Hello, good question, and I can't recall! One of the best sound mic preamps I have had running in the past decade. Large base tubes often sound better with more detail. This Bogen was actually a classic design, based on an earlier Presto design -- so it was very old, predating the Bogen production. Fabulous sound -- worth double or triple what it sold for!
@@HiFiTown I found the schematic. The first two gain stages are two 6F5 triodes, and the output stage is a 6J5 cathode follower. Kinda unusual arrangement. Usually the the 6J5 is pressed into service at the front line, the input and the 6F5 driving the output. There were three options for input transformers, 50 Ohm, 200 Ohm, and 500 Ohm, T157, T155, T156 respectively. Bogen tube equipment are all buzz boxes by now and require replacement of all caps. I am completely in agreement with your assessment.
Great job on research! Probably this method was more common during the prewar period, at least my guess. Yes caps, especially power supply filters are often a bit rough. Believe it or not -- the one above was all original, and pretty darn quiet (but could have been even better with normal attention). A surprising amount of this old equipment is out there with largely unserviced circuitry. Users and collectors are very cautious about which parts are changed! Oswalds Mill has a terrific article on noise floor on old tube recording gear and processes that I agree largely with -- it notes that there is allot going on psychoacoustic-wise in much of the really revered equipment -- in other words, dead silent as most shoot for is not always the best, strange, but true. Best Regards, Early @@vintageaudio7518
