Vectronics VEC-1680K - Manual (Page 22)

VEC-1680K Owner’s Manual Vacuum Tube Preamp
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coupled into gain control R9, which establishes the drive level for the following
stage. The second stage is configured for high gain, but its plate supply is set
intentionally "soft". This allows the stage to be driven into saturation easily in
order to create a variety of special effects and tonal colorings. The output level
of the second stage, which is many orders of magnitude stronger that "instrument
level", is attenuated back down to a useful drive level through a combination
fixed and variable attenuator network (R4, R5, and R10).
Foot-switch driven relay RLY1 controls signal routing by either connecting the
preamplifier circuitry on line, or bypassing it with a direct connection between
the Input and Output jacks.
The preamp is isolated from the AC line by a 12 volt AC adapter. This feeds
V1's filament circuit directly, and also energizes the preamp's two built-in DC
supplies. A simple low-voltage supply provides smoothed DC for the foot-
switch relay circuit (D5/C8). A second high-voltage supply develops plate
voltage for V1 by transforming 12-VAC back to AC line level through T1.
Diode bridge D1-D4 converts this energy to DC, where it is smoothed through
filter network C1, C2, C6 and R1, R11.