Help Me Identify This Preamp: Audio Radio Corporation - ARC 15

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SirCVH

Member
Joined
Aug 13, 2014
Messages
16
Hello
I picked up some tube stuff last week from a recycle/junk shop, one was an Ampex 403 preamp unit with UTC transformers and all the tubes and power supply! But I also picked up another preamp that I can't come up with anything on the internet and was hoping you guys can help.

It is labeled as a "ARC 15" by Audio Radio Corporation
- Input Transformer: UTC CG-134
- Output Transformer: UTC A24
- Power Transformer: UTC HP-123
- Tubes: 12AY7 and 12AU7 with 6X4 Rectifier

I am trying to figure out what kind of preamp this would be?

Who was Audio Radio Corporation?
Any other info, similar schematic, etc are much appreciated.

Pics Below....
Thank you!

 

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Put new filter caps in and fire it up, put it to work.  Should be a nice piece.    Not a lot to know. 
 
Radio Museum is a good site for info about old electronics. If you're going to find something about it, it would be there (although I did a quick search for "ARC 15" and didn't find anything).

You can tell a lot by the input transformer. CG-134 is a Consumer Grade line input transformer. So it's basically just an entry level line amplifier.
 
The UTC docs call the CG line "Commercial Grade" so hopefully decent quality. Wondering if this will have enough gain and reasonable impedance for a microphone? The primary can be 50, 200, 500 ohms, secondary is 80k to the tube.
Maybe I should wire a three way switch on the primary to give some input options?
 
Unfortunately no. It doesn't have the impedance, the frequency response and most importantly the gain will not be nearly enough for a mic pre.
 
squarewave said:
Unfortunately no. It doesn't have the impedance, the frequency response and most importantly the gain will not be nearly enough for a mic pre.

I'm not sure if we're reading the same thread, or if you're trolling.  It is very close to a mic pre.  The only thing that would make it NOT one would be if it's eq'd for tape or phono use, which is easily undone. 
 
Ok. Well my apologies then. The UTC catalog says it's a "consumer grade" line input transformer that's only good up to 10kHz. That didn't sound like it would make a good mic pre.
 
"Commercial Grade" not "Consumer Grade"! ;D

I did see that it is linear from 40Hz-10kHz, I'm wondering if this was mainly used for radio and maybe the higher register wasn't as important for that purpose? Interesting...

I am excited to get the Ampex 403 in shape, it has the UTC HA-100x and UTC HA-133 on it. I will probably use that for the Mic pre and maybe try this one for a color box (maybe for bass?). Will still try mics on it, and maybe will try to swap the 12ay7 with a 12ax7 or something too.

Thanks for your input guys!
 
Those are frequently better than the catalog spec, they just weren't making promises with that line.  Kinda like Edcor not testing their transformers before they ship them.......
 
I picked up one of those CG-134 input transformers at a surplus place for $5. Mine performs better than 40Hz on the bottom and better than 10Khz on the top....
 
Are you sure it's Audio Radio Corporation? Aircraft Radio Corporation made a whole lot of gear. I've never seen a LF amp though.
 
Conviction said:
Are you sure it's Audio Radio Corporation? Aircraft Radio Corporation made a whole lot of gear.

It looks clear in the picture.

Also Aircraft** was Boonton NJ, this Audio** lump says Los Angeles, Calif(?).

(They may look close from your house but they were a world apart. Before internet and overnight shipping, LA had a distinct electronic subculture, very different from the near-NYC shops.)

It's an utterly mundane utility amp. Microphone, mix make-up, in-house line driver. (Driving audio across Los Angeles wants something bigger than a 12AU7.) LA was full of radio, recording, film-sound studios who needed stuff like this. And small shops doing military and lab gear assembly. Somebody got a contract to do a few. Maybe cheaper than Langevin, or needed it fast. Probably just a few or we would find more traces.

 
The only internet reference I can find is on a quiz site: Who invented the vacuum tube: de Forest: aka Audio Radio Corporation.

Cheers

Ian
 
Started in 1955. Suspended now:

https://californiacheck.com/company/C0299794/audio-radio-corporation

That's about all.
 
After 2 years I am finally getting to this project, and I wanted to document the circuit here. I have started the schematic, but I have a question regarding the circuit.

- The circuit is a UTC-CG134 into the first half of a 12AY7, then 100k gain pot, into the second half of the 12AY7
-This gain stage is followed by a 12AU7 (to drive the output transformer, I think) and out through a UTC A-24
-My assumption until now is that this was used as a line level amplifier, possibly a mic pre where possible

My question is this:
The plate of the 12AY7 (pin 6) is connected through a blocking cap to the grid of the 12AU7 (pins 2&7). But ALSO, the cathode of the 12AY7 (pin 8) is connected to the plates of the 12AU7 (pins 6&1) through a cap and resistor. These are points "D" on my schematic that I have squared in red. Is this a normal arrangement for a line amplifier gain stage? The only places I can find with this arrangement of using both signals from the preamp tube (plate and cathode) have been "parafeed" circuits, phase inverter circuits, and RIAA phono correction circuits. Should I be able to use this circuit for recording as-is, or is this a phono preamp or other that I will need to make circuit changes for? If so, could I just disconnect connection "D" to correct for this? Are there any improvements or modifications I should make to the circuit for recording purposes?

Any insight is much appreciated!
I will clean up the schematic and post a final version as I work through the project.
Thanks!
 

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There's resistors. They do not have any Ohms in them?? Most circuits are a LOT about resistors and especially resistor-ratios.
 

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