Cosmetically pristine and working after being put through its paces. Pulled from a working environment from my uncle's studio after he passed. Tubes are in great shape. Left rack ear is slightly bent from being pulled in/out of racks.

From Pendulum's website:

The Quartet is an all-purpose recording tool designed with the ultimate in sound quality and flexibility in mind. We have combined four elements - Mic/DI preamp, Opto-compressor, EQ, and De-esser -into a fully integrated package that it makes it easy to track any source. In addition, the Mic/DI preamp, EQ and Compressor/De-esser are completely independent entities that can be patched out and used separately - a hand feature that makes it a valuable tool for mixing as well as tracking.

The Mic/DI preamp, borrowed from our acclaimed MDP-1, can handle mics, pickups, keyboards, or line-level signals. It features phantom power, input pad, phase reversal, and a switchable lo-cut filter. Independent gain and output controls permit overdrive of the tube stage to vary the harmonic content of the preamp.

The Opto-Compressor, derived from the popular OCL-2, uses our proprietary optoelectronic cell, and has a sound all its own. It can be used in one of four modes: Fast (for 'invisible' compression), Average (rms responding), a new Vintage mode (with program-dependent response), or full manual operation.

Our new 3 band tube EQ, a modifed Baxandall design, is simple but very effective. The HI and LO bands have up to ±10dB of shelving at 15, 10 and 7 kHz, and 50, 100 and 200 Hz respectively. The MID band has a broad ±10dB peaking response centered at 0.7, 1.6, 2.2, 3.3, 5.0 and 6.5 kHz. The EQ can be placed before or after the compressor with the flip of a switch.

The new De-esser, which is implemented within the opto-compressor, uses an entirely new approach. A highly selective inductor-based detector triggers a very fast opto-inductive notch filter. The user can control the depth of the notch, which in turn controls amount of sibilance to be removed. When not triggered, the de-esser is out of the signal path entirely.

Specifications

These are the general specifications. See page 15 of the Quartet operating manual for all detailed specifications– Mic/DI Preamp, Equalizer, and Opto-Compressor.

Circuit Type: Class A vacuum tube design with transformerless or transformer-balanced output
Vacuum Tubes: (3) ECC83/12AX7A, (3) 6922/6DJ8
Power: 120V, 45W (240Vac optional)
Power Supplies: +300Vdc, +6.3Vdc, +12.6Vdc and ± 18Vdc, fully regulated with soft-start warm-up and output muting