Sent: Monday, June 01, 2015 5:40 AM (revised 8 May 2022)

Subject: [AMPEX] Ampex 300 transports

Several people have asked recently about Ampex 300s.  Keep in mind that the transport design is quite old (going back to 1949 though there were incremental improvements).  Regardless of what electronics are paired with the transport, the transport itself requires special care and feeding and is going to require lots of maintenance and repair.

I would not recommend any indirect-drive Ampex as your first machine.  The drive system is complex and decidedly NOT user-friendly.  In the old days, when these recorders cost as much as a base-model automobile, institutions that owned model 300s had paid staff to maintain them.  The 300 transport is heavy, non-intuitive to service, and easily damaged when lifted from its console cabinet.  And console-mounted 300s with electronics underneath the transport included a fan (powered by a "refrigeration" motor) to provide much-needed cooling.  If you have such a model, make sure that fan is there and working (or install a more modern "boxer" fan).   This is important, particularly if you have electronics mounted underneath or if cabinet airflow is poor.

The indirect drive system used in the Ampex 300 and related transports does not age well.  This is why very few of these transports are working adequately today.  Many owners of such machines are in denial about this but there it is.  Servicing these transports is time-consuming and far more complex than most people realize.  Also, if you are thinking of obtaining a 300 transport, be aware that early versions do not have field-replaceable capstan assemblies.  Look for two holes in the flywheel.  These were used on later machines so that you could insert a T-handle hex wrench through one of the holes and remove the retaining ring (there are three socket-hex screws that hold the bearing retaining ring to the underside of the transport).  I do not know when this change occurred, but it was possibly in the mid/late 1950s.

Machines with non-service-removable capstan assembly are basically junk as the cost to retrofit such as transport is incredibly high due to the tremendous amount of labor involved.  I know because I have done this conversion and it's simply not worth it unless the machine has some special historical significance and you have a huge budget.  If the other parts are good, you can use the machine as an organ donor though be advised that early 300s did not have Jones plugs for the reel motors (they used barrier strips) so the motors are not directly interchangeable with later machines.  The older reel motors used a more primitive aluminum plate mount as well, not the more familiar die-cast alloy plate.

Even if you have a removable capstan assembly, it is difficult to remove (you have to remove the capstan motors and many associated parts).  Once it is removed, it is vulnerable to damage and you have to then have the rubber renewed.  Not cheap or easy.

As mentioned many times on this List, the weakness of the Ampex 300 indirect-drive capstan setup is that the rubber tire on the flywheel eventually needs renewal.  The rubber tire needs to exactly 5.975" in diameter and soft (45 to 48 durometer Shore A scale), without flaws, to ensure that the drive-motor pulley digs into the rubber to deform it as designed.  Otherwise the capstan cannot be adjusted to the correct speed (yes, there is minor speed adjustment possible but it is a very narrow range).  Over time, the plasticizers in the rubber gas off and you are left with a non-functional flywheel tire that is both too small in diameter and too hard to operate correctly.

Repair of the capstan assembly is possible but expensive, difficult, time-consuming, and requires specialized techniques.  Older capstan flywheels (without two drilled holes) are not worth repairing (see URL).

You can get maybe 20 years on a flywheel tire.  Less if the unit is operated/stored under non-ideal conditions.  Even if the rubber has no dents or obvious damage, the transport will never run at the correct speed if the tire diameter has shrunk more than a nominal amount.  You can also overheat the capstan motor and/or wear its bearings by adjusting the dig pressure too hard to compensate for a shrunken tire.

The stock setup for audio machines used an angled motor-paddle and a 1" pulley on the capstan motor (typically a sleeve-bearing Bodine or later, a sleeve-bearing Ashland motor).  Duplicators and instrumentation transports that operated at higher speeds used ball-bearing capstan motors mounted on a straight motor-paddle to accommodate the larger 2" motor pulley used.

The system, when working correctly, offered decent flutter specs and allowed the capstan diameter to be large enough to ensure good contact with the tape and pinch roller.  It also allowed for 15/30 operation (again, with the 2" motor pulley) and 30/60 operation (with the 2" pulley AND a capstan sleeve) for instrumentation and duplicator applications (note that these machines used different electronics but had transports similar to the original 300 mono machine).  Note that the use of a capstan sleeve is generally not a good idea under any circumstances but it was done for high-speed duplication.*

Note that 300-series instrumentation machines sometimes used "bathtub" electronics that appear to be similar to audio electronics.  They  are not. They lack audio transformers since they had response out to 60 kHz for instrumentation use and they use different tube types.  This has fooled many people over the years.

There are many home-brewed multichannel 300 machines out there so extra caution is advised.  The stock 300-3 used special 350 (at first) and then special 351 (later) electronics (catalog 30960-11 was typical).  I don't know if there were any 300-4s using 350 electronics.  The ones I have seen used special versions of the 351, typically with 30960-11 electronics and a transformer pod on the playback connector since the 4-channel playback heads used were low impedance.

Half-inch "homebrews" are often recognized by the lack of a special "turnaround" idler near the head assembly that was standard on factory-stock half-inch machines.  The turnaround idler had a half-inch rotating tape guide and was used in place of the quarter-inch spring-loaded guide on the reel idler assembly.

Usual disclaimers.

-dave

*To review, 300 mono and multichannel machines normally used a 1" motor pulley to provide a speed pair of 7.5/15 ips without using any capstan sleeve.  Duplicators (and some instrumentation machines which also used 300-type transports) used a 2" motor pulley to provide a speed pair of 15/30 ips (which was typically extended to 30/60 ips with the use of a capstan sleeve).  By the 1970s, some "clone" duplicators used high-speed direct-drive capstan motors (with sleeves) to duplicate tapes with speeds up to 120 ips