Vectronics VEC-1680K - Manual (Page 21)

VEC-1680K Owner’s Manual Vacuum Tube Preamp
21
Does not turn on: Is the power adapter plugged in? Does it have 12 VAC
output? Is the tube firmly seated in its socket?
Hum on Instrument: Are patch cables in good condition (no open grounds)?
Try flipping the polarity switch on the power amp.
Distorted Audio at all settings: Check condition of the tube. Is there a bluish
glow or does the tube make a strong ringing sound when tapped with a pencil
eraser (is it microphonic)? Tubes deteriorate with age and require periodic
replacement.
If you have the necessary tools and technical skills to troubleshoot and repair
high-voltage equipment, voltage analysis is a good way to localize trouble. Here
are the normal voltage readings for the 12AX7. Any variation over 10% from
these values may indicate a problem area in the preamp's circuitry.
Important Warning: Potentially lethal voltages are present in your preamplifier.
Never attempt to take "live" voltage readings on tube-type equipment unless you
are specially trained and qualified to do so!
12AX7 Voltage Chart
Pin-1 80 V
Pin-2 0.1 V
Pin-3 1.4 V Pin-6 90 V
Pin-7 .1 V
Pin-8 1.0 V
Pin-4 to Pin-5 13-VAC
Pin-4 to Pin-9 6.5-VAC
V1a V1b
If these checks fail to uncover the problem, repeat the "QC" check one more
time. Service records show that, for most malfunctioning kits, outright
component failure is relatively rare. In most cases, the culprit is a misplaced
part, reverse-polarized capacitor, or a faulty solder connection. If, despite your
best effort, you cannot solve the problem, kit repair services are available
through Vectronics. See the warranty on the inside front cover for complete
instructions.
THEORY OF OPERATION
The VEC-1680K preamp uses a 12AX7 high-mu dual triode at V1, a tube
designed especially for low-level audio preamplifier applications. The first stage
is optimized for linearity, with the grid-leak bias circuit (R6) chosen to present
minimal loading to magnetic instrument pickups. Output from this stage is AC-