RCA BA-31b Mic Preamp Restoration
Posted by DibS in 
I acquired a pair of these early solid state germanium transistor based mic preamps. They are fixed gain with an easy mod that gives you 6db more when you need it. The UTC iron on here is also key. From the numbers on the output trannies, I’m guessing these were built in 1965. I researched the caps that were in there and found modern equivilants. I’ve so far recapped one whole module and installed bypass caps across C12 (Output) and C6 (Output of gain stage). I’ll post a BOM when I’m done. (BOM posted in DIY Meta)
Before And After ReCapping.
A Pile of Old Caps
I plugged her in and she purred like a kitten… And not in a good way. It actually oscillated like a motorboat at first. I read on Prodigy Pro that these are finicky about output load so I put a 680 ohm resister across the output as per emrr’s recommendation. Oscillation stopped. Level came up. The SM57 I was testing with sounded dreamy. Now I’m gonna get started on #2.
EDIT 10/25/08:
Both modules are done and pigtailed. I’ve powered them up and have been using them for make-up gain on my passive mixer. They definitely have a sound to them. I can’t wait to use them on vocals and overheads. The next step will be properly mounting them in a Par-Metal case with phantom power.
More info including a BOM can be found in my DIY Meta.