RCA MI-11241 and RCA transformer questions

Pardon my ignorance. I'm one of those on this forum that know just enough to be dangerous. But trying to learn more....

I've studied this schematic: http://www.twin-x.com/groupdiy/albums/userpics/RCAMI11241PreBC2B.png

I had a RCA MI-11241 built to this schematic by a guy I thought knew what he was doing.  I've since discovered he used the wrong output transformer (UTC A-24). I've acquired a RCA F-1233 output transformer, but I'm not sure how to connect the secondaries for balanced operation. Specifically, what to do about ground? The A-24 has a ground lug, the F-1233 does not.  I'm thinking tie the OT case, output  XLR pin 1 and lug to chassis ground, attach OT pin 13 to XLR pin 2, and OT pin 12 to XLR pin 3. 

Next question. The input transformer on this build is RCA 901046-501. It came from the power amp (the module with two 6V6's) in a BC2B. I determined it's the power amp from Doug's BC2B restoration pics here  http://groupdiy.com/index.php?topic=53576.0 Does anyone have info on this transformer? Is it suitable for use as the input transformer on MI-11241?

The funny thing is with this input transformer and the A-24 on output, it didn't sound bad. Though the DC on the OT probably saturated that A-24, huh? Can any damage occur to this transformer due to the DC on the primary?

As always, I appreciate your guidance and advice. Happy New Year!

It's not the right transformer to be on a BC-2B monitor amp.  If it truly came from there, it was a replacement from used parts stock.  It's earlier generation. 

Output case grounds through the mounting screws.  The way it's drawn, 2 connections, that's hot and cold 150 ohm output. 

A-24, that's funny.  A-25 is cheaper and correct (enough) for the job.  If it still works there's no damage.  It probably just distorted like a mofo, and you might not have even noticed this sort of low harmonic content distortion as such. 

Thanks Doug!

I double checked the 901046 came from the power amp. I still have it here and compared it to your renovation pics. The module was stored in my attic and the power transformer has leaked black goo out. Wonder if it's still any good?

Larry Seyer and friends bought and parted out a BC-2B console in the early 90's. I got a mic pre module, power amp module and the frame.  Sold the mic pre a few years ago, big mistake. Sure wish I had hung onto all that stuff now. Along with a couple of 50's Strats I sold as well...

I think he just had the A-24 laying around, probably didn't know what it was worth.  Never noticed a lot of distortion with it, but the pre sure had lower output than one would expect from 30db of gain. Would always have to run it into the line input of my 1073 to get enough level, and I mostly use tube mics with the RCA. Looking forward to hearing the difference the F-1233 OT makes. A very experienced older electronics designer I know said the output transformer should make a much greater difference in sound in a circuit like this than the input transformer. What say you?

The other module transformer is not power, it's output. 

The input transformer that is supposed to be on that series is more linear response, so yes it makes a difference.  How much?  Your call. 

You should have 40dB out of this preamp strapped 600 out.  Around 34-36 with the stock 150 strapping. 

Lassoharp and I compared a BA-2 circuit with the correct output and an A-25, the A-25 had greater high frequency extension, the bottom looked the same, the sound was indistinguishable.  In that case I think the input would make a far greater difference.  There should be a thread around here somewhere. 

You are connected tangentially to the Seyer console!  I've always wondered if there was some clarity to be had around his commentary long ago; was his commentary meant to be accurate, or meant to cover his idea of an improved modification?  So many people have taken it as gospel truth about those modules, and it's just not correct. 

Regarding the Larry Seyer/RCA mic pre schematic - Larry won a few Grammies for his work with Ray Benson and Asleep At The Wheel. Ray owns a nice studio here full of cool vintage gear he collected when touring about the nation, visiting radio stations and picking up old gear they had decommissioned.  They have the API console from RCA that Elvis recorded on.  Anyhow, they became quite fond of these RCA mic pres from BC2B consoles and thought they were the best sounding pre in the building. So much so that they were going to tool up and manufacture a modern clone. They never got it off the ground, though my old studio partner Cris Burns was working there as a tech and learned enough to launch his own version: http://www.crisburnsaudio.com/cris_burns_audio_products.html

Just a guess, but I'm thinking they were trying to find suitable transformers to build clones. Since the original RCA spec'd UTC iron wasn't available,  the UTC LS10 and LS27 were probably deemed suitable replacements. That is just my educated guess. He might have gotten confused about the iron differences over time.  Probably posted that schematic just trying to share the love as best as he knew how. He's a good guy and great engineer, producer and musician.

Regarding the OT, I paid pretty dearly for the F-1233 and had to modify the case it's in to make it fit too. Sounds like I would have been better off with the UTC A-25, would have fit right in place of the A-24 and probably sounded better too. Damn. Oh well.  How do you strap these transformers, for 150 ohms or 600 ohms?  Seems either way would be OK for 15K ohm bridging inputs on interfaces and other gear. Can you please advise how to strap this OT for 600 ohms, or point me in the right direction?

Regarding the input, don't have an original F-1234 to compare to the 901046-501 in my preamp.

I've acquired a UTC A-10 and A-24 with shields. Thinking of using them to build a tube pre. What do you think?

I'd guess you are right in your assessment of Larry's meaning.  I had seen the clone that made it to market, and there was a K&K Audio clone kit too for awhile. 

You do have to pay dearly for that output, and a high percentage of them are bad too.  Easily bad from PSU failures in those consoles, such as deciding to not use the monitor amp and pulling the 6V6 pair out, thus raising the entire system B+ higher than should be allowed. 

The windings are strapped parallel for 150, strap the winding series for 600. 

Doug, thanks for your expertise with all that is RCA.

The more I dig around in this preamp build, the more I'm discovering. The PSU is delivering 5.8V for the heater and 245V for B+. I'm thinking that's too far out of spec. What do you think? Am I being too picky about this?

Those voltages are no big deal.  I'd rather run B+ low that risk the fragile output, and you will hardly find a noticeable difference.  Heaters will work fine there, getting low but fine.