RCA PA-90, first generation preamp for ribbon mics from the Public Address product line, 1933. 3RU, 35 lbs.
Single 76 tube, roughly 25dB gain - enough to get up to ‘high level mixing’ territory before…mixing…then on to a program amplifier and power amplifier(s).
130V B+ @ 2.3mA.
There’s an earlier version using the 56 tube with a 2.5V filament, you could use either here with the right filament supply.
4 capacitors are in the center rectangular case with a choke in the one to its right, with all wiring coming out of the potted cases and run directly to the tube socket - barely any space for new parts to fit in around the tube shield box, and brittle wires to disconnect and seal off. Thank goodness for smaller modern parts.
250 ohm input and output via coffee can sized transformers in 1/2” thick shielding, this was before they’d figured out multiple nested cans provided better shielding with small size and much lighter weight.
This one was probably custom order with a 500 ohm output - the transformer on the right with terminals on the front transforms 250:500 - and worsens the already antique response — we’re +/- 1dB 63-8kHz. The PA-103 and 41-A of the same era have much better response, so one might wonder if there some age deterioration of a transformer here.
I got this one up and running for a client, and I’ve seen one other on ebay in 25 years; pretty rare birds. I know of another in a collection in Hong Kong.
https://live.staticflickr.com/65535/53646355279_39eb08ef2a_b.jpg
https://live.staticflickr.com/65535/53646355319_3bbe84ca75_b.jpg
https://live.staticflickr.com/65535/53646229878_c9bc44673e_b.jpg
https://live.staticflickr.com/65535/53646229893_6147064267_b.jpg
https://live.staticflickr.com/65535/53646011421_207b30a5fd_b.jpg
https://live.staticflickr.com/65535/53646011446_7818a4775d_b.jpg
https://live.staticflickr.com/65535/53646229813_e5b523a528_b.jpg
Single 76 tube, roughly 25dB gain - enough to get up to ‘high level mixing’ territory before…mixing…then on to a program amplifier and power amplifier(s).
130V B+ @ 2.3mA.
There’s an earlier version using the 56 tube with a 2.5V filament, you could use either here with the right filament supply.
4 capacitors are in the center rectangular case with a choke in the one to its right, with all wiring coming out of the potted cases and run directly to the tube socket - barely any space for new parts to fit in around the tube shield box, and brittle wires to disconnect and seal off. Thank goodness for smaller modern parts.
250 ohm input and output via coffee can sized transformers in 1/2” thick shielding, this was before they’d figured out multiple nested cans provided better shielding with small size and much lighter weight.
This one was probably custom order with a 500 ohm output - the transformer on the right with terminals on the front transforms 250:500 - and worsens the already antique response — we’re +/- 1dB 63-8kHz. The PA-103 and 41-A of the same era have much better response, so one might wonder if there some age deterioration of a transformer here.
I got this one up and running for a client, and I’ve seen one other on ebay in 25 years; pretty rare birds. I know of another in a collection in Hong Kong.
https://live.staticflickr.com/65535/53646355279_39eb08ef2a_b.jpg
https://live.staticflickr.com/65535/53646355319_3bbe84ca75_b.jpg
https://live.staticflickr.com/65535/53646229878_c9bc44673e_b.jpg
https://live.staticflickr.com/65535/53646229893_6147064267_b.jpg
https://live.staticflickr.com/65535/53646011421_207b30a5fd_b.jpg
https://live.staticflickr.com/65535/53646011446_7818a4775d_b.jpg
https://live.staticflickr.com/65535/53646229813_e5b523a528_b.jpg
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