Powering the very high gain circuit tube filaments audio preamps is recommended to use DC currents. In the figure we give a circuit for this type of power, observing that the voltage adjustment potentiometer is of wire. This potentiometer must be adjusted to obtain 12 V with the valve powered. The transformer has 12 V or 6 + 6 V with a current of at least 600 mA. The diodes can be 1N4002.
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