Tesla logo

As far as I know, there was only one manufacturer of semiconductor devices in the former sovereign state of Czechoslovakia: the Tesla company. The company used a number of logos, most involving the word Tesla plus a wave symbol, such as the one shown. On semiconductor devices, they used just the word TESLA in capitals. There is disagreement about whether the company name has a connection to the Serbian-American electrical engineer and inventor Nikola Tesla.

I am indebted to a correspondent, Ivo, who provided me with fascinating information about Tesla. The company was formed in the years after the second world war by government directive. Research and development was based at the VUST Institute in Prague. (VUST stands for Výzkumný ústav sdělovací techniky, which means "Research Institute of Communication Technique"). The production was realized in the Tesla factory at Rožnov, a small Moravian town. In 1950, the experimental point-contact transistor was developed at VUST. Between 1953 and 1954, point-contact detector diodes and the development of alloy transistors was done. Production of the first PNP alloy transistors xNU40 started in 1955 followed in 1958 by production of the NPN alloy transistor series xxxNU70. In 1956, a demonstration transistor receiver was built with transistors xxxNU70, except for the oscillator, where the point-contact transistor was used because of its higher frequency. In 1958 a receiver "T58" was produced with transistors xxxNU70 as shown in this article.

There is also a comprehensive 'exhibition guide' in Czech on the Web, from an exhibition held about Tesla Rožnov in 2014 at the Wallachian Museum. It's a pdf that can be translated rather patchily by Google. On page 50 it has images of some interesting germanium diodes and transistors and it gives the following timeline for germanium devices:

And another for silicon devices, which I have truncated here as the more modern types do not interest me :

Rather oddly, most of Tesla's early semiconductor series are identified by two-letters plus a two-digit suffix, with a one- or two-digit prefix that denotes an individual type within the series. This makes cataloguing difficult! I have Tesla data books from 1968 and 1976, and Tesla devices are included in a Hungarian data book from 1960. However I have no proper data for the earliest types.

As with all Eastern European semiconductor manufacturers, Tesla went through major changes after the fall of communism. Some of its parts still survive as specialist manufacturers.

wanted transistor

Tesla made a small number of point-contact transistors type xNT40 about which I have no data. I would be very interested to find examples of these, or even just data about them.

If you know anything about these, or have data about them, please

wanted diodes

I am also seeking examples of Tesla's point-contact diodes, which are the two series 1NN40 - 6NN40 and 1NN41 - 7NN41. The first series is included in the Hungarian data book linked above. I have not seen an example of these, but old data books show the diagram on the right, which suggests a glass body with a band of rubber or plastic around it.

If you know where I could obtain any, please

xNN41 diodes This image shows examples from the nNN41 series. They have a typical glass body with a painted stripe around the cathode. The colour code used is a proprietary one:

You can find data for these here. The GA20n types use the same colouring but are physically smaller.

wanted diodes There are two more point-contact diodes in the NN41 series that seem to be rare:

These use a longer, thinner case than the lower-numbered NN41 types. Of course, I would be interested to obtain examples.

wanted transistors

I would also be very interested to find examples of Tesla's first junction transistors, dating from about 1955. I have an undated page from a Tesla Katalog that lists the PNP types:

If you know where I could obtain any of these, or have data about them. please

102NU70 transistor

Tesla's first production series of transistors is the xxxNU70 series of NPN germanium AF low-power transistors (102NU70 shown):

There are also 'bílý' versions of 105NU70 to 107NU70 that have lower maximum collector voltage and slightly different gain banding. This word means 'white'. Ivo thinks that this denotes a white spot on the top of the case. This might be a way of selling devics that did not meet the full spec.

156NU70 transistor

The series 151-156NU70 are "high-frequency" ultra-thin-base alloyed transistors used for AM receivers.

It seems that the early production of the xxxNU70 series, including these high-frequency types, used a flat black case like Tesla's first junction transistors. I am very keen to find examples of these.

If you know about the flat can varieties, or can provide data for the high-frequency types, please

There is also a series 101NU71 to 104NU71 of 250 mA types.

4NU72 transistor

There are three series of PNP germanium AF high-power transistors:

GC transistors

Tesla also manufactured germanium types in the GDR GC-, GD-, GF- and GS- series, but using higher serial numbers, as shown by my example of a GS507 (germanium NPN RF low-power) in a plain metal can and a GC519 (germanium PNP AF low-power) in a black-painted one. They also made a small number in the Cyrillic ГТ series.

Tesla OC70 transistor Tesla OC169 transistor

Tesla copied Western germanium junction transistors in the Mullard/Philips OC series. In the 1968 data book the following are listed:

  • AF types OC70, OC71, OC72, OC75, OC76, OC77
  • RF types OC169, OC170, OC171
  • power types OC27, OC28

Tesla went on to manufacture a large number of silicon transistors that do not really interest me. The 1968 Tesla Katalog contains silicon transistors in the standard BC, BD, BSX and BSY series and in the proprietary KC, KF, KD, KU, SU, KFY, KFW, KSY, KSZ, KDY, KUX and KUY series.

GAZ51 germanium diode

Tesla also made germanium diodes in the DDR GA and GAZ series. The image shows a GAZ51 in the ubiquitous TO-1A can; despite the Z this is not a Zener, it is a gold-bonded switching diode.

4GAZ51 germanium diode

This unusual construction is the 4GAZ51, a set of four GAZ51 diodes for use in modulators / demodulators. The diodes themselves are unmarked except for some indistinct paint markings. I believe that there is also a dual-diode 2GAZ51 although I have never seen one.

OA9 germanium diode

They also copied the Philips/Mullard germanium diodes OA5, OA7 and OA9 (shown) although they did not copy the standard SO-2 glass tube.

wanted transistor Tesla made a good number of germanium power rectifiers, listed here. I have none and would like to have examples of some.

KY701 diode

They also made silicon diodes in the KY and KYZ series. My image shows a KY701; it is not branded TESLA. Ivo informs me that these were made by Tesla Piešťany in Slovakia from 1967 onwards.

10PN40 photodiode

Tesla also made germanium photodiodes:

My image shows the very thin 10PN40. I am looking for examples of the others.

1PP75 photodiode

The 1968 Tesla Katalog contains one silicon photodiode: this 1PP75.

Tesla went on to make many more discrete silicon devices and integrated circuits, which I will not show here.