Wingfoot 813 Bias Supply Circuit Description and Schematic Diagram

The AA8V Wingfoot 813 Amplifier
High Technology Of The 1950s In The 2000s
by Greg Latta, AA8V

Bias Supply Circuit Description and Schematic Diagram

Power Supply Circuit Descriptions
Main Power Supply Schematic Page
Plate and Plate/Screen Supply	Bias Supply
Transformer T1 Primary Wiring	Inrush Delay Circuit
Power Connector Wiring

Bias Supply
Circuit Descriptions and Sub-Schematics
Transformer	Surge Resistors
Rectifier Diodes	Filter Capacitor
VR Dropping Resistor	Bleeder Resistor
Voltage Regulator Tubes	VR Base Diagram

Click On A Section of the Schematic Below for Information on That Part of the Circuit:

Bias Power Supply Schematic

Click here for a high resolution schematic.

Click here for a rotated schematic more suitable for printing.

Circuit Design Considerations:
The bias supply provides operating and cutoff bias for the 813 tube. Little current (less than 30 mA at -150 volts DC) is required, but the bias should be regulated for best linearity. Two voltage regulator tubes are used, giving the operator a choice of maximum bias. One tube is an 0B2 and the other is an 0A2. Maximum bias is determined by the tube mounted at V2. If the 0B2 is mounted at V2, the maximum bias is 108 volts. If the 0A2 is mounted at V2, the maximum bias is 150 volts. Currently, the 0B2 is mounted at V2.

Click here for pictures and information on the matching Wingfoot 813 Amplifier

Transformer T3:
The bias supply is powered by a 290 V secondary on transformer T3. T3 also has a 6.3 V secondary which runs the pilot lamp and a higher voltage, centered tapped secondary which powers the plate/screen supply.

Surge Resistors:
Early solid state rectifiers did not have high surge ratings and could be damaged by the turn-on surge that occurs as the filter capacitor first charges up. To limit the surge current to a safe value, extra 10 ohm and 47 ohm resistors were placed in series with the rectifier diodes.

In reality, these resistors are not really needed, since the transformer secondary provides enough resistance to limit the turn on surge to a safe value. Since they were already in the circuit, and they do no harm, they were left in place.

Rectifier Diodes:
To obtain a rectifier with a big enough peak inverse voltage (PIV) rating two diodes are used in series. To keep the reverse voltage evenly divided between the diodes, a 150k ohm equalizing resistors is placed across each diode. The diodes used in the bias supply are the original "top hat" style diodes.

Filter Capacitor:
The output from the rectifier is a stream of pulses that need to be smoothed out. The filter capacitor smooths out the pulses so that a steady DC voltage is obtained.

Another way to look at it is that the rectifier output is a DC waveform with an AC waveform superimposed. The capacitor short circuits the AC component to ground, leaving only the steady DC component.

Bleeder Resistor:
The filter capacitor can store a charge for a very long time, even after the supply has been shut off and unplugged. To drain off the charge and prevent possible shock, a 47k ohm "bleeder" resistor is connected across the capacitor.

Voltage Regulator Tubes:
Voltage regulator tubes have the property that the voltage across the tube is constant provided the current is within a relatively wide range. For the OB2 and 0A2, the current must be between about 5 mA and 35 mA.

Tubes may be placed in series to give a higher voltage rating as is done here. The OB2 operates at 108 V and the 0A2 at 150 V. Thus, the two in series operate at 258 V.

In the 813 amplifier it was eventually found that very little bias was needed, so the output is actually taken from across V2. In this case, V1 is really just used as a fancy dropping resistor, and could be replaced with a resistor. However, since the tube and socket were already in place, things were left as is.

Maximum bias is determined by the tube plugged into V2. It was eventually determined that 108 V was more than enough to cut off the 813, so an 0B2 is used at V2, and an 0A2 at V1.

VR Dropping Resistor:
To keep the current through the VR tubes at the proper value, a dropping resistor is placed in series with them. For intermittent loads, the resistor is selected to allow the maximum current of 35 mA to flow through the tubes when they are unloaded. A larger value can also be used, provided the tubes never extinguish when they are loaded.

VR Base Diagram:
The base diagram for the voltage regulator tubes is shown at right. There are multiple connections to the cathode and plate, which allow the tube itself to perform in some sense as a switch. For instance, it is possible to make connections such that the output of the supply is disconnected if one of the VR tubes is removed from its socket.

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