The Locomotive Audio Model 14B PCB is a single-channel, all-tube Vari-Mu compressor and limiter that combines fast peak control with rich valve tone, a smooth push-pull 6V6 output stage and highly musical gain reduction. Designed for vocals, bass, drums, guitars, buses and stereo work with a linked pair, it can move from gentle levelling to assertive limiting—or operate as a characterful tube line amplifier.
 
The Locomotive Audio Model 14B PCB is a boutique American Vari-Mu tube compressor and limiter for engineers who want dynamic control to feel musical rather than mechanical. Its circuit combines the fast, decisive attack behaviour associated with classic variable-mu designs such as the UA 175B with the fuller, smoother output character of vintage tube levelling amplifiers. The result is a compressor that can place a vocal confidently at the front of a mix, add weight and movement to bass, tame transient-heavy material or bring cohesive valve character to a bus.
 
At the heart of the 14B PCB is a fully tube-based gain-reduction and output architecture. A 6BC8 tube acts as the variable-mu gain-control element, followed by a 12AY7 driver and a push-pull pair of 6V6 output tubes. This arrangement contributes to the unit’s combination of responsive dynamics, harmonic richness and substantial output authority. Custom MuMetal-shielded input and interstage transformers, together with a custom output transformer, support low-noise operation and the strong, dimensional sound expected from a high-quality transformer-coupled tube processor.
 
The 14B PCB provides continuously variable attack from 300 µs to 20 ms and release from 100 ms to 2 seconds. Those ranges make it considerably more versatile than a fixed-time-constant vintage compressor: choose fast attack to restrain vocal peaks, percussion or sharp bass notes; use a slower attack to preserve drum impact and initial pick transients; then match release to the rhythm and density of the source. The two-position Compress/Limit switch provides up to 4:1 compression or up to 10:1 limiting, extending the 14B from smooth levelling to more forceful control.
 
A variable sidechain low-cut filter, adjustable from 70 Hz to 350 Hz or switched off, lets the compressor ignore part of the sub and bass energy in the detector path. This is especially useful on drum bus, bass-heavy material, stereo mix bus and mastering-oriented duties: the low end can remain powerful while the compressor responds more to the musically relevant midrange and high-frequency peaks. The filter affects the detector rather than functioning as a high-pass filter in the main audio path.
 
The Attack control has a further practical function. Turn it fully counter-clockwise to switch compression off and use the Model 14B PCB as a tube line amplifier. In this mode, the 6BC8 gain-reduction element is removed from active compression duty while the transformer-coupled valve circuit remains available for line-level gain, tonal density and analogue character. It is a useful option for DAW stems, synthesizers, keyboards, re-amping line-level sources or simply adding a more dimensional output stage without active gain reduction.
 
A hard-wire true-bypass switch offers immediate, level-aware comparison of processed and unprocessed signals. The illuminated custom Sifam VU meter can display either gain reduction or output level, supporting precise setup and critical listening. A front-panel line of control remains fast and tactile: set input and output level, select Compress or Limit, set attack and release, shape the detector with the sidechain filter, then use the meter and bypass to refine the result.
 
For stereo sources, two matched 14B units can be connected through the rear-panel TRS stereo-link connection. In a linked pair, the units can be used on drum bus, piano, stereo keys, mix bus or mastering chains while preserving a stable stereo image. Each unit remains a complete single-channel processor, so a pair also provides flexible dual-mono operation when stereo linking is not required.
 
The “PCB” designation identifies the printed-circuit-board construction version of the 14B. It is not a DIY kit or a stripped-down design: the unit retains the Model 14B’s professional tube complement, transformer-coupled audio path, 2U steel chassis and lifetime manufacturer warranty on parts and workmanship. Handmade in the USA, it is a compelling choice for engineers who want vintage-inspired Vari-Mu compression with practical modern control and a genuine tube line-amplifier option.
 
Key features
Single-channel, all-tube Vari-Mu compressor and limiter
Handmade in the USA
6BC8 variable-mu gain-control tube
12AY7 driver tube
Push-pull 6V6 output stage
Custom MuMetal-shielded input and interstage transformers
Custom output transformer
Continuously variable attack: 300 µs–20 ms
Continuously variable release: 100 ms–2 s
Compress mode: up to 4:1 ratio
Limit mode: up to 10:1 ratio
Sidechain low-cut filter: Off or 70 Hz–350 Hz
Compression-off / tube line-amplifier mode via Attack control
Input and output level controls
Rear-panel stereo-link connection for paired operation
Hard-wire true bypass
Backlit custom Sifam VU meter
Meter modes: Gain Reduction or Output
Balanced XLR input and output
Switchable 115 V / 230 V AC operation
Heavy 16-gauge steel 2U chassis
Lifetime manufacturer warranty on parts and workmanship
 
Applications
Vocal levelling, presence and controlled forwardness
Bass compression with tube weight and stable low end
Drum and room-mic shaping
Acoustic guitar, electric guitar and piano control
Synthesizer, keyboard and electronic-source thickening
DAW-stem processing and analogue mix enhancement
Stereo drum-bus, piano and mix-bus compression with a linked pair
Smooth gain levelling or stronger limiter-style peak control
Tube line amplification and harmonic enhancement without compression
Mastering-oriented colour and bus processing with two linked units