The Ampex 351 is also highly thought of for use as a mic preamp. It uses miniature noval tubes like the venerable 12AX7, printed circuit boards and lots of negative feedback. The input transformer was made for Ampex by several companies. Ampex 351 Vintage Tube Mic Pre with highly sought after input transformer. This is an amazing Mic Preamp! Recently gone through and given a clean bill of health. Items in the Worthopedia® are obtained exclusively from licensors and partners solely for our members’ research needs. Vintage Ampex 350 351 Mx10 Preamp Case. Ampex Atr-100 Atr-102 Extender Input Output Io Card New Rare. Ampex 58-0139-01. Ampex 58-0139-01 Ac/dc 10274 T402. Ampex 96134-01 Bridging Input Transformer Ampex Mx10/35 10 Available-vintage. Sealed Ampex Cassette Lot. Ampex 440/1100/1200 series studio recorders. The original Ampex head amplifier design has been combined with a new modern head input transformer. The line amplifier stage has been completely redesigned to provide improved specifications and sonic performance. The PCB’s are full ground plane types for improved shielding and circuit stability. One of the preferred platforms for the mod community has been the amplifier from Ampex 351 reel-to-reel player, as shown above. In fact, there's a fair amount of documentation online (especially at the Electric Audio forum, courtesy of Greg Norman), describing how to mod one of these units into a preamp.
Ampex 351 Input Transformer Diagram
From the research I have done the input transformer on the MX-10 is a Beyer TR/BV 351 015 006. This has a turns ratio of 1:15, and the input impedance is rated at 200 ohms. This is a little low for most of the mics I will be using, so I am wondering, how technically complicated it would be to swap it out with something like a Jensen JT-115K-E.
Ampex 351 Input Transformer Wiring
http://www.jensen-transformers.com/datashts/115ke.pdf