5) Adjust the GAIN TRIM potentiometer until the VU reads “0dB”
(All UnFairchilds have “0dB” on the
meter calibrated to a +4dB output level).
IMPORTANT! There is another aspect to the level gain trim adjustments; the gain trim control
adjusts the bias current of the 6386 tubes. It is a great way to adjust the level, but it will affect the
sonic character of the compression. This offers another opportunity for sonic flexibility!
Higher bias current settings will have a clearer, more focused sound to the compression. Lower
bias current settings will have a softer, thicker, more saturated sound. If you want a clearer, more
focused sound, set the INPUT GAIN to 14 or 15. This will force you to set the GAIN TRIM much high-
er to achieve unity gain with a +4dB output level. This can be better for using the UnFairchild on
your mix buss or for mastering. If you want the softer, thicker more saturated sound, set the INPUT
GAIN to 9 or 10. This will force you to set the GAIN TRIM much lower to achieve unity gain with a
+4dB output level. This can add some beautiful coloration when using the UnFairchild for tracking
or mixing individual instruments.
The higher bias current calibration settings can shorten the life span of the 6386 tubes. If you want
to avoid the possibility of shortening the life of the tubes, we advise you not to calibrate to an IN-
PUT GAIN setting lower than 12.
10
MATCHING CHANNEL COMPRESSION
The compression sensitivity on each channel is mostly determined by the ECC83S (12AX7) in the
side chain circuit. The influence of these tubes on the amount of compression can vary wildly. We
choose pairs of tubes that are already at least in the ballpark and then use the side chain calibra-
tion trim pots to fine-tune them. The trim pots are accessible via small holes on the rear panel (
see
figure 3
).
Channel A
Channel B
(figure 3)
