VAC PA90 User Manual
Page 10
10
INSTALLING NEW OUTPUT TUBES
Replacement tubes are available from VAC and other sources. Output tubes may be KT77
beam power tubes or EL34/6CA7 power pentodes. It is not necessary that they be matched
pairs, although an improvement in measured performance may be achieved in this way. Make
certain that each tube fits firmly in its socket. A tube that fits loosely may not make correct
contact on all pins and might "run away" (read on). DO NOT MIX TYPE KT77 AND TYPE EL34
TUBES.
Refer to Figure 4 for the location and types of tubes.
ALL POWER MUST BE OFF. Remove the old tubes after they have cooled down (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,
taking care to observe the direction of the locating ridge on the plastic center pin of each tube.
Turn the BIAS controls to the midpoint of their rotation, with the screwdriver slot
approximately straight up and down (12 o'clock position).
Follow the normal turn on procedure and begin the BIAS procedure. While doing this, keep an
eye on the plate (the outermost metal structure) of the output tubes. SWITCH OFF
IMMEDIATELY IF THEY BEGIN TO GLOW RED. This indicates that the tube is "running away",
being destroyed in a matter of seconds by conducting excessive current. (Note: with some
KT77s a slight dull orange glow may occur over a very small section of the plate, usually at an
edge. This is acceptable and not the same as running away, in which most of the plate will
become bright orange or red.)
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 and should be returned to its seller.
3)
The tube is defective.
4)
The bias pot has been turned too far COUNTER-CLOCKWISE.
5)
The power connectors between the amplifier and the power supply connector may not
be fully inserted and locked.
6)
There is a problem with either the amplifier or the power supply. Contact VAC or your
dealer to arrange service.
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.
Follow the BIAS procedure described previously in this manual to set the idle current at 54
milliamperes per tube, checking the idle current at 1 minute, 30 minutes, 1 hour, and 2 hours.
For further information, refer to Tips & Advice: A Word About Tubes in General and Tips &
Advice: A Word About Output Tubes.