Germanium transistors : TO1 case models

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…
Transistors manufactured in Eastern Europe used an elongated version of the TO1 case : some ACs, SFTs manufactured under license, and the national productions of these countries.

The TO1 standard was not strictly followed by all manufacturers, which can nowadays seem surprising. 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.


The original TO1 case

Composants - TO1 court

From left to right : Red point AC128 ; Genuine NKT275… On the right : ASY70 and AC151, in original Siemens case.


Philips-Mullard’s TO1 :
the classic of the classics

The referential TO1 case is cylindrical and short, matt metal, with outgoing leads through a narrow opening. Mullard-Philips imposed around 1964 something like a de facto standard with their new ACxxx series. It is often nicknamed “Red dot TO1 case” because of the colored mark which marks the collector of PNP models (AC125, AC128…). The less common term “Blue dot” marked  the NPN models (AC127, AC176 …).
These transistors are becoming rare and highly sought after, because they were the source of the famous creamy sound of the best vintage fuzzboxes – If properly selected, and if you know how to bias them…

One word on NKT275

It was the original transistor in the Arbiter Fuzz Face in 1966, before AC128s were adopted. Their British manufacturer Newmarket was a specialist in low cost components, and this cheap copy of the AC125 has been by chance at the origin of a cult pedal ! NKT275 were highly sought after in the 1990s, and today it is difficult to identify certainly authentic items. There are two reasons:

• Imitations of NKT275 are numerous. Their case can be sometimes close to the original one, sometimes different : Short Philips-like TO1, but with a wide opening for connections and badly faked fonts, long TO1 type from Eastern Europe, and even american TO5 type …
• Newmarket itself has blurred the situation. It is not impossible that this firm, often working piecemeal, has conditioned series of authentic NKTs in various cases, depending on industrial disponibilities. Given this confusion, some cloners declared that only the true TO1 boxes were originals. Not all manufacturers.

Special short TO1 cases

Siemens used a specific variant of the short TO1 case. It is distinguished by its bright metal and a more or less flared rim at the lower end of the case. The barred S logo confirms the identity of these items. Siemens transistors are quality items, with more gain and less leakage current than competitors, and generously amplifiy the treble.


The long TO1 case

Composants - TO1 long
From left to right : Generic AC128 ; AC128 and AC125 by Tungsram. Eastern Europe SFT308 and GT2307 ; GC301C (GDR).

This longer and thinner case was used in international productions, often no branded, and therefore it used to have a poor image among cloners and DIYers … However, one can find in this family very exploitable versions of AC125s, AC128s, AC188s, all the original SFTs – some are classics – and genuine productions of eastern countries.

A word on productions
under license

AC125s, AC128s, AC188s manufactured under license in Eastern countries had no specific brand printed on their case. These are the ancestors of today’s OEM generic components, which now equippy virtually all computers and common audio products…
They are not bad in principle : A no brand transistor that has been well chosen and well implemented will work better than a poor copy or a missed item in a prestigious brand.

Hungarian Tungsram transistors

Their AC125 and AC128 are authentics, with the positiv/negativ “T” logo. They have been snobbed as long as western originals and good generics were available. Now these models are no more available or jalously kept, Tungsram transistors are the lasts NOS transistors one can easily find and buy… So now everybody find them attractiv !
Here are some markers usefull to select them, and find out good models.

Black printed matt case models have often moderate leakage, and their mesured gain is nearly nominal. According to my experience, they are the most easy transistors to be exploited in classic fuzz and overdrive circuits.
• Green printed bright case models have more gain and leakage currents. One must carefully select them, espacially those with «HM» printed on the top of the case : these ones can be very leaky, and therefore hard to bias.
• Well selected items have an intersting sonic potential.
Sound granularity is not weak nor raspy, and they come well into saturation.
…AC128 has a typical midrange timbre, warm enough and clear, which can give a singing fuzz, even if it sounds not so creamy as the original Philips/Mullard models.
…AC125 his sharp and nervous : good for those who like more modern sounding fuzz, more radically on top of the mix.
I use them two in the brighter versions of my LikeYourFace and HiGeFuzz fuzzboxes.

Other specific models
from eastern countries

The long TO1 case was the common case of transistors from Hungary, ex-Tchecoslavie or ex eastern Germany (GTs, GT1s, SFTs, GCs, 10xNUxxs). Some of those transistors are quite correct, and can be used in fuzzboxes, and be very attractives.

• Audio preamplifier transistors
One can find good items in the SFT3xx series… For exemple :
SFT321, 322, 323 are good preamplifiers, each of them with its propper gain.
GT1321, 1322, 1323 are milatary versions of them, with improved quality.
• GC301 B or C is an eastern german equivalent of the western AC128.
According to my experience, it is the only one to give sound texture near to the creamy one can get with original AC128s.
His limited band x gain product causes a tonality that one can find a bit too dark when used in classic two transistors fuzzboxes. However, it works very well with the AC125 or AC128 Tungsram: GC301n wets and smooths their sharp granularity, while AC128 and especially AC125 come nicely kick its ass !
• Transistors from the czech brand Tesla.
This is a great brand, but I could never be able to get a nice saturated sound from them. Two examples that one may find on commerciel websites:
GC511 is intended as an AC128 equivalent… But he has a rattling voice… Love it or leave it !
104NU71 and his family are attractive given the scarcity of NPN germanium. Alas they reveal poor sounding in preamp overdrive or fuzz applications.

Just browse and try : more numerous transistors than you believe can sound well !
Guitar Poppa.
Bandeau fin gris moyen - L15

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