WORKING CONDITION: Excellent. We just had this serviced by Dr Bass here in Los Angeles, where each module and power supply were re-capped, tubes were checked and replaced as needed, and any related calibration and servicing was performed to get these session-ready.  All four channels sound fantastic, and the unit is ready for installation and years of reliable use. This is a gorgeous-sounding microphone preamplifier.

PHYSICAL CONDITION: Very Good. The rack chassis is solid, and the mic pre cards are all stable and in very good condition. The XLR pigtail leads have all been labeled, and the wiring harness has been bundled for neat and easy installation. 

INVENTORY #59329

BRAND: RCA

MODEL: BA-21A Preamplifier

FUNCTION: Microphone Preamplifier

PRODUCT DESCRIPTION: This listing is for an original rack of four vintage 1950s's RCA BA-21A Vacuum Tube Microphone Preamplifiers and their original and more rare-to-find BX-21 tube power supply! 

The RCA BA-21A is a highly revered, mid-20th-century vintage vacuum tube microphone preamplifier designed originally for broadcast and recording studio use. Known for its exceptional fidelity, fixed-gain architecture, and pristine iron-transformer coloration, it remains a gold-standard choice for warming up low-output microphones.

Tubes and heaps of UTC heavy IRON- what more could you want?

SPECS:

• Tube Complement: It runs a multi-stage circuit powered by two 12AY7 dual-triode tubes (often requiring high-grade, low-noise variants). 
• Gain Structure: Delivers a fixed gain of approximately 40 dB to 45 dB utilizing around 20 dB of negative feedback for highly stable operation. 
• Impedance Flexibility: Features highly flexible internal transformer taps. The input can be strapped for 37.5, 150, or 600 ohms, while the output accommodates 150 or 600 ohms. 
• Frequency Response: Remarkably flat, rated from 20 Hz to 20 kHz (down only 0.5 dB at 20 kHz). 
• Power Requirements: As a standalone plug-in module, it relies on an external power supply supplying 260–280V DC for the high-voltage plates and 12V DC for the tube filaments

ACCESSORIES INCLUDED: This item comes as seen in photos and includes its original RCA Instruction guide with schematics.

UNPACKAGED WEIGHT: 37 lbs.

UNPACKAGED DIMENSIONS: 19" x 6" x 14"