D.W. Fearn VT-3 - Manual (Page 20)

VT-I/F Vacuum Tube Instrument Interface
D.W. FEARN
22
The GND LIFT switch disconnects pin 1 of the OUTPUT connector from the
internal ground when the switch is in the up position.
Power Supplies
Primary power from the AC mains is connected to the VT-3 through a standard
IEC power input connector. The Power switch energizes all power supplies. A
fuse, accessible on the rear panel, protects the VT-3. and a recessed slide switch
provides quick switching between 120VAC and 220-240VAC operation.
The Pilot lamp is a type 1819 bulb, operated far below its rated voltage of 28.
The life of the bulb is lengthened, and the light output is more compatible with
other modern studio equipment.
The power transformer is custom made for the VT-3 and has primary taps for
100, 120, or 220 volt operation.
Filament supply
The power transformer output is rectified by a bridge rectifier and filtered
before being regulated to 12.0 volts by a three-terminal regulator. The negative
output of this supply is grounded. The tube filaments are rated for 6.3 volts; the are
operated in series-parallel at 12.0 volts, which reduces heat dissipation in the fila-
ment regulator.
B+ supply
Two separate regulated voltages are required for the plates of the VT-3. The B+
is filtered with long-life, low-leakage computer-grade filter capacitors before
being regulated and extensively bypassed and decoupled. The negative side of the
supply is grounded.