[Dept. of Computer Science]

Tesla 306U Talisman


Our Department and University.
See my other radios too! (Here is the next one.)

An attractive little bakelite radio from Tesla, the former Philips Elektra plants in Czechoslovakia.

Data for this radio
Type Tesla 306U "Talisman". Serial number is 217533.
Bands LW (1000-2000m), MW (187-571m), SW (16.5-51.5m).
Cabinet Bakelite and white plastic, WHD = 26x16x14 cm.
Tubes UCH21 (mixer), UCH21 (IF stage and audio preamp),
UBL21 (detector and output), UY1N (rectifier),
2 dial lights.
Controls Volume/off, Tuning, Band switch.
Produced Czechoslavakia, 1952.

Czechoslovakia after WWII

Marco Berti (Italy), Martin Hajek and Antonin Svec (Czechia) provided me with some some historical information about the radio plants and the political and industrial situation in Czechoslavakia at the time. Czechoslowakia had always been technically advanced, there were many radio factories and the population was among the most educated and culturally advanced of the world; only after the Second World War, under communism, there was a decline.

After the last Nazis escaped and the Russians entered Prague on May 8, 1945, the Czechs had free elections in early 1946, Benes was prime minister and the strongest party was the Communist. A couple of years of freedom followed, but in early 1948 Stalin decided to `normalize' Czechoslowakia, Benes mysteriously died and Klement Gottwald, a Stalin adept, took complete power. He was so bound to his big idol that he even died just one week after Stalin (in 1953).

The process of abolition of private factories had begun well before Gottwald's coup: in 1947 most of the formerly private enterprises had been nationalized. The Philips manufacturing infrastructure became the Tesla radio plants and started (or continued) the production of small bakelite radios using a then innovative Philips circuit.

The Four Tube Circuit

During the second world war, our national pride Philips managed to reduce the number of tubes in a standard radio from 5 to 4. Until then, a standard radio had a mixer, an IF stage, a first audio stage, an output stage, and a rectifier, each in a separate tube. The invention was a special design of the mixer tube UCH21 allowing a second indentical tube to serve as both the IF amplifier and the first audio stage. (For the technically inclined: there was no internal connection between the triode grid and the mixer grid, so the tube UCH21 has nine external connections. Detection took place in the output tube UBL21, fitted with diodes for this purpose.) Not only was the total tube count reduced from 5 to 4, but also two of the remaining tubes were identical; understandably, this reduced production cost and it was extra attractive in regions where radios were taxed on the number of tubes.

I do not know if the circuit is known by some special name; if you do, drop me an email.

The revolutionary circuit was adopted in many sets, starting with the Philetta 203U of 1941, produced in various countries including the then "Protectorate of Bohemia - Moravia". Also this Talisman 306U was designed, in the Tesla Hloubetin plant in Prague, on the basis of the UCH21, UCH21, UBL21, and UY1N tubes. However, what made this little radio so attractive and famous was not the circuitry inside, but its compact and elegant design; compare the radio to the Philips 208U (1944). Virtually the same design and the same chassis were used in four radio types (with different cabinets): the 305U, 306U, 307U, and 308U. Production of the 308U continued until 1958 and altogether over one million radios of this design were sold.

Rise and Decline of a Circuit

Meanwhile the 4-tube principle was used in many Philips radios produced all over Europe in the late forties; the ECH21/EBL21 tubes allow its use in radios with power transformers (see the BX462A), and in Italy (where locktal tubes were not produced) the circuit was adapted for the ECH4A/EBL1 tubes.

The introduction of Rimlock tubes (with only eight pins!) around 1948 meant the end of the circuit's use in the capitalist countries. In 1949 one could still buy radios with the ECH21 as the mixer, like the Philips BX690A, but its Rimlock successor ECH41 was already taking over, as in the Philips BX500A of the same year. Having few tubes became less important because of changing taxation laws and marketing dictated a reverse policy because a radio with more tubes could be sold for more money (regardless of whether the extra tubes actually produced extra quality). Philips' flagship of the fifties was this BX998A monster with sixteen tubes.

In Czechoslovakia, with no competition and no import-export with competitive countries, innovation was not a major issue, and besides, the radios were actually very good and well built. Tesla used the four-tube principle (with addition of a magic eye in some models) until 1956 with only minor modifications.

After only a few years the eight-pin Rimlock tubes were followed by the 9-pin Noval tubes and the good old Noval converter workhorse ECH81 has the triode grid and converter grid brought out separately. Some radios similar to the revolutionary circuit, but with Noval tubes ECH81, ECH81, EL84, and selenium rectifier do exist, but technical developments made other lineups preferrable. AM-only radios, like the Tesla Tenor, combined the audio driver with the output tube in an ECL82 or UCL82 tube (and later still, the ECL86). FM radios, like this Loewe Opta, required more tubes and usually combined the audio driver with the detection in the EABC80 tube. But if you want a nice Do-It-Yourself project, take the old circuit and four ECH81 tubes and build an all-ECH81 radio. Two tubes would make a balanced output stage, one triode is used as a phase inverter and one as a detector.

My Tesla Talisman

The Tesla 306U Talisman was almost identical to the 305U (produced in 1949-51) and it was produced in 1951 en 1952. I bought this Tesla in November 1996 in Prague for 700 crowns (50 guilders) and it had to wait until March 1998 for a complete cleanup. The radio suffers low sensitivity, but it receives the local stations very well and found a place in our living where I use it almost daily to listen the news.

Gerard Tel, gerard@cs.uu.nl.