Tube Vari-Mu compressor. Early to Mid 1960’s
The UA 175B pre-dates the 1176. The 175B includes UTC input and output transformers, Langevin attenuators. The tube array is as follows: 6BC8 (for gain control), 12AX7, 12BH7, (output gain) 6AL5, (sidechain) OB2, GZ34 (power).
Like the Fairchild compressor line, the 175 and 176 were based around a variable-mu tube gain reduction design. This means that gain reduction is controlled by highly unique variable-mu dual triode tube, which is no longer manufactured. Variable-mu tubes have the ability to reduce their mu (gain), as the input signal increases.
The 175 has a fixed ratio of 12:1—but similar to 1176, the 176 offers similar selectable ratios of 2:1, 4:1, 8:1 and 12:1; The multiple ratios are achieved by a multi-tapped transformer. Also similar to the 1176, Attack and Release controls are at their fastest when fully clockwise. The attack time is adjustable from 100 to 1000 microseconds, while release is adjustable from 27 to 527 milliseconds. Also, the compression circuit can be clicked off with a firm counter-clockwise twist, just like the 1176.
The meter is switchable to view Input, Output, or Gain Reduction. The Input and Output controls are both stepped at 2dB increments, and “Vernier” controls are provided for both Input and Output allowing fine-tune adjustment of the gain structure between the 2dB increments.
The front panel also has archaic “Double Tip” Input and Output jacks. The cables these inputs and outputs utilized actually shared two 1/4” jacks at the end of each cable.