This very dirty behemoth is the RCA designed and built RBA VLF receiver, and is awaiting restoration. This receiver is extremely well shielded, and is a TRF design with a twist. 3 RF amplifier stages, the third of which is operated at very low plate and screen voltages, 15 VDC, so that it acts as a limiter, followed by a triode plate-detector which is also fed a signal from a separate tuned heterodyne oscillator or BFO, then another fairly complex limiter stage, and two stages of audio amplification with extensive audio filtering. The audio output stage is configured such that one can connect from one pair to 20 pairs of headphones (why on earth would you do that?) with no change in output amplitude. It looks a lot like the companion RBB and RBC receivers, and can use their same power supply. This hulking receiver weighs over 90 lbs and is considered the very best of the TRF designs.