Endicott Research E700 series DC to DC voltage converter, nominal input is 5VDC and output is rated 48VDC, this is good for running vintage telephone equipment that runs on the old 48V central office power, as long as you don't need a lot of current. I have loaded it with 1000 Ohms (about 45 ma at 45 volts) and seems to hold this range with inputs between 4.5 and 6 volts. The output is only partially regulated. It says between 41 and 51 volts over the input range and load. It's also ideal for replacing the 45 B volt battery used in many vintage tube radios. It's small enough to install it inside the radio or inside the casing of a vintage "B" battery. You would have to arrange to switch the primary input. The output is isolated and two can be connected in series for approximately 90-100 volts. I ran a small radiosonde tube transmitter that way from 4 each AA cells in series. Running the 6.3V filament and B supply from the same 4 cells. Input and output is clearly marked. New old stock. General specifications:

https://ergpower.com/pdf30/DC_DC_Converters.pdf

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