Ampex Model 350
The machine that convinced broadcasters magnetic tape wasn't a toy - and made Bing Crosby open his checkbook.
Overview
The Ampex Model 350 is a two-speed quarter-inch magnetic tape recorder built by Ampex Corporation of Redwood City, California. It arrived when professional audio was still mostly a disc-cutting world, and it did more than any single machine to change that. The 350 came in console, two-case portable, and rack-mount configurations, and buyers could specify either full-track or half-track heads. A station could order a rack unit for the control room and a portable pair of cases for remote work, all sharing the same electronics architecture and the same tape format.
The transport runs at either 7-1/2 and 15 inches per second, or 3-3/4 and 7-1/2 ips, depending on how the machine was ordered. At 15 ips the response is quoted at plus or minus 2 dB from 30 to 15,000 cycles. At 7-1/2 ips the manual claims plus 2 dB from 40 to 10,000 cycles and plus 4 dB from 30 to 15,000. The 3-3/4 ips setting is the weak sister, with a quoted plus or minus 2 dB from 50 to 7,500 cycles - usable for voice and logging, not for music. Signal-to-noise tells the same story. Full-track at 15 ips manages 70 dB maximum record level to unweighted noise, dropping to 65 dB for half-track. At 7-1/2 ips, peak record level to unweighted noise is 60 dB full-track and 55 dB half-track. Flutter and wow run 0.2% at 15 ips, 0.25% at 7-1/2, and 0.3% at 3-3/4. Those figures were serious business in 1953 and remain respectable for a machine of this vintage.
What made the 350 a broadcast standard wasn't any single number. It was the control layout. Audio (August 1953, p.17) described the transport buttons as giving station operators a new "sureness" with tape - cueing exact, editing faster, fumbling eliminated. Remote control was available as an option, catalog number 5763 or 5763-I. The machine also stops automatically if the tape breaks or runs off either reel.
The 350 handles NARTB 10-1/2 inch reels as well as the smaller RMA 5-inch and 7-inch sizes. A Reel Size Switch adjusts tape tension for the reel in use. The manual is emphatic that operators should never mix a large reel on one turntable with a small one on the other, because braking becomes unreliable. The reel hold-down knobs (catalog #4402 for rack machines, editing knobs #1917 for console and portable) use a collet-and-pin arrangement. The manual warns against pressing them down fully, since excess pressure distorts reel flanges.
Editing and cueing on the 350 take a particular technique. Holding Fast Forward and Rewind simultaneously reduces tape motion to a slow creep, and alternating between the two buttons controls direction. It's a clever bit of mechanical improvisation, and it separated engineers who knew their machine from those who merely operated it. The manual also carries a blunt caution: never press Stop and Play in rapid sequence while the tape is traveling at high speed in Rewind or Fast Forward. The capstan idler will lock the tape to the capstan before it has time to stop, and the tape will almost invariably break.
The 350 was available for either 50 or 60 cycle operation, with the power line frequency indicated on the serial number plate. Rack-mount installations required care. The Electronic Power Supply Panel had to be mounted directly above the Mechanical Assembly to keep 60-cycle field from entering the electronics. If two 350s shared a rack, Ampex recommended an interleaved arrangement with spacer panels, and an extension cable (#3814) was available for the upper machine's power supply.
Half-track operation is documented in the manual as a distinct procedure, and the parts list confirms that head assemblies were orderable by catalog number - for example, a 475-17 half-track head assembly for a Model 350. That modularity is why so many 350s survived into the decades that followed. They could be maintained, not merely replaced.
Mix (October 1985, p.100) recounted how Bing Crosby, determined to produce his Philco show on magnetic tape permanently, backed Ampex with a personal check for $50,000 to mollify nervous network executives and get the professional magnetic tape effort moving. The 350 was the machine that made that bet pay off. Audio (June 1953, p.37) announced it as a new model from "the leader in tape recording," noting that the first Ampex - the Model 200 - had already made recorded sound come to life for the broadcasting and recording industries. ABC's assistant chief engineer, Richard Parks, went on the record in Audio (January 1955, p.2) saying the network could send the smaller Ampex 600 into the field and get results as good as what they recorded in-studio on 350s.
By the mid-1950s the 350 was everywhere the serious work was happening. Tape-Recording (October 1956, p.22) documented Ampex console recorders on site at the Democratic convention in Chicago, with recording centers installed at the Conrad Hilton and newsmen using the machines to interview delegates.
For collectors today, the 350 is awkward to own. It predates the stereo era, it's heavy, it runs on tubes, and it demands a working knowledge of tape tension, head demagnetization, and bias alignment. The manual's noise balance procedure shows what ownership entails: the Noise Balance Control should never be touched until the heads have been thoroughly demagnetized, and even then only with a 1 MFD condenser across the LINE OUT and a sensitive meter or speaker on the output. This is not plug-and-play gear. Examples that have been maintained, or better, restored by someone who understands the transport, remain the reference point for what quarter-inch mono tape sounded like at its best. The console versions are furniture, the portable two-case versions are the ones that travel, and the rack units are what broadcast engineers actually used.
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