Sonicraft A2DX Lab: Ampex MR70 Tech Notes

AMPEX MR70

The Ampex MR70 was specifically designed to dub EMI Beatles masters at Capitol for the US market.

Highly modified 300 transports in Ampex’ Standard Tape Lab that were designed to make flutter tapes, served as the prototype for the MR70 transport. Its massive die-case frame was designed to tight tolerances allowing the heads to be mounted for fixed azimuth. It uses a viscous damped reel idler with a booster motor to bring it up to speed, has varispeed fast wind and editing and constant tension tape supply.

The electronics use Nuvistor vacuum tubes and the best in resistors and capacitors. Because of the feedback used in the electronics, ultra-wide bandwidth audio transformers were mandatory. Mil-spec parts or better and gold or silver contacts were used throughout.

The MR70 supports settings for NAB, CCIR and AME equalization curves and the ability to switch between them.

In 1966, a standard MR70 quarter-inch two-track cost $4500, which translates to $28,000 in today’s dollars. So, it’s not hard to see why it wasn’t a hot seller. But today, it remains as the analog tape recorder that some say has never been surpassed.

In typical A2DX style, we started with five machines worth of transports and electronics. The entire lot was disassembled, the best parts chosen, and the ultimate machines were reassembled from these parts.

Today, we have access to precision parts and advanced testing methods that the 1960's designers of the MR-70 could have only dreamed of. Sonicraft master technician John Chester pored over the circuitry of these machines making important improvements wherever possible, advancing performance of the electronics well beyond basic restoration.

Ampex MR-70 1/2" 3-Track Vacuum Tube Recorder

All three A2DX MR-70 transports were completely overhauled and extensively upgraded by Bill Schlegel of International Recording Corporation. Bill did an amazing job of reworking these transports for better performance, easier servicing and rock-solid reliability.

Among the improvements were 4-speed servo capstan, varispeed, fingertip shuttle control of fast wind, modern relays and backlit controls. As a result, the A2DX MR-70 transports handle tape gently and predictably.

The MR70 3-track head assembly was refurbished with NOS Ampex MR-70 heads contoured to perfection by John French. Precision roller guides were also installed where stationary guides were previously used for the added boost in clarity and gentler tape handling they provide.

Note that the MR70 supports all of the various equalization curves in use during the golden years of 3-track recording including NAB, CCIR, and AME equalization.

The performance of this MR-70 is amazing and sonically, it is stunning.

The Sonicraft A2DX MR-70 one-inch four-track is truly one of a kind.

The head assembly had to be custom built from the ground up. Greg Orton of Flux Magnetics hand-crafted the heads to the highest possible standards. John French of JRF Magnetics came up with an ingenious design to ensure rock-solid tape guidance and head contact.

Custom MR70 1" 4-track head assembly

The track width and spacing chosen for the A2DX MR-70 1" 4-track was designed to match perfectly with tapes made on the European standard for 1" 4-track recording - the Studer J37. Conveniently, tapes made with any of the other 1" 4-track variants will also track properly.

The performance of the A2DX MR-70 one-inch 4-track is extraordinary. At 15 ips using IEC / CCIR equalization, frequency response remains flat to almost 30 kHz before falling off. The electronics are amazingly quiet and the sound is pristine yet warm and full.

Support for all of the various equalization curves used by one-inch 4-track machines both in the United States and in Europe is built into every MR-70.

Ultimate Ampex MR-70 1"4-Track Vacuum Tube Recorder

If you are looking for the ultimate vacuum tube sound for your 1/4" 2-track transfers, look no further.

A2DX Ampex MR70 1/4" 2-track

This Ampex MR-70 machine has been given the same kind of exhaustive restoration and enhancement as the A2DX 1/2" 3-track and 1" 4-track machines previously discussed.

If you have quarter-inch 2-track, half-inch 3-track or one-inch 4-track masters to transfer, these are the ultimate machines to do them on.

We are continuing to restore and enhance MR70 components. Other formats can and will be made available. Contact us for details.

To go back to the A2DX Tech Info Page, click here or hit the "back" arrow on your browser.

footer