PowerTube™ Preamp
True to the Music
®
Radial Engineering Ltd.
6
THE HIDDEN SWITCHES
There are two mini slider switches located above and below the 15-pin card edge connector
plate. The lower one is used to balance the power supply current draw for the 12AX7 heater
when multiple PowerTubes are being used. The upper one is used to lift the ground at the
input. These ‘set and forget’ switches are only used in special situations.
POWER ASSIGN
SWITCH
INPUT
GROUND LIFT
g
y
POWER ASSIGN
SWITCH
INPUT
GROUND LIFT
Power assign switch
This switch is used to select the (+) or (-) from the 16V power rails inside a Workhorse or API
Lunchbox equivalent racks when multiple PowerTubes are used together. The intent is to
balance the power draw on the supply to reduce opportunity for brown outs which of course
would limit the PowerTube performance. The Power Assign switch comes factory set in the
up (- 16V) position. The switch position is not critical when only one PowerTube is being
used. When using multiple PowerTubes, simply toggle the switches with one down (-16V)
and the next up (+16V) so that they even out the power draw.
Ground lift switch
In many ways, a ground lift switch is somewhat superfl uous on the input of a microphone
preamp as lifting the ground will disconnect the 48V phantom. This of course means that if
you lift the ground, phantom power will no longer be able to feed the microphone or power
an active direct box. Make sure that the ground lift switch is set in the up position before
you insert the module into your rack. So why have it? Tubes can make just about anything
sound warmer. When connecting a keyboard or pre-recorded track to the PowerTube to add
excitement and character leave the ground lift in the up position, unless you encounter noise
caused by a ground loop.
AC POWER GROUND
GROUND LOOP PATH
AUDIO CABLE GROUND
LIFT
AC POWER GROUND
AUDIO CABLE GROUND
