What to do if the transformer delivers a higher heater voltage as required? Use a resistor to reduce the voltage you need. In fact you create a voltage divider... The dropping resistor is the first and the Tube - Heater the second resistor of this divider.

Calculate Dropping Resistor

New Tube filament voltage :    Volts *   New Tube filament current:    Ampere *   Transformer voltage:    Volts *     * Required values       Dropping Resistor:  Ohms   Voltage dropped by Resistor:  Volts   Power dissipated by resistor:  Watt     Recommended Resistor power rating:  Watt

Heater Values of some Power Tubes

Tube	Vh (Volts)	Ih (Ampere)
EL34 / 6CA7	6.3	1.5
EL84 / 6BQ5	6.3	0.76
KT66	6.3	1.27
KT77	6.3	1.4
KT88 / 6550	6.3	1.6
211	10.0	3.25
2A3	2.5	2.5
300B	5.0	1.2
5998	6.3	2.4
6AS7	6.3	2.5
6HB5	6.3	1.5
6L6	6.3	0.9
6V6	6.3	0.45
7027	6.3	0.9
7189	6.3	0.76
7558	6.3	0.8
7591	6.3	0.8
801	7.5	1.25
805	10.0	3.25
807	6.3	0.9
811A	6.3	4.0
845	10.0	3.25

Heater / Filament Supplies extern Link

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