I read in the Neumann forum a couple of years ago that the EC71 (5718) would also be a very good replacement - here’s a quote from that forum post:
„As an alternative there are three different tubes that work and sound the same. RFT Funkwerk Erfurt in east Germany made the AC761, identical system but grid and plate are reversed, heater voltage is the same, but there are same as rare than the Ac701. Than there are the EC70/71 two UHF sub miniature triodes that were made by Telefunken and other western companies as well EC70/6778 and EC71/5718. Those tubes are very similar to the AC701, u=20/27 Ri=3.65/4.65 with better interelectrode capacitance than the AC701. They fit without a problem in any Mic that asks for an AC701, with minor modification in the cathode set up, the 6.3V filament is no problem at all. Most Neumann power sup. have a potentiometer to adjust the filament from 10 to 3V. Aside the different heater and pin layout the nice price of about $5.00 is way better than the $100 that the AC701 goes for these days. Here is the AC701 data: Bp= 80V Ug= -1.5V Ip = 2.1mA S = 2.8mA/V u = 22 Ri= 8.5 k Ohm Max. Values Bpc = 250V Bp = 120V Pmax = 0.8W I max = 5mA“
I bought like 5 or so of the EC71 back then for my M49 builds, but since I still haven’t received my mic body yet, this project was on hold for a while.
Besides I didn’t find any info in advance on how to bias it correctly...
Didn’t mean to crash your thread, just wanted to throw in some more options...