Ampex 400

The Ampex 400 reel-to-reel tape deck, produced during the early 1950s (approximately 1950–1953), is a tube-based, professional-quality audio recorder designed for half-track mono and stereo tape recording on ¼-inch tape. It supports two tape speeds: 7½ and 15 inches per second (ips), with a frequency response from 30 Hz to 15 kHz at 15 ips, placing it firmly in the high-fidelity category for its time.​

Technical features include:

• Tape format: ¼-inch magnetic tape on up to 7-inch reels, using half-track recording for maximizing tape usage by recording two separate audio tracks on the standard tape width.

• Head assembly: A single plug-in lug-in head housing containing erase, record, and playback heads made of permalloy, enabling easy maintenance and consistent head alignment.

• Controls: Four pushbuttons for Start/Play, Stop, Fast Forward, and Rewind, plus a separate Record button; electrical relay-operated controls allowing remote operation and reducing mechanical wear.

• Transport system: Three motors for capstan, take-up, and rewind, offering quick response times with tape acceleration to stable speed in under 0.5 seconds and stopping with less than two inches of tape movement at 15 ips.

• Wow and flutter: Very low flutter (under 0.2% RMS at 7½ ips and well under 0.2% at 15 ips), ensuring stable tape motion and sound quality.

• Tape timing accuracy: Approximately 0.2%, amounting to 3.6 seconds drift over a 30-minute recording, critical for broadcast and studio consistency.

• Rewind speed: About 1.5 minutes for a full 2400-foot NARTB reel, which was fast for the era.

• Weight and size: Around 90 lbs (41 kg), making it relatively compact and transportable compared to later console models.​

The Ampex 400 was regarded as a highly capable, reliable machine at an affordable price point for professional studios and high-end home use. Its half-track design allowed efficient tape use suitable for broadcast applications. The modular head assembly and electronic separation facilitated maintenance and alignment. This model played a significant role in making magnetic tape recording more accessible to a broader audience during the early 1950s.​

In summary, the Ampex 400 is a mid-century tube-based reel-to-reel tape deck offering solid audio performance, precise tape handling, and operational convenience, marking an important step in the development of magnetic tape recording technology. Manuals and service documents are available for those interested in restoration or technical maintenance.