VAC PA100/100 - Manual (Page 4)

Installing New Output Tubes
Replacement tubes are available from VAC and other sources. Output tubes are KT88 beam power Kinkless
Tetrodes (6550 may also be used - see Tips & Advice, Output Tubes). 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). 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.
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.
Replacement tubes are available from VAC and other sources.
Ultra-linear or Triode?
The input amplifier, phase splitter, and driver stages are triode tubes operated in Class A with no cut-off or grid
current. The output stage is connected for partial-triode, well known for low distortion, good damping factor,
and high efficiency. This technique was invented by Alan Blumlein of EMI in England, and was popularized in
the US as "ultra-linear." VAC can convert amplifiers to triode connection for a small charge.