marelli: The strange volume control of the 'Chiliofono'
ID: 153366
This article refers to the model: Chiliofono I (1) (Marelli (Radiomarelli); Sesto San Giovanni (MI))
marelli: The strange volume control of the 'Chiliofono'
21.Nov.07 22:26
2324
Dear all,
in the Forum thread ‘Old capacitor marking’ Prof. Dietmar illustrated several non-conventional volume control circuits used in the early age of the radio. This review recalled me the strange solution I found in the model ‘Chiliofono’, produced in Italy by Radiomarelli, in the very late twenties. This huge TRF receiver used a voltage divider, from the screen grid voltage (about 120VDC) to ground, to give a positive bias to the cathodes of the first two RF amplifier stages. The volume control is encircled in the diagram below. Considering the combined action of the cathode currents and of the current flowing from the power supply, the DC voltage on both cathodes roughly varied from 2V to 40V. The result was similar to the grid bias illustrated by Prof. Dietmar in the circuit of Fig. 172, the transconductance of the tubes decreasing when the positive cathode voltage was increased. The relatively high current passing in the volume control potentiometer asked for a wirewound type. By the way, the radio used a double section potentiometer, the second section wired as a variable attenuator on the phono pick-up.
in the Forum thread ‘Old capacitor marking’ Prof. Dietmar illustrated several non-conventional volume control circuits used in the early age of the radio. This review recalled me the strange solution I found in the model ‘Chiliofono’, produced in Italy by Radiomarelli, in the very late twenties. This huge TRF receiver used a voltage divider, from the screen grid voltage (about 120VDC) to ground, to give a positive bias to the cathodes of the first two RF amplifier stages. The volume control is encircled in the diagram below. Considering the combined action of the cathode currents and of the current flowing from the power supply, the DC voltage on both cathodes roughly varied from 2V to 40V. The result was similar to the grid bias illustrated by Prof. Dietmar in the circuit of Fig. 172, the transconductance of the tubes decreasing when the positive cathode voltage was increased. The relatively high current passing in the volume control potentiometer asked for a wirewound type. By the way, the radio used a double section potentiometer, the second section wired as a variable attenuator on the phono pick-up.
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