Wow! an RCA transformer I don't know! MI-38482

[emrr]

RCA MI-38482

Nine pin plug-in transformer, fatter case than the usual RCA and UTC 9 pins. 

No trace of what this is for/from.  Anyone know? Solved - see below  I was thinking one of the late 60's early 70's SS remote amps, but I'm not finding this in any documentation. 

125/600 primary
pins 1-3 7.5Ω DC
pins 1-4 16Ω DC

A separate 10K bridging input winding
pins 5-6 139Ω DC

pins 8-9 91Ω DC = 10K
pins 2-9 349Ω DC = 60K

Useful for tube input, mic or bridging. 

 

[mjrippe]

You stumped me.  First thought was BN-6 or BN-7 type mixer input, but nope.

 

[rackmonkey]

From an early 70’s PA unit. Found it in a catalog.

Interestingly, this is from a period when RCA appeared to be rebadging McMartin PA equipment. I have several plug-in mic pres that I’ve confirmed go into PA units similar to these that are housed in the exact same metal cans with octal plugs, some labeled McMartin and some labeled RCA. The circuit boards inside are identical. The PA unit shown in the catalog page below looks like a McMartin unit. The number escapes me, but you can find it in one of the McMartin catalogs from American Radio History.

my bet is that this transformer is a rebadged McMartin MT-4.

 

[emrr]

Bingo!  A winner!  Thanks!

 

[mjrippe]

Excellent work Rackmonkey.  I have seen one of those RCA branded McMartins before but never thought much about them.  So the transformer is probably a McMartin MT-4, drawing attached.

 

[emrr]

a McMartin MT-4, drawing attached.

Any other published specs?

 

[rackmonkey]

Any other published specs?

For most of its life, it was used to make a high impedance input into a low impedance one. Primary 600/secondary 50k. So they can be used as tube pre inputs. Level is mic level. I have the specs somewhere around here. I'll dig them up.

Later on, McMartin had a 600/600 plug in that replaced the MT-6 (600/600) which they also called MT-4. No idea why. Only seen 1 of them and I have seen/own many MT-4s.

 

[alexc]

Sup-oib !

 

[rackmonkey]

Okay, it's been a long time since I'd messed with any of these MT-4s. I'd forgotten just how hard it was to come up with any more published specs on it. Lots of testing results by individuals, but all of it was around determining DCR/winding info prior to the diagram Mr Rippe posted above being widely available. That diagram is the sum total of published info, other than what I've attached here. The image below is from the manual for a McMartin LT-80B PA amp, and is the closest thing to acknowledging the max signal level. It's probably not much above 0dbm, given that there are instructions on a 47k padding resistor for high output mics.  Although that note below seems to reference the circuit itself. So the transformer may not be the issue there.

In my experience with them, they are full audio spectrum units, if a little on the darker side. I have plots of one w/150 and 600 ohm source impedances in my Google Drive transformer response curves folder (I did a post a long time ago with a link). Level handling is as expected based on the information in the image, IIRC. I ended up using one in a single ended 6N7/6C5 pre I threw together that has an octal socket for swapping input transformers, to allow for color changes. I haven't used that pre since I bought some SA-70s from Doug.

So that's what we've got. I've seen a lot of posts about those transformers across many different forums. They're obviously abundant as a lot of folks have had questions about them over the years.

 

[spicemix]

Schematic for McMartin LT-300A, which featured one of these included on the one I bought, may be of some use. Germanium era. Now in the technical docs:
https://groupdiy.com/threads/mcmartin.90217/#post-1193464