WhitakerAudio 60 W Stereo Power Amplifier, User And Assembly Manual

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60 W Stereo Power Amplifier
between pin 7 of V2 on the right channel amplifier and ground. Vary the Hum balance
control and confirm similar, but reversed, functionality; i.e., when pin 2 is at maximum
voltage, pin 7 is at minimum voltage, referenced to ground.
22) Using the procedure detailed in the previous step, set the Hum Balance control so that the
voltages between ground and pins 2 and 7 are approximately equal (~3 V ac). Check the
voltages between ground and pins 2 and 7 of the other output tubes. They should be the same
(~3 V ac).
23) Check the rectifier tube filament voltage. Using an ac voltmeter, measure the voltage
between pins 2 and 8 of V51. A value of approximately 5 V ac should be measured.
24) Using an ac voltmeter, measure the voltage between ground and pin 2 of V51. A value of
approximately 5 V ac should be measured.
25) Using an ac voltmeter, measure the voltage between ground and pin 8 of V51. A value of
approximately zero volts should be measured.
26) Check the plate voltage. Using an ac voltmeter, measure the voltage between ground and pin
4 of V51. A value of approximately 400 V ac should be measured. Next, measure the voltage
between ground and pin 6 of V51. A value of approximately 400 V ac should be measured.
Note: do not measure the plate voltage between pins 4 and 6 as the potential difference (800 V
ac) exceeds the safety rating of many common voltmeter test leads.
27) Switch off the amplifier and remove the socket from the ac power source.
28) Install V51, the rectifier tube (5U4).
29) Reconnect the socket to the power source. Switch the power on and observe the rectifier tube
for proper operation. The filaments should be lit.
30) Using a dc voltmeter, measure the voltage between ground and pin 3 on each of the 7027A
output tubes. A value of approxi540 V dc should be measured after the rectifier tube
has warmed up (about 30 seconds).
Note that under this test condition the power supply will be operating at its maximum
potential. As a capacitor-input filter power supply, under no-load conditions the voltage across
the input capacitors will rise to the peak value of the ac waveform. With the 400 V ac voltage
between the plates of the rectifier tube and ground, the potential at the positive terminal of C51
will rise to about 560 V dc. The actual value is somewhat less because resistors R56, R57 and R58
load the supply somewhat. The values for R56, R57 and R58 were chosen assuming normal