VAC Vintage High Power - Manual (Page 4)

Tube Types
The input/splitter tube V1 and the driver tube V2 may be type 12AU7A/ECC82/E82CC or type 12BH7. The output
tubes V3, V4, V5, and V6 may be type KT88, KT88 Super, or type 6550A. Golden Dragon tubes are generally
recommended and have been used extensively during the development of this amplifier. Please feel free to contact VAC
if you require additional information or advice.
Installing New Output Tubes
Replacement tubes are available from VAC and other sources. Output tubes may be KT88 Beam Power Kinkless
Tetrodes or 6550A Beam Power Tetrodes. 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 KT88 AND
TYPE 6550A TUBES. DO NOT 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 correct idle current for each tube. Recheck
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.