StewMac 57 MINI TWEED, Assembly Instructions Manual

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STEP 70
Test the dangerous DC voltage
The dangerously high DC voltage that
passes through the 5Y3 rectifier tube
and the filter caps is referred to as “B+” .
The next step is to test this B+ voltage .
Plug the power cord back in . The pi-
lot lamp should light, along with the
filament inside the 5Y3 tube .
Again, spend a few minutes watching
for smoke or smells .
At this point, dangerous voltages
are forming in the filter caps. They’ll
hold a potentially lethal charge until
you discharge them, even after the
amp is unplugged .
For safety, only have one hand
on the amp during DC tests.
Keep your other hand behind your
back when you need to probe a com-
ponent . This way, 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 500V DC and
connect the black (negative) probe to
ground . Once that probe is secured
to ground, measure the DC voltage
at eyelet 16 . This B+ voltage should
be roughly 370V DC .
If this reading is correct and the amp
continues to behave normally, unplug
the amp .
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STEP 69
Test the standard AC voltage
Set your multimeter to 20V AC . Check
the heater voltage across pin 9 and
pins 4+5 on the 12AX7 socket . This
should read between 5-7V AC . If this
reading is drastically different, unplug
the amp and check your connections .
If the multimeter reading is good, and
the amp seems normal,
unplug it and
install the 5Y3 rectifier tube . Use one
hand to open the tension clip while
while you put in the tube with your
other hand .
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 mak-
ing 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 .
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STEP 68
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
volume knob to 3, which turns the
on-off 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 in-
dication 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, unplug the amp immedi-
ately and carefully review all your
connections .
12AX7
6
4
3 2
1
9
8
7
5
15 16 17
8μf
16μf
Gr
ound
Testing and troubleshooting
Seriously, keep one hand
behind your back!