SOLD: RCA BA-21A triode tube mic preamp

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emrr
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SOLD: RCA BA-21A triode tube mic preamp

Post by emrr » Mon Sep 18, 2017 11:08 am

RCA BA-21A triode tube microphone preamplifier with copy of manual. Custom UTC input and output transformers. Mid 1950’s circuit is single ended input stage with a push-pull output stage. I checked all the caps with a cap meter and then fired it up briefly, response looked right and it ran fine with voltages looking correct everywhere. Wired for 150 ohm mic input and 600 ohm output, input can be wired down to 50 ohms or up to 600 source Z. Actual output impedance is 100 ohms, so these have excellent line driving capability, actual input Z is bridging. Uses two 12AY7 tubes, no tubes included. Rated for 40 db of fixed gain with max input of –22 dbm, and output of +18dbm with 0.75% or less total RMS harmonic distortion at 30Hz. You will need to wire input and output connectors, and connect 250-285VDC @ 10mA and 6.3V AC or DC @ 0.6A power supply. Factory drilled front panel mounting hole is for a bridging input volume control (20K ohm input to 600 ohm input transformer winding); nice spot to connect a –20db mic pad switch. Note this one does not say BA-21 anywhere on it other than the backside of the PCB, and has a different MI #, but I have confirmed it is the same. This version was installed as part of the first video record deck systems.


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