power cord from the AC source. Leave the amp's power and standby switches in the “on”
position.
High Voltage (B+) Testing
Step 1
– With the amp still disconnected from AC power, install the tube rectifier.
Step 2
– Select the “tube rectifier” position with the rectifier switch (if installed).
Step 3
– Plug the amplifier's AC power cord into the AC power source at the wall.
Step 4
– The panel indicator should illuminate. Visually verify that the filament inside the
rectifier tube is glowing. Monitor the amplifier for any unusual smoke or smells or blown fuse.
If anything unusual occurs, disconnect power immediately and review connections.
Step 5
– With your multimeter on the
500 volt
DC
range, connect the meter's black lead to
chassis ground. Placing the meter lead in one of the captured nuts is fine. Touch the meter's
red lead to one of the positive terminals of the can cap mounted to the side of the amp,
closest to the power switch. The voltage here should be 492 V dc (+/- 10V).
Step 6
– Remove AC power by disconnecting the AC power cord from the AC source.
Preamp Tubes, Installation and Testing
Step 1
– With the amp still disconnected from AC power, install all three 12AX7 preamplifier
tubes (no power tubes just yet).
Step 2
– Plug the amplifier's AC power cord into the AC power source at the wall.
Step 3
- The power light should illuminate. Monitor for any unusual smoke or smells or blown
fuses. If anything unusual occurs, disconnect power immediately and review connections.
Step 4
– Let the amplifier warm up for 2 minutes. With the multimeter on the
20 volt range
,
connect the meter's black lead to a chassis ground as before and touch the red lead to turret
T6. The voltage here should be close to +1.7 Vdc (+/- .5V).
Step 5
– Touch the red meter lead to turret T4. The voltage here should be +.9 V (+/- .5V).
Step 6
– Touch the red meter lead to turret T18. The voltage here should be +1.0V (+/- .5V).
Step 7
– Adjust the meter range to the
200V range
. Touch the red meter lead to T20. The
voltage here should read 157V (+/- 25V).
Step 8
– Touch the red meter lead to turret T36. The voltage here should be +35 V (+/- 10V).
TubeDepot.com 31
Within a minute, the rectifier will have heated up and provided a slowly increasing high
voltage to the power supply. This slow voltage will properly form the high voltage filter caps.
NOTE
Whenever testing voltages, it is recommended to keep your free hand off of the chassis. In
this way, there isn't a path for significant current to flow through the body to ground in case
the measuring hand accidentally comes in contact with the high voltages.
The presence of voltages at the following steps 4 - 8 indicates that the preamp tubes are
correctly sourcing current (it's working). The tolerances for these voltages are +/- 10%.
NOTE
The previous voltages were measured without power and preamp tubes installed. Those
previous voltage values will all decrease with the added load of the tubes.
NOTE
8.2
8.3
CAUTION
Elevate the amp off of the work bench with a thick book(s) under the transformers so that
the rectifier and power tubes are not damaged during the following tests.
WARNING
Содержание JTM45+
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