RCA PA-103 (PA103A1). 1932ish mic preamp from the sound reinforcement catalog. Fixed gain with unusually high output and B+ current requirements, almost qualifies as a line amp. I had the opportunity to make a restoration plan for a pile of these several years ago, so observations are based on multiple units. These are super scarce units, I’ve not encountered any others.
250Ω in/out, 58dB, 300V @ 27mA, 2.5V @ 3A
Output looks pretty clean to at least +16dBu, definitely distorting by +22dBu.
Lots of quirks here.
- Early telco style design with things protruding off the front and back of the panel, somewhat helps with rack weight distribution. The tube deck is shock iso’d from the rest of the chassis. 0.5 inch thick transformer case shields.
- mechanical layout DEFINITELY NOT with service in mind. The caps are all in a sealed pack (5 caps, 1 inductor) with wire leads which looks like a transformer case, have to disconnect and isolate those wires and find new spots for caps. It's a very tight sandwich between the resistor board and the shield plate.
- not being broadcast line, it’s carbon resistors. “Dog bone resistors” as the old timers call them. All new modern carbon comp in keeping with the clients wishes, except for the 1W R.
- 2.5VAC tube types, not a lot of obvious 2.5V/3A supply options.
- input low pass cap to limit upper bandwidth, also tames a big transformer resonance. You can take it out and live with the inaudible resonant peak for a surprisingly modern sound. Larger cathode caps also afford more bottom, an octave. Response is best with a 250R load resistance, mainly treble improvement.
- input center tap is grounded, and must stay that way, out of control resonance if lifted, typical of early RCA input transformers.
- does not run quietly or with expected frequency response without the tube cover box in place. In fact you can reseat the cover on these multiple times and watch the response plot settle differently each time! There’s a shield plate between the tubes and the resistors too, which also affects response and noise.
- you could put an interstage volume control in, tests showed no response changes across the full throw of a gain pot. Where to put a gain pot is another matter, practically and non-invasively.
The tube current meter points were someone's DIY addition.
Restored they sound really good, and a bunch of channels would heat a room and run the power bill up in no time. Full info can be found in the Rider manual, lucky break there.
The associated PSU is a crazy brute force CLCLC unit that can handle 4 of these, if you use less than 4 PRE there are (3) 100W wire wound resistors in a cage to be strapped as loads replacing any less preamps, so a PSU with one PRE is set up to burn 78ish mA of B+ as heat!
250Ω in/out, 58dB, 300V @ 27mA, 2.5V @ 3A
Output looks pretty clean to at least +16dBu, definitely distorting by +22dBu.
Lots of quirks here.
- Early telco style design with things protruding off the front and back of the panel, somewhat helps with rack weight distribution. The tube deck is shock iso’d from the rest of the chassis. 0.5 inch thick transformer case shields.
- mechanical layout DEFINITELY NOT with service in mind. The caps are all in a sealed pack (5 caps, 1 inductor) with wire leads which looks like a transformer case, have to disconnect and isolate those wires and find new spots for caps. It's a very tight sandwich between the resistor board and the shield plate.
- not being broadcast line, it’s carbon resistors. “Dog bone resistors” as the old timers call them. All new modern carbon comp in keeping with the clients wishes, except for the 1W R.
- 2.5VAC tube types, not a lot of obvious 2.5V/3A supply options.
- input low pass cap to limit upper bandwidth, also tames a big transformer resonance. You can take it out and live with the inaudible resonant peak for a surprisingly modern sound. Larger cathode caps also afford more bottom, an octave. Response is best with a 250R load resistance, mainly treble improvement.
- input center tap is grounded, and must stay that way, out of control resonance if lifted, typical of early RCA input transformers.
- does not run quietly or with expected frequency response without the tube cover box in place. In fact you can reseat the cover on these multiple times and watch the response plot settle differently each time! There’s a shield plate between the tubes and the resistors too, which also affects response and noise.
- you could put an interstage volume control in, tests showed no response changes across the full throw of a gain pot. Where to put a gain pot is another matter, practically and non-invasively.
The tube current meter points were someone's DIY addition.
Restored they sound really good, and a bunch of channels would heat a room and run the power bill up in no time. Full info can be found in the Rider manual, lucky break there.
The associated PSU is a crazy brute force CLCLC unit that can handle 4 of these, if you use less than 4 PRE there are (3) 100W wire wound resistors in a cage to be strapped as loads replacing any less preamps, so a PSU with one PRE is set up to burn 78ish mA of B+ as heat!
