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STEP 161
Test the dangerous DC voltage
The dangerously high DC voltage that
passes through the rectifier tube and
the filter caps is referred to as “B+” .
The next step is to test this B+ voltage .
With the amp still unplugged and
turned off, install the 5AR4 rectifier
tube while spreading the tension clip .
Plug the power cord back in and turn
the power switch to ON . The pilot
lamp should light, along with the
filament inside the 5AR4 rectifier tube .
Again, spend a few minutes watching
for smoke or smells before flipping
the standby switch to ON .
For safety, use only one hand to
probe the amp during DC tests.
Keep your other hand behind your
back when probing a component,
so you can’t be a path between B+
voltage and ground—a mistake that
would send a dangerous charge
through your heart .
Set your multimeter to 600V DC and
connect the negative lead to ground .
Once the negative lead is secured to
ground, flip the standby switch to ON .
Measure the DC voltage at eyelet 46
for your B+ voltage . This should be
roughly 490V DC .
Turn the power and standby switches
to OFF and unplug the amp .
Any
multimeter
will work fine for
the two types of tests we’re about to
do: checking
continuity
and reading
voltages
. The instructions that came
with your meter will be helpful .
Continuity testing
is simply making
sure current flows between two
points successfully . Touch the meter's
red lead to one end of the section
being tested, and the black lead to the
other end . If the continuity is good,
your meter will beep or register this
on the display .
Voltage testing
is where you need
to be careful . Some steps require the
amp to be plugged in and turned
on . This becomes dangerous if you’re
not cautious . Respect the voltages
and follow the directions, and these
tests are safe and easy .
STEP 159
Perform a safe power-up
At this point, there should be no tubes
installed, and the speaker should be
disconnected .
Before plugging the amp in, turn the
power switch to ON . Switching the
amp on before the first power-up
protects you from shock if a mistake
in your wiring has created a short
to the chassis . If this short exists, an
indication would be that the pilot
light will not turn on, since the AC
current is going directly to ground .
Plug the power cord in . The pilot lamp
should light .
For a few minutes, watch for smoke
or unusual smells . If anything seems
unusual, disconnect the power
immediately and carefully review all
your connections .
STEP 160
Test the standard AC voltage
Set your multimeter to 20V AC . Check
the heater voltage across pin 9 and
pins 4+5 on the V1 12AX7 socket .
This should read between 5-7V AC .
If this reading is drastically different,
disconnect power and check your
connections .
If the amp seems normal, unplug
the power cord and turn the power
switch OFF .
Testing and troubleshooting
V1
6
4
3 2
1
9
8
7
5
D
F
4
3
8
7
6
5
4
3
8
7
6
5
38
28
1
2
3
76
29
M
46
64
65
75
104J 600V
ALWAYS discharg
e the filter cap
before working on th
e circuit. See how
to use a snuffer stick o
n page 6.
And seriously, keep o
ne hand behind
your back!
