FILAMENT DC SUPPLY

1. Know how much total current the circuit will draw

Add up all the various filament currents.
Add 50% factor to the required transformer current rating.
Use a transformer rated at the required current draw or higher.
Never use a transformer rated less current than required.

2. Voltage Drop

A solid state bridge rectifier will be used and a voltage drop can be expected across the rectifier, usually about .7 (seven tenths) of a volt. Since we are working with a low voltage, this voltage drop must be considered.

EXAMPLE
8 - 12AX7 amplifier stages @ .3 amp filament each 12AX7 = 2.4 amps
add 50% factor = 3.6 amps required transformer current rating
desired DC voltage = 6.3 volts DC
Transformer filament voltage = 6.3 volts AC
6.3VAC X 1.414 = 8.9VDC - .7V (loss across rectifier) = 8.2VDC
Need to drop 1.9 volts
Actual filament current draw = 2.4 amps
Value of resistor to drop 1.9 volts = 1.9 volts divided by 2.4 amps (ohms law) = .8 ohms
Watt rating of resistor = current squared times resistance = 2.4 X 2.4 = 5.76 X .8 = 4.6 watts
Double the wattage rating for safety and use a 10 watt resistor.

NOTE
Use a bridge rectifier rated at 100 volts and at least twice the required current
In the example above, at least 8 or 10 amps

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