FM pre stage with triode

In 1953 a special circuitry for FM pre stages with triodes was invented by Rudolf Cantz from TELEFUNKEN. Later this was named "Zwischenbasis Schaltung" Intermediate Basis toplogy. This circuit has some advantages with respect to Cathode basis circuits as well as to Grid basis circuits. 

Because the FM broadcast band in Europe then was 87.5 to 100 MHz, no tuning of the first circuit was necessary, and a two gang capacitor was sufficient.

American radios seldom used this circuitry, and so a question came from Joe Sousa concerning the function of this circuit. Hans Knoll who is a fomous former developper of Grundig radios has some German literature about FM circuitry, and he supported us with this information. So a very fruitful colaboration began, and finally succeeded with a translation of the Cantz paper to English.

In this paper Cathode basis, Grid basis , and Intermediate Basis topologies are discussed and explained. In a companion German thread on "Die UKW Vorstufe mit Trioden" additional material can be found.

In a mail Joe says:

Since learning about ZB, I have looked at the front ends of American FM radios and ZB was rarely used. One contributing reason was that the US FM band was twice as wide, making the fixed-tuned operation of ZB less effective, as compared to straight grounded grid. However, a modest x value of 0.5 might have improved the gain of a grounded grid stage while maintaining adequate bandwidth for the entire 88-108MHz band. Pentodes were used a lot in American FM radios, despite the partition noise. However, some tetrodes, like the 6C9 were designed with a shadow grid for reduced partition noise. There were also triodes with shielded grids, like the 6HA5, which had about 0.3pF-0.5pF at C_ga. One gets the sense that there was more design activity in FM/VHF tubes than in FM/VHF circuits in the US.

Find here the Cantz paper "RF Amplification with Triodes" (.pdf 1MB)

Regards,

Dietmar

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