The Pentagrid Converter

The pentagrid converter is a commonly used tube you'll find in the converter stage of many radios. The five grids allows it to function as both, local oscillator and mixer stages. The image below shows one typical configuration.

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Grid 1 (pin 1) is called the oscillator grid because it's used as the control grid for a triode oscillator circuit. Grids 2 and 4 form the screen grid of the mixer and are connected internally (pin 6). This dual grid also functions as the anode (plate) of the triode oscillator, operating with grid 1 and the cathode (pin 2). Grid 3 (pin 7) between the dual grid is the signal grid because it's the control grid of the r-f input signal. Grid 5 is the suppressor and connects internally to the cathode. The plate (pin 5) is for the pentode mixer function.

Secondary winding of T1 resonates with C1 to produce the desired oscillator output frequency. T1 primary acts as a tickler coil to provide the necessary feedback voltage to keep the oscillator going. Antenna and C1 form the r-f tuned circuit that feeds the desired r-f signal to grid 3 of the converter. Note that tuning capacitor C1 of the signal circuit is ganged with C1 of the oscillator. This causes the oscillator frequency to always differ from the signal frequency by the intermediate frequency as the radio is tuned to different stations. As a result of the heterodyning action in the converter, the output in the tuned plate circuit TC1 is the desired i-f signal. This is coupled to the i-f amp by transformer Z1, which is tuned to the i-f frequency.

Heterodyning

The idea of producing a new frequency by the process of heterodyning is similar to a more familiar example of beat frequencies commonly heard with musical sounds. When two tones of nearly the same frequency are produced at the same time, the ear detects a change in sound intensity. This beat is produced as the two individual tones alternately reinforce and cancel each other out. When the two tones are exactly in phase (peaking at the same time), they reinforce to produce sound with a higher amplitude. As the tones shift out of phase, the amplitude will lessen as the tones begin to cancel each other out. The result is a sound at the beat frequency, which is exactly equal to the difference between the two original frequencies. For a radio with an i-f frequency of 455khz tuned to a station at 1000khz, the proper oscillator frequency would be 1455khz.


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