Just got to poke around in one of these, and satisfy my curiosity about them. You see them listed as mic preamps from time to time, and I can say definitively that they are not, and cannot be. By appearance, they appear to be a single transformer and a pair of 6AK6 tubes. The 700A transformer houses both input and output, and it's a basic PP circuit. Basically two transformers, two tubes, cathode resistance. Literature describes it as a unity gain bridging amplifier with +24dbm output capability, 225VDC@45mA. There is a mention about the development of the amp in a few tech mags, and the UN installation contract seems to have been the design genesis. There are series bridging resistors on the input primary, and from the input secondary to the grids, and it's been suggested that strapping those out yields a mic preamp. The input DCR's are about 1.5K:5K, so forget that idea. You will get some gain with strap outs, but not much, and the input resistance is more representative of a 10-15K ohm winding. The connector and package make these difficult to work with. Without the mating connector and tray they will either sit on your shelf doing nothing, or it could be best to do a partial tear-down for mounting and wiring. The transformer set appears ideal for a custom limiter adaptation.
carry on.
carry on.
