1940s American-Style Mono Preamp with Octal Tubes – Designed for a Big, Full Sound

The 40AM preamp is your gateway to achieving that sought-after vintage sound of the 1940s. Inspired by iconic models like the RCA BA-11A and Gates SA-70, this preamplifier combines design elements from both into a single solution. Capable of powerful drive yet extremely versatile, the 40AM allows you to achieve recordings that embody the warmth and character of a bygone era.

The 40AM Experience

While it can be driven into distortion by saturating the input or utilizing maximum output headroom, the true strength of the 40AM lies in the subtle nuances achievable by adjusting its various sections. Between the choice of input transformer windings and the 5-position "Response" selector, you can precisely shape the sonic character of any source.

Additional features include:

• 20 dB Pad
• 85 Hz Low-Cut Filter
• High-Impedance DI Input

making the 40AM an extremely comprehensive unit, capable of sculpting sound in a targeted way.

Transformers

MuMetal shielded input transformers and internally wound nickel and steel output transformers. These components are fundamental to the unit's sonic character.

Tubes

Two 6SJ7 octal tubes (one configured as a pentode and one as a triode) amplify the signal, offering up to 52 dB of maximum gain.

Response Knob

A 5-position rotary selector allows you to shape the overall tonal balance of the signal.

• Position 5: emphasizes high and low frequencies, rich and more saturated sound
• Position 1: attenuates extremes and highlights mids, with greater harmonic character
• Position 4: recommended starting point for a more neutral sound

Output LED Indicator

3-color LED indicator for signal level:

Green → Yellow → Red

Sound Controls

Response Knob

Controls the frequency response and harmonic behavior of the signal:

• Higher settings: brighter and fuller sound, less perceived distortion
• Lower settings: attenuated highs, softer lows, and greater emphasis on mids
• Recommended neutral setting: position 4

Level Knob

Allows precise adjustments of the output level. It interacts slightly with the Response control: reducing the level may result in a slight darkening of the sound, which can be compensated for using the Response selector.

Project Inspiration

Combining vintage circuits requires time and attention to achieve the right balance. When well executed, it allows for the integration of strengths from multiple historical designs.

The 40AM is born from the combination of:

• Pentode input stage of the RCA BA-11A
• Triode output stage and feedback structure of the Gates SA-70

The result is a rich, compressed sound that integrates perfectly with the 86B and WT-72 models from the same line.

Unlock Your Creativity

Finding the right piece of hardware that can give sound that "something extra" is often one of the hardest parts of studio work. Having a combination of stimulating tools available is one of the best ways to foster creativity.

The 40AM is designed precisely for this: to help unleash creative potential and allow you to focus on producing high-level audio.