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Newly updated Rev 4 adds integrated level switching to the exceptional Zero Loss loop. On board switch allows selection of pedal level or line level (-20db or +4bd) to accommodate anything from pedals to high end rack gear at the flip of a switch.
We officially support this loop when installed in Marshall 1959, 1987, 2203 and 2204 models, also authentic replicas of these amps. The installation instructions are specific to this style of amp, with the long tailed phase invertor circuit. The loop will also work great in many, many other tube amp circuits, but we do not supply specific instructions. Please download the instructions from the link below to determine if the loop is compatible with your amp. Also, any good tube amp tech/designer can tweak this loop to work in most applications. It has been used successfully in tube amps from 18 to 200 watts.
Download the instructions here: FX Loop Instructions V 1.4
And here's a link to more info on the MetroAmp forum: