CBS 30th St. 2-1 Preamp/Mixer
« on: September 12, 2020, 02:33:00 PM »
I just picked up this box that was apparently built at 30 St. Studios in 1950.

The person I got it from bought it at auction from RCA 24th St. when they closed, and they gave him all the info on it. Apparently it was moved to RCA sometime in the early '60s.

It's a 2-1 preamp/mixer, obviously all tube, with a Thordarson output transformer. Originally they had two external boxes with input transformers in them, one with Thordarsons wired for 30:50k and the other with UTCs for 600:50k. I just bought a couple of OP-6 transformers from Doug Williams, thinking those will do nicely, at least until I can find the original Thordarsons.

Supposedly they used this in the '50s for ribbons but also to DI electric guitars and basses. As far as I know that wasn't common practice until much later? The two inputs on the front labelled "Microphone" are essentially the same circuit, but the two non-labelled ins are not. The mic inputs run through 4 triode stages in total, with the attenuators between the 2nd and 3rd (mixer) stages. However, the left non-labelled input goes directly to the attenuator, while the right goes to the 2nd stage. Maybe this was to have 2 lower gain inputs that were out of phase with each other? Any other thoughts?


Re: CBS 30th St. 2-1 Preamp/Mixer
« Reply #1 on: September 12, 2020, 02:34:30 PM »
Obligatory gutshot!

Re: CBS 30th St. 2-1 Preamp/Mixer
« Reply #2 on: September 12, 2020, 02:38:06 PM »
Question - does anyone know what these components are? The ones that look they're built into the terminal strips, looks kind of like a silver top hat with a weird kind of arm on it. These are wired between the grid leak resistors of the first stage and ground. The first stage is zero-biased, maybe that has something to do with it? I also assume that the loading resistors for the input transformers were in the box with the transformers, because they're not in this one!

EmRR

Re: CBS 30th St. 2-1 Preamp/Mixer
« Reply #3 on: September 12, 2020, 03:46:26 PM »
Question - does anyone know what these components are? The ones that look they're built into the terminal strips, looks kind of like a silver top hat with a weird kind of arm on it. These are wired between the grid leak resistors of the first stage and ground. The first stage is zero-biased, maybe that has something to do with it? I also assume that the loading resistors for the input transformers were in the box with the transformers, because they're not in this one!

Those are bias cells, I've posted about them somewhere here.    1.5V I think. 

Because of that and the resistors, I suspect it's older.  Those resistors are all prewar types. 

The missing badge is about exactly the size of the RCA/NBC in-house equipment badges. 

The input transformers likely never had load resistors, American stuff almost never does unless it's for line use. 

Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Re: CBS 30th St. 2-1 Preamp/Mixer
« Reply #4 on: September 15, 2020, 03:05:58 PM »
Wow - thanks so much for the info! Seems like the bias cells should probably be replaced, but right now I’m thinking I’ll recap it and see if the cells are still ok. I don’t have a VTVM, and it seems like that’s the only way to check the cells?

I was wondering why this thing was loaded with pre-war value components! BTW the input tubes are shock-mounted, which I thought was a bit odd for something that lived in a studio...  maybe it was something from earlier that they repurposed? Or it was used for location recording? I haven’t worked on too much prewar stuff but I’ve never seen shock-mounted tube sockets.

I do believe it was at CBS and then RCA (the seller was a well known musician/engineer who gave me the info), but all the evidence points to it being built earlier. Is it possible one of the engineers brought the box with them from a previous job?

Very interesting about the loading resistors. Wouldn’t that make the input impedance (especially with a bias cell or grid leak bias) incredibly high?


 

Related Topics

  Subject / Started by Replies Last post
12 Replies
6070 Views
Last post August 30, 2006, 01:10:46 AM
by mattmoogus
5 Replies
3109 Views
Last post December 28, 2004, 05:21:26 AM
by zebra50
CBS EQ"S

Started by wilebee The Lab

6 Replies
1943 Views
Last post January 08, 2005, 03:39:19 PM
by wilebee
15 Replies
5000 Views
Last post December 05, 2005, 11:17:52 AM
by jdiamantis