[quote author="NewYorkDave"]So, since you've owned just about every tube piece out there (or so it seems), what's your own impression of the BC... umm, MI-11241 preamp? A lot of people talk about it as if it's the best preamp ever, but I suspect many of them have never heard one and they're just parroting what they read on the 'Net. I'm not sure if I've never used one myself--if I did, I didn't take note of it at the time.
Seyer drew those cathode voltages on the RCA schematic? Fooled me... I thought they were original.[/quote]
Seyer used to have a B+ of something crazy like 380VDC; he's removed that. Not LS series iron; custom UTC spec'd by RCA. He's got UTC LS pin #'s listed next to the transformers, and he's got the RCA part #'s listed along with LS #'s rather than the real UTC #'s.
A lot of people have never heard much of anything and like to say the only thing they have heard is the best thing ever. What does it sound like? Very very very close to the BA-11A. Big surprise; same transformers. I have an original MI-11241, and original BA-11A's. 10.2 db feedback in the MI-11241, 27.2 db feedback in the BA-11A. I think Seyer had a BC-2B console, and has never really compared against the tons of other similar choices that are out there to be found and explored. And the gospel was born.
For fun, I restored a beater BA-11A and built in the MI-11241 circuit with NOS resistors, and recapped the BA-11A section with the same NOS caps I used for the MI-11241 section. The 9-pin socket goes in easily where the plug-in can filter cap was. I put in switching that made it possible to flip-flop either circuit in between the iron, for an absolute minimum of possible differences other than the circuit.
I ran a bunch of program material through it and recorded same passages through each circuit. Lined them back up in the computer, lined up the volume difference (part of a db) with polarity reversal method, and then did A/B listening. Only with perfectly matched A/B samples could you tell any real difference. Even the very slight volume difference would make any other type of comparison questionable since it skews the results. I doubt you could make a realistic comparison between two separate units due to the closeness of the result. Certainly not between any kind of clone using other parts.
I gave one person a blind comparison to study, and they came back with the following, which matched my observations:
"MI-11241 has more presence , better definition. Not as good a low end. Doesnt seem as balanced in the low end.
BA-11A is darker sounding, warmer, more dimensional, more balanced thru out. Would pick it most of the time between the 2."
It sounds to me like a cross between the BA-11A (pentode/triode/27 db feedback) and the BA-2A (triode/triode/no feedback), which makes sense being triode/triode 10 db feedback. Slap 10 db of feedback into a BA-2A and leave the volume control full up and you probably have the same result; you have the same gain under that condition.
I thought the mids were nicer on the MI-11241, with more presence and softer top and bottom. The BA-11A was slightly harder in the mids; sounded like a less favourable harmonics blend. The top, bottom, and overall imaging (soundstaging for you Dave :wink: ) won easily. None of these differences were detectable in a clear manner if you threw the levels off from one another by even 1/4 db; that's what a fine hair we're splitting here. The other way to look at it would in terms of what the source is, and which way you want to steer the outcome. It's interesting, but not anything to lose sleep over.
Anyone who cuts up a BA-11A (or any other RCA preamp) to make an MI-11241 is an idiot. I would never waste the time to do the dual-circuit mod again, though it was fun as a curiosity project. It doesn't stand out over the others. They are all different and all useful. If you're building a clone, then why not? Just buy Kevin's kit, unless you're gonna try to track down original or vintage UTC parts. At this level of hairsplitting they are all gonna sound different.