Heptod

According to international terminology, any electronic component with seven electrodes can be called a heptode. In fact, under this name there are two classes of lamps.

One of them is a completely independent converter, which allows to perform a local oscillator and a mixer using the same electrode structure and has the second name “pentagrid” (translated as “five-grid lamp”). In the nomenclature of lamps of the USSR, the most typical example is the 6A8 lamp.

The second type of heptode is designed to use a separate local oscillator, for which later they began to produce combined triode-heptode lamps - in the USSR 6I1P, 6I3P.

The appearance of the heptode was preceded by an invention that fundamentally changed the entire technique of radio reception - the principle of superheterodyne reception .

Converting a signal of any received frequency to a certain constant signal of an intermediate frequency sharply increased (in comparison with direct amplification receivers ) selectivity and sensitivity - the main quality indicators of any receiver. Simultaneously with the transition to superheterodyne reception, a need arose for special dual-frequency frequency converting lamps.

The conversion of the received frequency signal into an intermediate frequency signal can be carried out in two ways: according to the combined circuit and according to the separate local oscillator circuit.

With the combined scheme, the functions of the local oscillator and the mixer could be performed by one special five-grid lamp in which the local oscillator and the mixer were turned on as if in series, that is, the two networks closest to the cathode formed a local oscillator triode , and the following networks were part of the input signal amplifier. The mixing of these two signals occurred due to the fact that the anode current of both lamps turned out to be common and approximately equal to the product of these currents.

In a circuit with a separate local oscillator, the local oscillator frequency voltage was generated by a special cascade on a triode or pentode , and the signals were mixed in another lamp with two control grids (pentode or hexode ). Initially, preference was given to the first option as more economical (one lamp with one incandescent cathode instead of two). For this, a special five-grid lamp was developed - pentagrid.

The number of grids for both lamps is the same, but it is easy to see that their purpose is different. The first grid closest to the cathode of both lamps is the control in the composition of the generator triode - the local oscillator. In the pentagrid, the second network performs the function of the anode of the same triode, but in the heptode this network does not.

The next step was the appearance of two shielding nets instead of one. This was due to the fact that tetrodes and pentodes had only one control grid, which was fenced off from the anode by a screen grid.

In the new lamp - the pentagrid - it is still preserved: it is the “lower” of the two screen grids. But then the second control grid was near the anode, that is, it turned the mixing part of the lamp into a regular triode with its inherent main drawback - a large anode-grid through passage capacity. To eliminate it, between the second control grid and the anode an additional, second screen grid was connected, connected inside the lamp with the first screen grid, since both of them performed the same function.

The disadvantage of such a pentagrid is the absence of an antidinatron network . This drawback was eliminated in heptodes in which there is an antidinatron network, but the network that acted as the anode of the heterodyne triode in the pentagrid was removed. And his role began to fulfill the integrated screen grid.

Unlike conventional circuits, where the screen grid is shorted to ground by a capacitor of a sufficiently large capacity, the circuit coil of the local oscillator loop (providing positive phase shift feedback) is sequentially included in the power supply circuit of the screen grids in this circuit solution, and by the local oscillator generator.

Both pentagrides and heptodes have been used for a long time, performing the functions of frequency-conversion lamps. The first domestic pentagride of type СО-183 was released back in the 1930s, and heptodes of type 6A2P were used in broadcast receivers until the 1970s.

The combination of a local oscillator and a mixer in one lamp, as well as a relatively large interconnect capacitance, cause the local oscillator to “leak” into the receiver’s antenna and, accordingly, to broadcast it, which in some cases (a radio receiver mounted on a military ship or aircraft) made it possible to detect even a radio station that works only on reception.

Five-grid lamps are characterized by a number of additional parameters that other types of lamps with fewer grids do not have. First of all, this is the steepness of the conversion S _pr - the ratio of the variable component of the anode current of the intermediate frequency I _{a pch} to the alternating voltage of the RF signal on the second control (signal) grid U signal. In other words, the steepness of the conversion shows what amplitude of the intermediate frequency current creates the signal voltage, with an amplitude of 1 V at a given alternating voltage on the local oscillator grid.

Another specific difference of five-grid lamps from triodes, tetrodes, and pentodes is that two graphic characteristics — anode and grid — are not enough to evaluate their properties. This is because the usual anode characteristic as a function of voltage on one of the control grids varies greatly not only depending on the voltage on the screen grid, but also on the voltage on the second control grid. Therefore, five-grid lamps, as a rule, are accompanied by at least four families of graphic characteristics.

• Levitin E. Pentagrid. // Radio Front, 1934, No. 22, p. 22-24
• Gendin G.S. All about radio tubes.-M.: Hotline-Telecom, 2002.-296 pp .: ill .- (Massive radio library; Issue 1258).

• Conversion lamps