A Primer on Pentagrid Frquency Converter Tubes

Frequency converter tubes are used in superheterodyne receivers to change the radiofrequency (RF) of the incoming amplitude-modulated (AM) signals to an intermediate frequency (IF) which is now typically equal to 455kHz. This IF frequency is then amplified by fix-tuned amplifiers and demodulated by a diode detector which removes the 455kHz IF component and passes the audio component to amplifiers (1^st and 2^nd audio) which eventually drive a loudspeaker.

Converter tubes change the frequency of the incoming signal by mixing it (beating it) with a second injected frequency from a tunable local (meaning inside the radio) oscillator. If this process happens in one tube, that tube is called a “converter” tube. If this process happens with 2-tubes, one tube is called the “mixer” and the other the “oscillator” (Radio Amateur’s Handbook, 1957, p. 94). Some of the older literature refers to converter/mixer tubes as “first detectors” since some type of detection process is required to extract the intermediate frequency (Frye, p. 128).

In the All-American 5-tube radio (AA5) of the 1930’s-50’s, the converter tube is the first tube to see the RF signal from the antenna which can be a long-wire, loop, or loopstick. Its effect on the signal-to-noise ratio (S/N) is great, especially if that converter is a pentagrid (5-grid) tube. As the number of grids or controlling elements within a tube increases, the amount of tube noise generated likewise increases (Kiver, p. 67). The principal source of internal noise in converter tubes is what is termed shot noise, so named because it’s amplified effect in a loudspeaker sounds like shot (i.e. buckshot) hitting a metal plate. This shot noise (or “tube noise” as called by some) increases with the number of elements (grids) in tubes making the pentagrid converter the noisiest of all (Ghirardi & Johnson, p. 28). In some superheterodyne receivers, pentagrid-converter noise is even strong enough to be heard above the noise resulting from and amplified by a preceding RF amplifier stage (Ghirardi & Johnson, p. 142).

In the process of conversion, a certain amount of IF amplification takes place. The ratio of the IF voltage developed in the output of the converter to the RF signal input voltage is called the conversion gain or conversion efficiency. For a converter tube, this gain is typically 0.3-0.5 times that of a similar tube operated as an IF amplifier (Slurzberg & Osterheld, p. 447). In the RCA 45X18 AA5-radio, for example, the conversion gain from a 12SA7 converter tube is measured at 35X while that of the 12SK7 IF amplifier tube is listed at 150X with both tubes at –3V fixed bias (Marcus & Levy, p. 253).

Increasing the gain of a converter tube, especially if it is the first tube in the radio (like an AA5), decreases the noise referred to its grid and provides for a better overall signal-to-noise (S/N) ratio (Radio Amateur’s Handbook, 1957, p. 94). There is a distinct advantage in obtaining maximum stage gain from the converter as no increase or decrease of gain following the converter has any effect on the signal-to-noise ratio (Radiotron Designer’s Handbook, 3^rd Ed., p. 107).

The figure of merit for converter tubes is a characteristic quantity called the conversion transconductance or g_c (RCA Receiving Tube Manual-1960, p.12). It is defined as the limiting value of the quotient of the IF current in the IF transformer divided by the applied RF signal voltage producing it. As this quantity is a current divided by a voltage, it takes on an inverse-resistance value commonly known as a mho (ohm spelled backwards). For most tubes (converters and others), values are typically expressed in a millionth of a mho or micromhos (mmhos). When the performance of a frequency converter is determined, conversion transconductance is used in the same way as control grid-plate transconductance (g_m) is used in simple frequency amplifier tube calculations.

Converter gain (a numerical ratio) is expressed in terms of g_c, plate resistance (r_p), and the load resistance of the tuned plate circuit to the IF frequency (R_L).

For plate voltages of 100V, the following data are listed for the 12SA7 and 12BA7 converters (RCA Receiving Tube Manual, 1960):

With r_p the same for both tubes and, assuming R_L to stay the same in the same radio, replacing a 12SA7 converter tube with its 12BA7 equivalent and maintaining the same fixed grid bias in both cases results in a gain ratio of 900/425 or approx. 2.11.

This means that, independent of the volume control, the signal-to-noise ratio (S/N) of the demodulated audio signal is approximately doubled on going from a 12SA7 to a 12BA7 converter when both tubes are maintained at the same fixed bias.

1.      The Radio Amateur’s Handbook, 34^th Ed., American Radio Relay League, Inc. (1957).
2.      John T. Frye, Basic Radio Course, Gernback Library, Inc. (1951).
3.      Milton S. Kiver, F-M Simplified, D. Van Nostrand Co., Inc. (1947).
4.      Alfred A. Ghirardi and J. Richard Johnson, Radio and Television Receiver Circuitry and Operation, Rinehart Books, Inc. (1951).
5.      Morris Slurzberg and William Osterheld, Essentials of Radio-Electronics, 2^nd Ed., McGraw-Hill Book Co., Inc. (1961).
6.      William Marcus and Alex Levy, Elements of Radio Servicing, McGraw-Hill Book Co., Inc. (1955).
7.      F. Langford-Smith, Radiotron Designer’s Handbook, 3^rd Ed., The Wireless Press (1941).
8.      RCA Receiving Tube Manual, Radio Corporation of America (1960).
9.      F. Langford-Smith, Radiotron Designer’s Handbook, 4^rd Ed., The Wireless Press (1953).

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