4
TR7 and TR10 provide a constant current source for Z3 the 15 volt zener.
The output voltage is set by the ratio of R119 and R120 which provide the feedback to TR8
via TR11 and TR12 to stabilise the whole circuit. TR9 with R121, R122 and R123 form a
current limiter. This allows the supply voltage to ramp up at switch on when the heater
filaments of the preamp valves draw considerably more current while cold.
R117 and R118 halve the supply voltage to provide the opamp bias voltage, and as shown on
the circuit diagram the DC supply is routed through the filaments of two of the valves in series.
This will result in each filament having the nominal 6.3 volts across them.
Paul Stevens
30 June 1999
