Stable Filament Supply

Posted on Feb 5, 2014

Valves are enjoying increasing popularity in audio systems. With the European E` series of valves, such as the ECC83 (12AX7) and EL84 (6BQ5), the filament voltage is 6. 3 V. Depending on how the circuit is wired, the ECC 81 83 series of twin triodes can also be used with a filament voltage of 12. 6 V. In earlier times, the filament voltage was usually

Stable Filament Supply
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taken directly from a separate transformer winding, which (in part) was responsible for the well known valve hum`. With regard to the signal path, current valve circuits have hardly experienced any fundamental changes. In high-quality valve equipment, though, it is relatively common to find a stabilised anode supply. Mains hum can have a measurable and audible effect on input stages whose filaments are heated by an ac voltage. The remedy described here is a stabilised and precisely regulated dc filament voltage. The slow rise of the filament voltage after switching on is also beneficial. The exact setting of the voltage level and the soft start have a positive effect on the useful life of the valves. Diagram shows a voltage regulator meeting these requirements that is built from discrete components. The two sets of component values are for a voltage of 6. 3 V (upper) and 12. 6 V (lower). Thanks to the fact that the supply works with a constant load, it can do without special protective circuits and the additional complexity of optimum regulation characteristics for dynamic loads. The circuit in Figure 1 consists of a power MOSFET configured as a series-pass regulator and a conventional control amplifier. Zener diode (D5) sets the reference potential. A constant voltage is thus present at the emitter of the BC547 control amplifier (T3). The current through D5 is set to approximately 4 5 mA by series resistor R5. The output...

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