User`s manual

Adjust VR3 for minimum indication on the VU meter. Sometimes this will not be a full
dip. Only an extremely matched 6BC8 will dip fully.
Set Balance Test Switch to Up (Static Test). Adjust VR2 for minimum indication on the
VU meter. The meter should dip to −20 dB at the null. Continue to the next step to
balance the amplifier stage.
5.4 Balancing the Amplifier Stage
Only perform this adjustment after balancing the 6BC8 stage. Continue by setting the
Interstage Switch to OUT. Adjust VR4 for minimum indication on the VU meter. The
meter should indicate −10 dB or less. If it does not, suspect the 12AX7 and 12BH7. An
unbalanced amplifier can cause thumping on attack in the Interstage OUT operating
mode.
Upon completion of balancing, BE CERTAIN TO PLACE THE TEST SWITCH BACK
IN THE CENTER POSITION or you will hear hum modulating the audio signal.
5.5 Noise and Distortion Tests
To check the noise of the 176, send a 1 KHz tone into the 176 and adjust for 3 dB of
compression. Compression Ratio should be set to 12:1. Adjust the Output Level for a 0
dB reference on your analyzer. Remove the input signal and decrease the Input Level to
0. Read the residual noise on the analyzer. It should be less than –76 dB. If it is not,
remove the 6BC8 and see if the noise remains. If the noise vanishes, suspect the 6BC8
tube or 0B2 tube. If the noise remains, suspect the 12AX7 or 12BH7.
Distortion can be measured by feeding +4 dBm signal into the 176. Adjust the Input
Level for the desired amount of gain reduction, then measure the distortion at the output.
Higher amounts of gain reduction increase the distortion slightly. Faster Attack and
Release times cause increased distortion at low frequencies.
5.6 Frequency Response Tests
Frequency response tests should be performed below the threshold of gain reduction to
keep the compression from adjusting the amplifier gain. Frequency response should be
flat within +/ − 0.5 dB from 20-20,000 Hz at all settings.
5.7 Hum and Microphonics
Hum in the 176 is generally due to tube degradation or failure. Many tubes don’t make it
out of the gate, so remember that when replacing them. Tubes used in the 176
amplification are dual-section. If they are not matched well or one side fails before the
other, the 60 Hertz AC filament voltage can induce hum into the amplifier.
Noise tests will show that the 176 has effects of microphonics like all tube equipment.
Microphonics are caused by vibration in the tubes. Sometimes tubes can whistle, ring,