RCA MI-12241 preamp: kind of like a BA-2A with EQ addition

[emrr]

I just dug out a set of these RCA MI-12241 preamps I've had sitting around for a while, and thought I'd see if anyone had the official schematic laying around. Figure I might end up having to draw it out like so many other antiques (EDIT: see below link). It fits the footprint of the BA-1A or BA-11A, but is from the industrial line (PA's for factories, schools, etc.) instead of broadcast line. Next biggest identifying factor is front panel gain and compensation controls. You see people list these on ebay as compressors, since compensation is only spelled out so far as 'comp.''. Pair of 6J7's wired in triode, maybe like the BA-2A, using the BA-1A / 2A output transformer, and a smaller input more like the OP-6 can. Please let me know if you happen to have the official info. There is also an MI-12241-A version that looks like it's the same thing.

Thanks!

 

[rafafredd]

Figure I might end up having to draw it out like so many other antiques.

Yes, most probably you will have to draw it, and off coarse, let us see it!

I never heard of this one, but maybe you might get lucky and somebody will come to your help, and our pleasure!

It seems that it does use a gapped output transformer, just like the RCA BC2B mic pre. Would be nice to see how they implemented gain control.

 

[emrr]

Visually, it appears to be trans / tube / EQ / gain pot / tube / trans. I have yet to see a parallel feed output on an RCA tube piece other than the OP-6.

 

[emrr]

As I thought, more or less the BA-2A audio circuit with a tilt EQ added, and no power supply.  Wiring to the connector is different from the standard broadcast line BA-1A or BA-11A modules; I re-wired mine to match the BA series so I can swap out in my RCA rack system.  The rated output and frequency spec at the top is quoted from an RCA system install manual, and not from actual measurements.  The voltages and level per stage are from actual measurements. The input is the same as found on most of the industrial series equipment from this era.  The output is basically the same as found on the BA-1A, BA-2A, and various other theater preamp systems from the late '40's-early '50's. 

RCA MI-12241 preamp schematic

 

[12afael]

Doug, you can upload it here.

http://groupdiy.twin-x.com/

12afael

 

[emrr]

Plots of the RCA MI-12241 preamplifier from the industrial / PA series equipment. Modified 'EQ bypass' response shown, and BA-2A response included for comparison.

 

[clintrubber]

Hmm, a q&d simulation I ran for that tilt-section didn't gave as smooth curves as your measured ones. The tilt-freq is in agreement, but the slope ends well before 10kHz, from there it's flat.

And below the tilt-freq the lows are more an 'S' than sloped.

(A picture would have said more... :wink: )

 

[emrr]

Be curious to see your sim plot, if you have a screen capture. I wouldn't be surprised at all if the iron signature is affecting the extremities of my plot. My flattest plot (actually -2 setting on the knob) pretty much looks like the plot in the 1950 RCA catalog.

 

[clintrubber]

Did just a simple simulation this afternoon, just the passive tilt-section.

Assuming a certain output-resistance of the preceeding tube, too lazy to calculate or look up, but I tried both something like ~5k & ~40k IIRC and (understandably) it didn't bring the weird curves the sim showed in better agreement with your graph. Will post a screengrab.

 

[abby normal]

Thanks for posting that Doug. Cool stuff for sure. :thumb:

 

[Murdock]

Sorry for reviving this old thread, but I'm interested in building this preamp.
Is the 1Meg EQ pot a log or linear pot?
And are the two wired in "reverse"?
And the Gain pot is probably audio tapper, right?

 

[Sorr]

Sorry for reviving this old thread, but I'm interested in building this preamp.
Is the 1Meg EQ pot a log or linear pot?
And are the two wired in "reverse"?
And the Gain pot is probably audio tapper, right?

The dual 1meg pot is Lin. Gain Log.

 

[emrr]

All info is on the schematic

 

[Murdock]

Thanks!
I know, and I simulated it in spice.
But what is the modified 'EQ bypass' you wrote about?
Because, like you plot shows, the 0 position is not a bypass.

 

[emrr]

You lift the eq section