Balanced Audio REX II Preamplifier - Manual (Page 7)

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They incorporate two types of vacuum tubes, user selectable for the desired sound: 6C45 and 6H30.
To change the AC Shunt tube type remove the Power Module top cover and move the board-mounted
switch S1 to desired position. The switch is located close to the center of the board.
Only the selected tubes will have their filaments running at any time.
INTRODUCTION
Thank you for your purchase of the Balanced Audio Technology REX II
preamplifier. Please read this owner's manual to obtain the full benefit of the REX II
in your system. It will provide you with the needed safety information and
operating procedures for this exceptional unit.