor must be modified afterwards to match the theoretical technical data of the set of 2 or more transistors.

2- This a posteriori adaptation must be made while monitoring the tone of the resulting distortion in order not to drift away from the prototype.

Nowadays cheap transistor tester are avaible widelyso transistor selection is a tad easier.

It is therefore easy to understand that this kind of transistor is not industry compatible. Only handmade and hand-tuned devices can benefit from this nasty gremlin. Beauty has its price.

The hottie

This strange device is exactly the same transistor BUT with a factory mounted radiator, useful in other applications where it receives its 32v possible supply. The AC128 can be as hot as a soldering iron in certain cases with no damage. Compare this to a ts9 chip.

You will find these square beatuies often in a matched pair with a AC127. Those pairs were used in the not-so-distant past for making small amps that will go in radio, record players etc … remember the Queen deacy amp was powered by ac128 as power transistors.

I feel the radiator somehow tightens the fuzz tone, while not in a big way, making it a valuable successor.

The modern

Well, they were the last poduction AC128 and can be found easier in new old stock. Their data is even more erratic , with hfe gain going from floor to roof in the same batch. I don’t recommend them for beginners.

Unfortunately,while really good in booster / one-transistor pedals, this young component gives a higher even/odd harmonics ratio in fuzz pedals than its pain-in-the-ass ancestor. So the resulting distortion is , exaggerating, closer to a metal zone than a fuzz face.

They are precious anyway, as they fit in the original fuzz face design without any change and give a very acceptable approximation, even for a purist. They really are pure germanium after all!

The little sister

The AC188 pictured here, is a scaled down version of the modern AC128, made for lower voltage applications. And what are our 9v fuzz box ? In fact , this little sister does miracles with electric guitars.

Pushed to their limits, they deliver a full frequency grawl that is hard to beat. A very interesting transistor indeed, the AC188 is too rare to give its full dimension but remains one of my fave if you know how to use it.

the OC family

OC prefixed transistors were made by Mullard and Valvo , and are famous among pedals aficionados because they were present in famous classics such as Marshall supafuzz and eraly vox tonebenders. They usually are good quality and good sounding. The oc75 seems to have been manufacured or selected for good and constant Hfe , making it a perfect choice , while the oc76 must be sorted but yet is also a great choice for fuzz pedals. I can imagine OC75 were selected OC76 , just like Philips were offering different selection for their great dac chip tda1541 .

The Frenchies

Made in Aix-en-Provence SESCO 324ti transistor in front of a Jacques French Fuzz Face

The first transistor to be produced in France was invented by Doctors Welker and Mataré of the Société des Freins et Signaux Westinghouse (F & S Westinghouse) in mid 1948 and announced to the World in May, 1949

inside a Elka Dizzy Tone charged with french sft352

In the world of fuzz, some french transistors made their way to history such as the SFT352 present in the famous Elka Dizzy Tone which was at my time a modest copy of the tonebender.

But in 1962 , the two companies present in the early days,
Thomson-Brandt and CSF merged into two new firms , COSEM in the French Alpes and SESCO in Aix-en-Provence , a few miles away from my hometown, Marseille.

So it’s no surprised I found here nice supply of germanium transistors from SESCO, including my fave, the 324TI which has perfect HFe and noise level for fuzz pedals.

These means I can do a nearly 100% made in Marseille pedal, which I call the F.F.F. ( French Fuzz Face ) . Please send me a note if you’re interested.

EPILOGUE : When supply will dry…

In a very close future, all stocks, including mine, of original germanium AC128 or any usable germanium transistor will dry.


After this extinction, that only a few aficionados will notice, there will be no other possiblities to obtain THE original fuzz tone than a used Germanium fuzz box or a digital model.

Make your choice.

From my point of view, I feel my ’65 strat deserves more than a plastic computer. And in my experience, no digital guitar tone can sound good live or even recorded.

” How well you know me

You’ve seen me cry

I’m just a shadow

In a rock’n’roll sky “

Deep Purple 1973 “Super Trooper”

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