Step 6
– There should be about 6.6 VAC as measured at these two turrets (photo 8.1a)
Step 7
– After 5 min. of trouble-free operation, remove the AC power by disconnecting the AC
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
– Plug the amplifier's AC power cord into
the AC power source at the wall.
Step 3
– 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 4
– With your multimeter on the 500 volt DC range, connect the meter's black lead to
chassis ground (photo 8.2a) and carefully touch the meter's red lead to the “B” terminal of the
can capacitor (photo 8.2b). The voltage here should be 413 Vdc.
Step 5
– 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 panel indicator should illuminate. Monitor
for any unusual smoke or smells or blown fuses. If
anything unusual occurs, disconnect power
immediately and review connections.
TubeDepot.com 47
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.
WARNING
The presence of voltages at steps 4, 5 & 6 indicates that the preamp tubes are correctly
sourcing current (they are working). The tolerances for these voltages are +/- 10%.
NOTE
The previous voltages were measured without power and preamp tubes installed. Those
voltage values will decrease with the added load of the tubes.
NOTE
photo 8.2a
A
B
Photo 8.2b
photo 8.3b
8.2
8.3
Photo 8.3a
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.
