The Federal AM-864/U is a rare example of early American tube compression technology. Originally manufactured by Federal Telephone and Radio Corporation for military communications applications, the AM-864/U features a fully balanced audio circuit, transformer-coupled input and output stages, and a tube-based automatic gain control system.

Unlike modern compressors with extensive parameter controls, the AM-864/U uses a straightforward design built around variable-mu gain reduction. Its 6SK7 tubes provide automatic attenuation in response to incoming signal levels, delivering a characterful compression response with the harmonic richness associated with vintage tube circuitry.

The compressor features a front-panel input attenuator and an analogue meter that can be switched between output level and gain reduction indication. Internal adjustments allow calibration of the compression threshold and control behaviour.

With its distinctive circuitry and vintage construction, the Federal AM-864/U is an interesting choice for recording engineers seeking authentic early tube compression characteristics, particularly for vocals, bass, drums, and creative analogue processing.

Main Features

* Vintage all-tube variable-mu compressor
* Originally designed for military communications applications
* Fully balanced audio signal path
* Transformer-coupled input and output
* 600-ohm input and output impedance
* 6SK7 variable-gain tubes for automatic attenuation
* Tube-based compression control circuit
* Front-panel input attenuator
* Analogue meter with selectable output level and gain reduction indication
* Internally adjustable compression threshold and control settings
* Classic vintage tube compression character

During the service all vacuum tubes have been tested. The 5Y3 rectifier tube replaced. The electrolytic capacitor in the rectifier anode section replaced.
The circuit has been modified to make the VU meter work in GR mode.
Controls and potentiometers have been cleaned.
General operation and functions have been tested.
The compressor has been modified; "Attack" and "Release" controls have been mounted on the front panel.

Serial no: #1368

PSU works on 115/220 V AC internally switchable. EU power cord included.

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