Reader T.F. sent me this scan from AUDIO ENGINEERING c. 1952: the introduction of the RCA BC-2B Console.
LEFT: The BC-2B incorporated the RCA MA-11241 dual mic pre-amp unit; a two-stage circuit, each channel used a single 12AY7 for, I would suppose, about 25 – 30 db of gain. The schem for the 11241 is posted below here. Notice that, similar to the earlier octal-pentode based RCA mic pres, the full B+ current flows through the output transformer; this severely limits your choice of output transformer: the only vintage full-fidelity units that I am aware of are the UTC A-25 and LS-27. Lundahl makes a modern unit that satisfies this spec, as does Hashimoto (HL-20K-6); very expensive pieces tho! Any of you fellas know of other 15k:600 1/2 watt transformers that handle 8ma unbalanced DC and still pass 40 – 20K?
Here’s a dude that’s cloning the BC2B preamp; price is $650 for the preamp plus another $450 for the power supply. Assuming that the build-quality is good, $1100 is a pretty fair price for this thing; I know how much those Lundahls cost ( I use the same O/T in my BRDCSTR as well) plus phantom power is a pain in the ass to build in.
What is the B+ you are operating at?
300 vdc or thereabouts.
The old RCA “Radio Age” magazines are available online at http://www.vacuumtubeera.net/RadioAgeMagazine.html now.
40 dB gain, output is 30K:600, they are wired 150 out in the consoles. 285VDC for B+. Passes well below 40Hz. That particular schematic should be banned from the universe, I have no idea where it came from, definitely not the official schematic from the manual.
Hi Doug. Do you have a more accurate schem for this device that you can send us or link to? c.