Stable Filament Supply
Description: Valves are increasingly popular in audio systems. The European E series of valves, such as the ECC83 (12AX7) and EL84 (6BQ5), operate at a filament voltage of 6.3 V. Depending on the circuit configuration, the ECC81 and ECC83 series of twin triodes can also function with a filament voltage of 12.6 V. Historically, filament voltage was often derived directly from a separate transformer winding, contributing to the well-known "valve hum." The signal path in current valve circuits has not undergone significant changes. However, high-quality valve equipment frequently features a stabilized anode supply. Mains hum can significantly impact input stages where filaments are heated by AC voltage. To address this, a stabilized and precisely regulated DC filament voltage is proposed. The gradual increase of filament voltage upon activation is advantageous. Proper voltage level adjustment and soft start contribute positively to the longevity of the valves. The accompanying diagram illustrates a voltage regulator constructed from discrete components, with two sets of component values for 6.3 V (upper) and 12.6 V (lower). The circuit operates under a constant load, eliminating the need for special protective circuits and the complexity of optimal regulation for dynamic loads. The circuit features a power MOSFET configured as a series-pass regulator and a conventional control amplifier. A Zener diode (D5) establishes the reference potential, providing a constant voltage at the emitter of the BC547 control amplifier (T3). The current through D5 is set to approximately 4-5 mA using series resistor R5. The output voltage UO (the controlled variable) influences the base of the control amplifier (T3) through a voltage divider (R6/R7). A drop in output voltage decreases the collector current of T3, reducing the voltage drop across load resistors R1 and R2, which in turn increases the voltage at the gate of the MOSFET, closing the control loop. The resistor values in the voltage divider are selected based on typical tolerances of Zener diodes and must be adjusted if the diode is out of specification. The load resistance of the control amplifier is shared between R1 and R2, with the current through the load resistance effectively mirroring the collector current of T3, as the MOSFET draws negligible gate current. Filter capacitor C2 is connected at the junction of R1 and R2 to minimize residual hum. Electrolytic capacitor C4 and power supply filter capacitor C1 serve the same purpose. The level of hum voltage is also influenced by the load current magnitude. The voltage drop across the series-pass regulator remains nearly consistent for both 6.3 V and 12.6 V output voltages. For example, with a BUZ11 and a 1 A load at 6.3 V, the average voltage across the source-drain channel is about 7 V, resulting in a power dissipation of 7 W, necessitating an appropriate heat sink. The gradual rise of the output voltage is facilitated by the timing network composed of R3/C3 and T1. Upon power activation, T1 initially keeps the gate of the MOSFET near ground level. As C3 charges, T1 conducts progressively less current, allowing the control transistor to ultimately regulate the gate voltage. The mains transformer must be selected based on the required load current, with the necessary input voltage indicated in the chart. The transformer should possess a power rating at least 30% greater than the calculated load dissipation. When feasible, a filament voltage of 12.6 V is preferred, as it results in lower power dissipation in the series pass transistor when using twin triodes from the ECC81 and ECC83 series.
The described circuit serves as an effective solution for providing a stable filament voltage to vacuum tubes in audio applications. By utilizing a power MOSFET in a series-pass configuration, the circuit ensures that the voltage remains constant despite variations in load current. The inclusion of a Zener diode for reference voltage and a conventional control amplifier allows for precise regulation, essential for maintaining audio fidelity. The design also emphasizes the importance of minimizing hum, which can detract from sound quality. The use of filter capacitors further aids in reducing noise in the output. The gradual rise in voltage upon startup not only protects the components but also extends the operational life of the valves, making this circuit an ideal choice for high-end audio systems. Additionally, careful selection of the transformer and other components based on load requirements ensures optimal performance and reliability in various audio applications.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 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 voltage UO (the controlled variable) acts on the base of the control amplifier (T3) via voltage divider R6/R7. If the output voltage drops, the collector current of T3 also decreases, and with it the voltage drop across load resistors R1 and R2.
The voltage on the gate of the MOSFET thus increases. This closes the control loop. The values of the resistors forming the voltage divider are chosen for the usual tolerances of Zener diodes, but they must be adjusted if the diode is out of spec (which can happen). The load resistance of the control amplifier is divided between R1 and R2. The current through the load resistance and the collector current of T3 are practically the same, since the MOSFET draws almost no gate current.
Filter capacitor C2 is connected to the junction of R1 and R2 to reduce residual hum. Electrolytic capacitor C4 and power supply filter capacitor C1 serve the same purpose. The hum voltage also depends on the magnitude of the load current. The voltage drop over the series-pass regulator is nearly the same for an output voltage of 6. 3 V or 12. 6 V. With a BUZ11 and a load of 1 A at 6. 3 V, for instance, the average voltage across the source drain channel is approximately 7V. The power dissipation of 7 W requires a corresponding heat sink. The slow rise of the output voltage is due to the presence of timing network R3/C3 and T1. When power is switched on, T1 holds the gate of the MOSFET at nearly ground level. As C3 charges, T1 conducts increasingly less current, so ultimately only the control transistor affects the gate voltage. The mains transformer must be selected according to the required load current. The required value of the input voltage can be read from the chart. The transformer should have a power rating at least 30 % greater than what is necessary based on the calculated load dissipation.
Where possible, preference should be given to a filament voltage of 12. 6 V. When twin triodes in the ECC81 83 series are used, for example, the power dissipation in the series pass transistor is lower with a voltage of 12. 6 V.
The described circuit serves as an effective solution for providing a stable filament voltage to vacuum tubes in audio applications. By utilizing a power MOSFET in a series-pass configuration, the circuit ensures that the voltage remains constant despite variations in load current. The inclusion of a Zener diode for reference voltage and a conventional control amplifier allows for precise regulation, essential for maintaining audio fidelity. The design also emphasizes the importance of minimizing hum, which can detract from sound quality. The use of filter capacitors further aids in reducing noise in the output. The gradual rise in voltage upon startup not only protects the components but also extends the operational life of the valves, making this circuit an ideal choice for high-end audio systems. Additionally, careful selection of the transformer and other components based on load requirements ensures optimal performance and reliability in various audio applications.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 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 voltage UO (the controlled variable) acts on the base of the control amplifier (T3) via voltage divider R6/R7. If the output voltage drops, the collector current of T3 also decreases, and with it the voltage drop across load resistors R1 and R2.
The voltage on the gate of the MOSFET thus increases. This closes the control loop. The values of the resistors forming the voltage divider are chosen for the usual tolerances of Zener diodes, but they must be adjusted if the diode is out of spec (which can happen). The load resistance of the control amplifier is divided between R1 and R2. The current through the load resistance and the collector current of T3 are practically the same, since the MOSFET draws almost no gate current.
Filter capacitor C2 is connected to the junction of R1 and R2 to reduce residual hum. Electrolytic capacitor C4 and power supply filter capacitor C1 serve the same purpose. The hum voltage also depends on the magnitude of the load current. The voltage drop over the series-pass regulator is nearly the same for an output voltage of 6. 3 V or 12. 6 V. With a BUZ11 and a load of 1 A at 6. 3 V, for instance, the average voltage across the source drain channel is approximately 7V. The power dissipation of 7 W requires a corresponding heat sink. The slow rise of the output voltage is due to the presence of timing network R3/C3 and T1. When power is switched on, T1 holds the gate of the MOSFET at nearly ground level. As C3 charges, T1 conducts increasingly less current, so ultimately only the control transistor affects the gate voltage. The mains transformer must be selected according to the required load current. The required value of the input voltage can be read from the chart. The transformer should have a power rating at least 30 % greater than what is necessary based on the calculated load dissipation.
Where possible, preference should be given to a filament voltage of 12. 6 V. When twin triodes in the ECC81 83 series are used, for example, the power dissipation in the series pass transistor is lower with a voltage of 12. 6 V.
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