6. 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.

Clement Goossen had an article on this circuit (now no longer available) with a lot of information, including the first publication of the diagram from 1941. This first design uses the UY21 tube, while the later radios made to the design usually had an UY1N rectifier. 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". In The Netherlands the circuit was used in the well-known Philips 208U (photo), the radio that started the post-war productions in Eindhoven. Also the Talisman 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. Virtually the same design and the same chassis were used in four radio types (with different cabinets): the Tesla Talisman 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. All radios based on the U-series are transformerless, but simultaneously with this series a corresponding E-series was brought out. The ECH21/EBL21 tubes allowed its use in radios with power transformers, and in Italy (where locktal tubes were not produced) the circuit was adapted for the ECH4A/EBL1 tubes. The Music Board on the right also uses the ECH4 tube (and the AZ1 as the rectifier).

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.
The main drive for the introduction of the circuit was cost, but the quality of the circuit was such that it could be employed in higher class sets as well. Depicted on the left is the 1949 Siera 223, a five band radio with a balanced output consisting of two EBL21. The RF section contains two ECH21 tubes and with a magic eye and rectifier, the total tube count is six.

In the fifties 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, like the ECH21 had five years before. Orion had the ECH21-based Orion still in production and modified the Orion R-636F (photo left) to use 2xECH81 plus an EBL21. Some other radios similar to the revolutionary circuit exist with all Noval tubes ECH81, ECH81, EL84, and selenium rectifier, but technical developments made other lineups preferrable. The Philips car radio N3X94V of 1959 is equiped with 2xECH81 and EL95.
AM-only radios, like the Tesla Tenor (photo right), 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.

Gerard Tel