VAC Vintage Mk II User Manual
Page 5
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
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 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." It is
simple for a qualified service technician to connect the output stage for triode operation.
Directions will be provided upon request.