The Retro 176 is based on a Bill Putnam classic, the UA 176.
The 176 design shares many functional details with the more commonly used 1176LN. Similarly to the 1176LN, it has a fixed threshold and input/output potentiometers to control compression and output level. This is a very intuitive design, which has proven its value over the course of more than half a century. The 176 offers four ratio settings (2:1, 4:1, 8:1, 12:1), and thus is capable of fairly mild compression as well as heavier limiting. Attack and release are fully adjustable via front-panel potentiometers, and, just as with those on the 1176LN, their fastest settings are in their fully clockwise position.
The large backlit VU meter, made by the American manufacturer Hoyt, can display gain reduction as well as input and output levels. The unit has a power switch (accompanied by a red status light), and also, just as you'd expect on a modern high-end unit, a switchable hard-wired bypass.
The Retro 176 uses six vacuum tubes, a 6BC8 dual-triode tube provides the variable gain stage, the actual compression element. A negative bias (the detector or side-chain signal) is applied to the grids of the triode elements, determining the output gain of the valve. The signal is then, via the inter-stage transformer, fed to the output amplifier, which is based on a pair of 12AX7 and 12BH7 dual triodes, operating in a push-pull configuration. The other three valves are not placed directly in the signal path, but they perform some very important duties nonetheless. Firstly, a 5Y3GT power rectifier valve is used in the power supply, a task that nowadays (apart from in certain guitar amps) is most commonly handled by silicon diodes. There's also a second, smaller 6AL5 rectifier valve for the side-chain signal and, finally, an OB2 voltage regulator valve, which feeds the 6BC8.
