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STEP 71
Test the 12AX7 preamp tube
Install the 12AX7 preamp tube .
After this tube is correctly installed,
plug the amp back in . The pilot lamp
should light up . Let the amp warm up
for a few minutes . If you notice smoke
or smells, unplug immediately .
Connect your multimeter’s negative
lead to ground . Because the power
cord’s ground lead is connected to
the metal chassis at the three-lug
terminal, any mounting bolt on the
chassis is a ground .
Set the multimeter to 5V DC and
check eyelet 26, which should read
around 1 .65V .
Set the multimeter to 200V DC and
check eyelets 22 and 24, each should
read around 165V .
If all of these voltages come within
approximately 10% of their expect-
ed values, unplug the amp before
moving on .
If the readings at eyelets 22 or 24
show no voltage or low voltage,
unplug the amp
to do a continuity
test . With your meter set for reading
continuity, make sure you’ve properly
installed the behind-the-board jump-
er from eyelet 18 to eyelet 14 .
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STEP 72
Test the 6V6 power tube
Install the 6V6 power tube .
Connect the speaker . Perform these
next tests with the speaker turned
away from you . If the amp starts to
oscillate and squeal, this will help
protect your ears .
Plug in the amp . After a few moments
you should hear a low hum . If the
hum becomes very loud, unplug the
amp immediately and review your
connections .
Allow the amp to warm up for a few
minutes and attach your multimeter’s
negative lead to ground .
Set your multimeter to read 5V DC
and test for voltage at eyelet 12 . This
voltage should read around 1 .65V .
Set your multimeter to read 20V DC
and test for voltage at eyelet 20 . This
voltage should read around 19V .
Set your multimeter to read 400V DC
and test for voltage at eyelet 17 . This
voltage should read around 325V .
If your voltage readings are correct,
plug a guitar in and begin playing at
low volume . If the amp is behaving as
it should, keep increasing the volume .
It should start to break up nicely as
you increase the volume .
If there are any strange oscillations,
squeals, or the amp seems at all unsta-
ble, use a wooden chopstick to probe
for loose connections:
n
from the input jacks to the
eyelet board
n
from the tube sockets to the
eyelet board
n
from the tube sockets to the
front panel controls .
It usually takes just a minor wiring ad-
justment, perhaps resoldering a loose
joint, to correct this sort of distortion .
Let it rip!
If the amp’s stable and your tests
match the voltages specified, unplug
the amp and install the back panel . It’s
time to rock!
Play with your guitar’s volume and
tone knobs to see how they interact
with this amp . A single-coil pickup set
to 10 creates gorgeous clean tones
with the amp volume set between
3 and 6 .
Between 6 and 8, you’ll find early tube
distortion, and there’s full-on tube
saturation from 9 to 12 .
Use the controls on your guitar to
shift between distorted and clean
when the amp’s turned up past 6 .
Humbuckers and P90s push you into
distortion a lot faster than standard
single-coils . Enjoy your tone quest!
Tube life
The life of the power tube is affected
by how hard you drive the amp . If you
are overdriving the amp for hours
every day, expect the power tube to
have a shorter life span .
We encourage you to experiment
with different tube brands and find
the brand that is most favorable to
your ears and your wallet .
StewMac is here to help
For more than fifty years, StewMac
has supplied instrument builders
and repair shops with high-quality
tools, parts, and expert advice . Our
customer service team really knows
how to help if you run into questions .
Call 800-848-2273 from 9AM–6PM
Eastern time, Monday–Friday .
Email: service@stewmac .com
Thanks for choosing this StewMac
kit, and welcome to the world of amp
building!
GO!
GO!
Always discharge the filter caps
before working on the circuit.
See how to use a
snuffer stick on page 6.
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