6.3
Valve Complement
6.3 V winding
3.90 A
5 V winding
1.90 A
Heater power
34.1 VA
Spec (+20%)
4.7 A
5 V: 2.3 A
Current Budget
6.3 V winding3.90 A
3×12AX7
2×EL34
5 V winding1.90 A
1×GZ34/5AR4
DC Heater Dropping Resistor
Series resistor
333 mΩ
Dissipation
5.07 W
use ≥ 15 W
Raw supply from 6.3 Vac
≈ 7.3 V
bridge, before sag
Engineering Notes
How it works
Every valve has a heater (or a directly-heated filament) that must be supplied with its rated voltage, ±10%. Most audio valves use 6.3 V; rectifiers and some power triodes use 5 V (and the 2A3 uses 2.5 V). The total current a winding must deliver is simply the sum:
I_total = Σ (n × I_heater) VA = Σ V × I
The 12AX7, 12AT7 and 12AU7 have centre-tapped heaters: wired in parallel they draw 300 mA at 6.3 V; in series, 150 mA at 12.6 V. Valve rectifiers need their own 5 V winding because their filament sits at full B+ potential.
Add 15–25% margin when specifying a transformer: a winding run at its limit gets hot and its voltage sags. For low-hum preamps, heaters can be run from DC: a bridge rectifier and capacitor on 6.3 Vac produce roughly 7–8 V, which a series resistor drops to 6.3 V:
R = (V_supply − 6.3) / I_total