CBS 30th St. 2-1 Preamp/Mixer

[gnomefather]

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?

 

[gnomefather]

Obligatory gutshot!

 

[gnomefather]

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]

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. 

 

[gnomefather]

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?

 

[emrr]

Thought about this again....pre-war parts all around, pre-war VI meter too. 

 

[abbey road d enfer]

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

Some mic manufacturers specifically recommanded unloaded xfmrs for best response.
Specs for the BK6 clearly indicate the load to be an unloaded xfmr.
There is an RCA document that explains that, but unfortunately it's too big to upload.

 

[Winston OBoogie]

I don't know this particular schematic but I'm assuming the bias cells just provide either grid bias of negative 1.5V voltage, or a positive 1.5V bias to the cathode.

Little modern rechargeable batteries will work fine as replacements.
Or go to cathode bias and just put an led under the cathode that has a 1.5V drop at the current in the valve.  Red LED will get you in the ballpark.  Assuming there's enough current in the valve stage to turn the LED on.

If not enough valve current to turn on the LED, then put a resistor from B+ straight to the cathode LED that sources about 5mA at the H.T. to 1.5V drop.

 

[gnomefather]

Some mic manufacturers specifically recommanded unloaded xfmrs for best response.
Specs for the BK6 clearly indicate the load to be an unloaded xfmr.
There is an RCA document that explains that, but unfortunately it's too big to upload.

Does this hold true for more modern mics? I don’t own any old ribbons, mainly using this with dynamics and some new ribbons (Coles 4038 reissue, Royer 121s, Cascade Fathead).

I found a schematic in an old Thordarson catalog on which my pre was definitely based. If I remember correctly the mixer stage is the only addition... I’m about to head to my studio, I’ll upload the schematic and info pages when I get there.

I don't know this particular schematic but I'm assuming the bias cells just provide either grid bias of negative 1.5V voltage, or a positive 1.5V bias to the cathode.

Little modern rechargeable batteries will work fine as replacements.
Or go to cathode bias and just put an led under the cathode that has a 1.5V drop at the current in the valve.  Red LED will get you in the ballpark.  Assuming there's enough current in the valve stage to turn the LED on.

If not enough valve current to turn on the LED, then put a resistor from B+ straight to the cathode LED that sources about 5mA at the H.T. to 1.5V drop.

I ended up reworking the first gain stage for the same bias point using cathode bias, just a resistor. Love the LED idea tho, I have a stash of HLMP-6000 red LEDs that I’ve used in the past.

I would also love some help integrating my external PSU and input xformer box, mainly how to deal with grounding. The preamp itself has a star ground point hooked up to the chassis. I’ve been really liking the sound of this box (esp on electric bass) but the hum is higher than I’d like... is it worth separating the audio ground from the chassis and running a separate ground back to the PSU? Right now I just have one ground wire connecting the PSU and the pre, going from the star ground of the pre (again, tied directly to the chassis) to the star ground of my PSU.

 

[abbey road d enfer]

Does this hold true for more modern mics?

Somewhat, yes. Many current ribbon mics are just remakes of old designs and thus have the same idiosyncrasies.

I don’t own any old ribbons, mainly using this with dynamics and some new ribbons (Coles 4038 reissue, Royer 121s, Cascade Fathead).

Experience has told me that I prefer higher impedance with all dynamic mics. the same electric principles being at work, it makes sense that ribbon and dynamic mics behave similarly.

I would also love some help integrating my external PSU and input xformer box, mainly how to deal with grounding. The preamp itself has a star ground point hooked up to the chassis. I’ve been really liking the sound of this box (esp on electric bass) but the hum is higher than I’d like... is it worth separating the audio ground from the chassis and running a separate ground back to the PSU? Right now I just have one ground wire connecting the PSU and the pre, going from the star ground of the pre (again, tied directly to the chassis) to the star ground of my PSU.

Some data is missing from your description, particularly how the PSU is laid out, and are the preamp and PSU chassis also grounded, e.g. via rackmount?