This unit sounds extremely good on the 50ohm vintage ribbon microphones, the transformers are all Gardners we think & the ppm meter is an Ernest Turner, the 771847 input transformers are 600 ohm, others were available see circuit diagram. The 2G-309 transistors were all replaced with RCA 2613 type by the previous owners, ex BBC stock, it came from the original Dr Who studio so I was told by the seller on a Merton Abbey Mills car boot sale in 1996 sold to me for £20.
There is a short video describing this unit coming up soon.