Anyone know the B+ on an RCA BC-2b?

Just found a schematic for it, but didn't have a B+ in it.

http://www.larryseyer.com/content/view/59/39/

Thanks.

My bad, the schematic that was posted here is on that page you linked to. It's likely the same as the BC-3C, which is 275V. You could verify by taking a look at the tube's plate curves, and figuring out where the 1k cathode resistor will idle the plate.

Since both stages are running off the same supply, it'd be easier to draw the DC loadline for the first stage, since the value of the plate resistor is known (220K). The cathode voltage is shown, and that tells you both the grid bias and the plate current. The rest falls into place pretty easily.

So, when you've figured out the voltage of the supply on the preamp side of the 2.7K decoupling resistor, you can take the value of total plate current from both stages (again, easily deduced because cathode voltages are shown) and from that figure out what the voltage on the input side must be.

the good part with tubes is the gross intolerance to supply voltage.

this in mind, try for a mojo synergy between volts, amps available, I/O iron and tubes.

Bueller? Bueller?

:razz:

OK, plate curves on the RCA datasheet are drawn in one-volt steps of grid bias from +1 to -7V. -4V is close enough to -3.8V, so we'll use that curve.

3.8V across 8.2K = 0.46mA.The -4V curve intersects half a milliamp at about 160V. So there's our plate voltage.

Voltage dropped across plate resistor = 0.46mA * 220K = 101V.

101 + 160 = 261V supply after decoupling.

Total plate current of V1A and V1B is 0.46mA + (3.5 / 1K ) = 4mA

Voltage dropped across decoupling resistor is 4mA * 2.7K = 10.8

261 + 10.8 = 271.8

A few volts more or less... It's not critical. So Skipwave's guess is right.

If you look where the -4V curve intersects 260V (which is not far from where the plate of V1B will sit due to the low resistance of the transformer primary) it lands right around 3.5mA. So it all checks out.

the thing ya gotta remember with these plate curves is that it's all ballpark--but usually close enough.

If you're planning to DIY one of these, good luck finding a suitable output transformer that doesn't cost lots of money.

use search here. i mentioned it along with a lot of other gory details in several different posts. I looked at BC-2B and thought you meant the whole console, because the preamp in the BC-2B is the MI-11241. Seyer's transformer and voltage info is wrong; it's just been taken as gospel since it's been posted for so many years.

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.

http://www.groupdiy.com/index.php?topic=15107&highlight=bc2b

http://www.groupdiy.com/index.php?topic=2232&postdays=0&postorder=asc&highlight=bc2b&start=15

[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.

Thanks for the comments... It's a nice reality check.

Don't know if you've seen it, but my "One-Bottle" pre started out as my sort of Madman Muntz take on the MI-11241. Actually, this thread got me thinking about the One-Bottle again and now I'm considering some refinements. But that's a subject for another thread.

I remenber reading somewhere that the BC2B B+ was 365v.

The nominal B+ for the preamp is 285V. This is directly from the manual (which I just found).

Schematic with correct voltages, adapted from the manual:

LINK

As you can see, the plate curves and simple arithmetic got us pretty close to the actual values.

I left the transformer info out, since it wasn't explicitly spelled out in the manual. But someone on Usenet who owns an original BC-2B measured his preamp transformers as 30/150:33.7K input and 12K:600 output.

[quote author="rafafredd"]I remenber reading somewhere that the BC2B B+ was 365v.[/quote]


That was Seyer's f***d up value I referred to earlier. Might be 365VDC if you don't power the PP 6V6 monitor amp in the board, and use the same PSU without compensating for the load change. :shock: The PSU is RC filtering only, no inductance at all, and there's a resistor in the preamp branch to strap for either four or six preamp channels worth of current load. So, start powering an original board with various parts disconnected (and no manual in hand) and you'll arrive at a different VDC result.


[quote author="NewYorkDave"]
I left the transformer info out, since it wasn't explicitly spelled out in the manual. But someone on Usenet who owns an original BC-2B measured his preamp transformers as 30/150:33.7K input and 12K:600 output.[/quote]


I had an NOS/NIB input xfrmr once, and the RCA box stated 30 ohm is 1:46 ratio, 150 ohm is 1:20.5 ratio. That would appear to be a 63K secondary.

12K:600 for the output would be only a 13 db ideal loss. Most 15K:600 outputs I've checked measure close to a perfect 14 db loss. I measured 18.6 db loss through the output (BA-11A in operation with 1K tone) with a 620 ohm load on the 600 ohm secondary. Not accounting for losses, that would appear to be roughly 42K:600. Considering 1.1db of winding loss, that would translate into 33K:600, and a 2 db winding loss would be 27.5K:600.

I'm more sure about my math on the input than the output; someone double check.


In real life, on a BA-12A with 285VDC at pin 1 of the output trans, I measured 245VDC on the output plate (xfrmr pin 5) and 100VDC on the input stage plate. The BA-12A skips the 2.7K isolation resistor, being a stand-alone BA-11A with PSU. Haven't noted real life on a BA-11A, but the manual states V1 plate 80VDC and V2 plate 256VDC with 285VDC PSU.

This will make you feel wacky: The MI-11241 / BA-11A / BA-12A output transformer has DC resistances of 1750 : 440 ohms. Yes, 440 ohms for a 600 ohm rated output. I measured four of them to get this average value, and they were all from different batches.

The input looks more standard at 11 (30 ohm) and 22 (150 ohm) to 5K (63K ohms) DCR.