Oh, right, RCA.
They did everything at 250Ω for a long time.
The earliest are mic level mixers before a single program amplifier, for carbon or condenser mics.
Like everything before a certain point in the mid/late 1930's, the earliest are passive mix panels with no amplifiers, those being external. Balanced H or ladder attenuators with a multistep matching transformer on the output, tap for the number of mix channels, then a master output. The largest I've seen was 8 channel. The film version PB-37 (see other post here) also had a bonus HPF.
The early racks of broadcast amps had a 4 position rack panel mixer with no master, that being the control on the program amp.
The 1937 76-A console used the standalone program (40-C/83) and monitor amp (82/BA-4/14) circuits of the day, many basic RCA designs were tweaked and subtly changed from 1934-1941, with the basic BA-4 monitor amp circuit having a bunch of different part #s from 1937-1956. The preamp was a 3x1 type that also existed as standalone rack equipment.
The 1939 76-B changed the preamp to the individual 85-B circuit and continued on with 40 and 82 circuits through the end of that line. So a 1938 rack of amplifiers with a 78 console surface is really the same as a 1950 76-D for the most part.
The 1951 BC-2B (never seen any reference to an 'A') had the most patching functionality of any RCA tube console, monitor amp like much the BA-24, a new program circuit that is much like the BA-3/13 but with a 6V6, and a 6072 preamp using the BA-11/12 transformers.
The 1956 BC-3/4/5/6 moved fully to PCB construction and over to high impedance mixing, and out of necessity reduced the patching options. Highest amount of negative feedback, all 9-pin tubes except for the monitor amp outputs, and much smaller transformer types. The least sexy on the inside, and more of a pain to service because of early gen PCB's with lots of hand wiring to/from. Same external case and footprint as the BC-2B, at least with the BC-3. Varying widths on the 5/6, and then there's the rarer BC-4 which seems to have a different case style. The BC-3 is still in the 1967 catalog.
The Photophone film and recording studio consoles did have basic HP/LP EQ's and some different concepts such as input and output side mixers, one version is documented thoroughly in an RCA Recording Manual that's online. There are pictures of larger and later variations though no info has really surfaced. Many of the components in those consoles are the same as the standalone equivalents, in many cases just mounted in a desk with an overall faceplate. There are some more advanced filter sets and EQ's, some that are sort of 'tilt' shelves (so whoever trademarked 'tilt' recently shouldn't have qualified).
Film and recording employed some fairly severe HP and LP filters on the program outputs, to tailor for the capture mediums of the time. If those are removed, bandwidth is excellent. As example, I just worked on a large number of 1933 RCA pro PA system preamps, and they all measured as full bandwidth as is ever needed for rock and roll, maybe not dog whistles (almost) or pipe organs.
The pro line broadcast and film stuff until WWII used custom wire wound resistors done on multi chamber bobbins, so noise as good as is possible, essentially the tube noise floors. Resistors like that don't drift either.
. I assume part was to match their transmission standards but that is just an assumption.
