VAC Phi 300 - Manual (Page 12)

10
INSTALLING NEW OUTPUT TUBES
Output tubes are type KT88. Replacement 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, determined by the plastic “keyway” in the center of the base. Do not use excessive
force. Replace the top glass cover before operating the amplifier.
Adust the bias control fully counterclockwise.
Switch on the amplifier and follow the bias procedure outlined later in this manual. Bias must be
checked and adjusted when the amplifier first turns on, at 60 seconds, 90 seconds, two minutes,
and five minutes. In the earliest checks, set the bias so that the meter read a bit to the left of center.
Final setting should be made after 15-30 minutes. Recheck new tubes daily for the first week.
Whenever a tube is changed, the bias settings must be checked and adjusted.
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.
For further information, refer to Tips & Advice: Tubes in General and Tips & Advice: Output Tubes.