VAC PA35/35 - Manual (Page 4)

Tube Types
The input/splitter (frontmost) tube may be type 12AU7A/ECC82/E82CC or type 12BH7 (typically richer
sound). The driver tube (next toward the back) is type 12AT7A. The output tubes are type EL34, pentode
version only. Some 6CA7/EL34 have been made as "beam power" tubes, and will damage the PA35/35, as
will any output tube that internally ties pins 1 and 8. 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 are EL34 pentodes only. Some
6CA7/EL34 have been made as "beam power" tubes, and will damage the PA35/35, as will any output tube
that internally ties pins 1 and 8. 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.
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.