The bias power supply provides the negative bias for the grid block keying circuits and the modulator. Very little current is required, but the output of the supply must be very clean and hum free. The supply uses two taps ("Y") on the power transformer in a classic full wave center tapped configuration.
Because the supply is positive ground, the rectifier tube cannot have a common cathode. The 6AL5 7-pin dual diode has two totally independent diodes and is perfect for this low voltage/low current application. The supply uses a capacitor input filter and the output is fed to the keying circuit and, after passing through a voltage divider, to the modulator circuit.
Click On A Section of the Schematic
Below for Information on That Part of the Circuit:
|6AL5 Rectifier Tube|
|Power Transformer T1 Secondary|
|Voltage Dropping Resistor|
|Modulator Voltage Divider|
6AL5 Rectifier Tube:
The bias supply has a positive ground, and this means that the rectifier tube cannot have a common cathode, A rectifier with separate cathodes must be used. However, the output current and voltage of the bias supply are relatively low.
The 6AL5 is a small 7-pin tube with two completely independent diodes and is ideally suited for this application. You can click here for a 6AL5 data sheet.
Power Transformer T1 Secondary:
Power transformer T1 powers all of the power supplies in the Ranger. Three pairs of secondary taps for the bias, low B+, and high B+ supplies are provided, along with a secondary center tap.
Voltage Dropping Resistor:
The voltage needed by the transmitter circuits is less than the output of the bias supply, and resistor R54 is used to drop the voltage down to the correct value.
Under key up conditions, less current is drawn from the bias supply, so the output to the keying circuits and the modulator increases during key up. This puts more bias on the modulator tube grids, lowering the idle current in the modulator tubes. When the key is closed, more current is drawn, and the bias drops to the correct value.
The output of the rectifier is filtered by capacitors C90A and C90B, which are in parallel and effectively form a 30uf, 150 volt capacitor. The capacitors smooth out the variations in the output of the rectifier tube to produce pure DC.
Modulator Voltage Divider:
The voltage needed on the modulator grids is half of that needed by the keying circuits, so a voltage divider consisting of resistors R52 and R53 is used to cut the voltage in half.
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