Locomotive Audio 40AM-1

Locomotive Audio 40AM-1 is a single-channel tube preamp in a 500-series module built on octal tubes, combining the pentode input and triode output stage of the RCA BA-11A with the feedback topology of the Gates SA-70 console, both 1940s American broadcast designs. It's the single-channel counterpart to the two-channel 40AM-2 from the same octal Locomotive preamp family.

Octal tubes instead of a miniature 12AX7

Two 6SJ7 tubes (one strapped as a pentode on the input, the other as a triode on the output) deliver a maximum gain of 52dB and the heavy, dense character typical of American broadcast consoles from eighty years ago. MuMetal-shielded input transformers and nickel-and-steel output transformers, wound in-house at Locomotive, finish the signal path.

A 5-position Response switch instead of one fixed tone

A 5-position Response switch reshapes the circuit's tonal balance, and a 5-position input switch selects the transformer winding configuration for mic or line mode. A -20dB pad, an 85Hz high-pass filter, a high-impedance front-panel DI input and a 3-colour LED meter round out the control set packed into one 500-series slot. Octal tubes are less common in 500-series preamps today, which makes the 40AM-1 a distinctive pick for engineers specifically chasing that eighty-year-old heavy character, rather than another 12AX7-based replica.

KEY FEATURES

• Single-channel octal tube preamp, 500-series format
• Tubes: 2x 6SJ7 (pentode input, triode output)
• Design combining the RCA BA-11A and Gates SA-70 (1940s USA)
• 5-position Response switch
• 5-position mic/line input switch
• -20dB pad and 85Hz high-pass filter
• High-impedance front-panel DI input
• Hand-built in the USA

TECHNICAL SPECIFICATIONS

• Type: single-channel octal tube preamp, 500-series
• Tubes: 2x 6SJ7
• Maximum gain: 52dB
• Transformers: MuMetal input, nickel-and-steel output, in-house wound
• Pad: -20dB
• High-pass filter: 85Hz