VAC Phi 70 - Manual (Page 11)

9
INSTALLING NEW OUTPUT TUBES
Output tubes are type 300B. Replacement 300B output tubes should be purchased from VAC. It
is important that the tubes be checked for any tendency to mechanical or electrical shorts (see
Safety Notice at the front of this manual). It is desirable that tubes be in matched quartets for each
channel, and be close to the "bogey" values for the major parameters. Make certain that each tube
fits firmly in its socket.
ALL POWER MUST BE OFF. Remove the top glass and the old tubes after they have cooled down
(THE COVER BECOMES QUITE HOT IN NORMAL USE; TUBES BECOME HOT ENOUGH TO CAUSE
SERIOUS BURNS WHEN IN OPERATION AND MAY TAKE SEVERAL MINUTES TO COOL DOWN). Install
the new tubes firmly and fully in the sockets, observing that the tube will only fit into the socket in
one orientation, as two of the pins are of larger diameter. Do not use excessive force. Replace the
top glass cover before operating the amplifier.
A slight violet glow in the tube is not cause for concern. If at any time the plate (the outermost metal
structure) of the output tubes begins to glow bright orange or red SWITCH OFF IMMEDIATELY. The
red glow indicates that the tube is "running away", being destroyed by conducting excessive
current. Tubes may run away for several reasons:
1) The tube is not fully inserted in the socket.
2) The tube fits loosely in the socket and thus can not make correct contact. Such a tube is
unusable.
3) The tube is defective.
In the event that trouble is encountered check connections and/or try another tube. Stop if the
problem persists and consult with your dealer or VAC.
Switch off immediately if any part visible through vent slots at the back of the chassis glows orange.
See Safety Notice at the front of this manual.
For further information, refer to Tips & Advice: Tubes in General and Tips & Advice: Output Tubes.