VAC PA160 Mk I - Manual (Page 11)

11
Installing New Output Tubes
Replacement tubes are available from VAC and other sources. It is not necessary that they be matched pairs,
although a slight 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). It is best not to mix brands 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). Handle tubes by their bases, not the glass. 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.
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 rapidly by conducting
excessive current. (Note: with some KT88s 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. (Note: British KT77 normally have pins with slightly smaller
diameter, so pay close attention to fit.)
3) The tube is defective.
4) The bias is misadjusted.
5) There is a problem with the amplifier. Contact VAC or your dealer to arrange service.
In the event that trouble is encountered, 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 70 milliamperes
per tube, checking the idle current at 1 minute, 30 minutes, 1 hour, and 2 hours.
Replacement of Low Level Tubes
All power must be switched off. Allow tubes to cool down. Remove and replace with new tubes of the
appropriate types, noting the location of holes in the socket and pins of the tubes. Handle tubes by the bases,
not the glass.
Replacement tubes are available from VAC and other sources.