In figure A there is a line plug from which two wires connect to the input of a variable transformer (Variac). The output side of the Variac is taken from the bottom input connection and the rotating contact. These two wires connect to the primary of a 6.3 volt filament transformer. The secondary connects to pins 2 and 7 of the 6AX5. A voltmeter is connected in parallel with the tube heater. In Figure B there is a large rectangle labeled 'Tube Power Supply'. Two wires come from the power supply which are labeled 6.3 V A C. A pot has one side connected to the top wire. The wiper of the pot is connected to its other side. The other side of the pot also connects to pin 2 of the 6AX5. The lower wire from the power supply connects to pin 7 of the 6AX5. The pot is labeled 10 ohm 5 watt. Pins 2 and 7 are the heater connections. There is a voltmeter connected across the tube heater pins 2 and 7.
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