WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

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WhitakerAudio
1.1.2 Power Supply
The transformer-coupled ac input power is converted to dc operating power for the amplifier
stages by the 6CA4 (V51) full-wave rectifier tube, the output of which is delivered to choke L51
and then to the filter bank. Filter capacitors for the B+ supply are provided on both the right and
left channel amplifier boards.
The rated output voltage of the B+ power supply is 345 V dc. With the recommended bias
voltage on the grids of the push-pull output tubes (–30 V), the steady-state no-signal current draw
is 130 mA. When producing 10 W per channel (the rated output), peak current draw is 210 mA.
The maximum output of the power supply is 300 mA. The power transformer (T51) and choke
(L51) are conservatively rated for this application. The 6CA4 rectifier tube is rated for an average
output current of 180 mA; the rated peak current per plate is 500 mA.
Bias voltage has a significant impact on the steady-state current requirements of the amplifier
stages. An operating value of –30 V is strongly recommended. Lower (less negative) bias will
result in shorter tube life from the output devices and rectifier.
Fixed bias operation of the output stage requires that the power supply provide good voltage
regulation as the plate current of the 6973 tubes vary considerably with signal level. The
conventional choke-input type of power supply used provides the required stability. The fixed
bias for the output stage is obtained from a 50 V ac tap on the high voltage secondary winding of
power transformer T51. This voltage source is rectified by a silicon diode and filtered.
Potentiometer (R57) is provided for adjustment of bias within a range of approximately –20 V to
– 50 V. Never set the operating bias on the output tubes for less than (more positive than) –20 V
dc. Component damage may result.
Potentiometer R55, connected across the 6.3 V secondary winding of transformer T51,
provides a hum balance adjustment for the amplifier. The wiper arm of the potentiometer is
connected to the junction of a resistive voltage divider across the output of the power supply. The
resulting positive voltage applied to the tube heaters minimizes heater-to-cathode leakage and
substantially reduces hum. Zener diodes are provided in this circuit and the bias supply to prevent
excessive voltages due to component failure.
Power thermistor VR51 is included in the primary ac circuit to limit the inrush current when
power is first applied to the amplifier. When at room temperature, the resistance of VR51 is about
10
Ω
but as it heats due to current through the device, the resistance drops to a fraction of an ohm.