RCA BA-11A Anyone have a schematic?

Found plenty of info on Google for BA-1A and other RCA tube preamps, but I couldn't find a clean schematic for the BA-11A.

Does anyone have one they could post?

thanks

I am also looking for this schematic as well... but no luck as of yet.

Thanks.

Best I've got is here.


Variations are an earlier and more common input transformer with 150 / 30 ohm only, and I've seen stock 150K feedback resistor in some units.

Anyone who wants to host this should go ahead; I'm about to hit my bandwidth limit this month so it will probably dissappear.

Here

EDIT:Ha! We posted at the same time.

You guys are awesome. Thanks a lot !!!!!! :grin:

Now if I could find some RCA 1620s. A few years ago when I last played with these preamps 1620s were all over the place. Seems they've dried up.

If anybody has a few they're willing to part with feel free to email me here:

22busy at cox dot net.

Thanks again!

p.s. Hi Greg S., it's BB trying to help Mike get these things up and running.:wink:

1620 = 6J7

Dave's schematic is the earlier and more common input transformer.

Presented in neither are the recommended options for bass and treble boost. Those exist for the BA-1A also, and some others.

Thanks, Dave & Doug. This place is an information gold mine.

Cheers, BB !!! :guinness:

[quote author="emrr"]Presented in neither are the recommended options for bass and treble boost.[/quote]

Look here.

That's the part never seen; thanks Dave.

Those changes give you +1 db at approximately 40 Hz and 15 kHz according to the chart in the manual. + 1/2 db appears to be about 100 Hz and 11 kHz. This is mainly about compensating for bandwidth losses in long signal chains with many amps. I shot RTA's on my BA-11A's, and would think the low portion might sound like a boost, while the high portion might sound more like an extension of response. The lows on mine remained very flat to a very low frequency, but the highs started to fall significantly after 15 kHz.

The 2 different schematics have been eye opening. I have a few different RCA preamps, a couple of BA-1a and a few BA-11a. But, I was always under the impression the input transformer on my BA-1a had been changed to something non-original.

All the photos I've seen of BA-1a show 2 circular transformers (UTC?). Also the only schematic I'd seen indicated an input transformer with 7 terminals like the one on Dave's schematic. My BA-1a have the proper output transformer but the input is in a large square case and it has 9 terminals as shown in Doug's schematic. So now I'm beginning to wonder whether they are original. The input is definitely not UTC but the pin structure may be correct and it's substantial in size. In fact it's about the same size as the large, square UTC input tx on the BA-11a. Hmmm.

Anyway, thanks again for the invaluable help.

1) the square transformer on the BA-11A next to the connector is the output, not the input. Same with that position on the BA-1A.

2) I have seen both ADC and Hollytran square inputs with the same RCA part # as the round transformer on the BA-11A.

3) I have never seen a square transformer on a BA-1A, though a BA-11A input would be perfectly fine on a BA-1A.

4) The BA-11A schematics we've posted have nothing to do with the BA-1A.

5) Everyone wants to assume the BA-1 and previous RCA generation RCA transformers are UTC, solely because starting with the BA-1x series about 1950 you see RCA iron with both RCA and UTC stamps. I believe the earlier iron to be made either by RCA themselves, or another manufacturer. My pet theory is Ferranti, and I've found a little supporting evidence. It bears little resemblance to the UTC designs other than basic footprint; different terminal boards, different style lugs, different fine details of the case structure.


What #'s are on your square BA-1A transformer? That answers the question.

Hmmm, don't know why but I assumed RCA changed the placement of input and output transformers between the BA-1a and BA-11a series. Don't have 'em here but I'm sure you must be right that the rear transformer in both is the output.

That makes it even more confusing. I'm pretty certain the square, rear tx on the BA-1a is an ADC. I'll try to get the number off of it.

Been using my BA-11a for years and love the sound of 'em. Big, ballsy and natural. If I had to pick a desert island preamp, that would be it. Neve, API and the like are very nice but IMO don't hold a candle to the BA-11A particularly when it comes to acoustic recordings.

I'm pretty certain the iron on the BA-11a is UTC because of the UTC stamp on each tx. Curious if anyone has compared the BA-1a and BA11a. The circuits are somewhat similar. Biggest difference I could see was in the configuration of the output stage. Both have octals wired as pentodes for the first stage. The BA-1a uses the second tube wired as a pentode too, but the BA-11a configures it's second pentode as a triode.

The BA-1as have been sitting unused ever since I got 'em years ago. It looked like someone had been futzing with the circuit and I thought the tx had been changed. So rather than just power them up I put 'em aside as one of those get-to-someday projects. I sold them to a friend and we're trying to get them up and running. Also trying to decide whether to keep them stock or convert them to the BA-11a circuit which I know sounds great. So, they're at my friend's place now but I'll try to get pics to post in case anyone is interested.

Thanks again.

I've compared them, and shot frequency response graphs. Used same set of tubes to minimize differences, and both had signal and electrolytic caps replaced (BA-1A oil filter caps left alone). BA-1A was flatter a little farther out in both directions, and I suspect there's more feedback in that circuit and it accounts for the difference. Which wasn't much.

The transformer #'s are posted on the BA-1A schematic, though I've never seen the XT- #'s on the actual transformers, just the 6 or 9 digit #. RT-# migrated to XT-# migrated to 000000-000 type numbers along the way, with XT-# pretty much falling by the wayside by 1950. The 6 and 9 digit #'s are the RCA drawing #, where the earlier XT-# was a part # and also had a separate drawing #. You'll note the RCA part # in the manuals is different from the 6 digit drawing # on the transformer.

UTC is standard on BA-11/12/13/14 on up through BA-31/71 of the SS era. You do see some Thordarson outputs on BA-4's and 14's sometimes. BA-6's seem to be a mix of different manufacturers also.

I've wondered if the ADC's could possibly be service parts made at a much later date. The one I've seen looked out of place, but had the right part #. I've not seen ADC iron as standard on any RCA I can think of ATM. Though there are ADC / RCA repeat coils which were sold as raw units.

The square silver Hollytran transformers (which don't say Hollytran on them) are on products from the Hollywood RCA Photophone division. I've seen BA-11A's that were not labeled as BA-11A, but only with an MI-# and a slightly different feedback circuit. There's a BA-1A circuit shown in another Photophone equipment package manual, and it too was only identified by an MI-# variant.

The BA-11A is rated for 6-8 db more output level than the BA-1A, and I am pretty sure the BA-1A output transformer can't take the same output level as the BA-11A. I don't think you'd find the transformation worth the effort; better to sell BA-1A's and replace with BA-11A's if you are certain of the preference. But I'd definitely listen first. Nothing wrong with the sound of the BA-1A. Only thing that makes sense for a BA-1A transformation would be the BA-2A which uses the same transformers and has an interstage gain control since there's no loop feedback. All triode, too.

All BA-1A's I've seen have the 7 terminal input. The BA-11A has the 2 impedance 7 terminal version at first, and then the 3 impedance 9 terminal version later on.

They would look futzed with if someone's added the frequency compensation additions; they were likely not ever supplied that way.

I have a BA-11A on the bench that belongs to a friend of mine. I don't think this thang has seen power in many years. I have a few questions before I fire it up.

1. I don't trust the original Aerovox plug in cap. The cap is a 40Uf/450v, and a 40uf/25v built on a octal base. The cap itself says pin 1 is a common ground, pin3 is the HV cap, and pin 5 is the LV cap. It has a 4th pin (7) that I assume in not used. My concern is that there are wires tied to pin 7. I built a replacement and I ignored pin 7. Is this OK?

2. All the wiring and components appear to be original. The only signs of a soldering iron touching anything inside are the primary of the input transformer, and the secondary of the output transformer. All other connections are clean and have a pink QC paint dot on them. The output has been rewired for 150 ohms. Should I leave it or switch it back to factory 600 ohms?

3. The input transformer does not match the one shown on NYDave's (miss that dude) schematic. It is a 9 pin UTC transformer marked G-9634 on the outside and B-149102-1 next to the pins. The secondary doesn't look like it's been jacked with. Pins 4 & 9 are grounded, and pin 8 goes to the grid of V1. Right now pin 2 is tied to the hot side of the input and pin 6 is tied to the low side of the input. There are obvious signs that pins 1 & 7 have been desoldered. I suspect (ok I'ma guessin) the input has been changed from 600 ohm to 150 ohm and it should probably go back. Any clue as to the actual wiring diagram of this transformer?

Any answers, comments, suggestions, or slaps upside the head are greatly appreciated.

Thanks!
-Richard

Pin 7 on the cap might just be a cheap sub for a terminal.

Was it connected to the metal can internally?
If not, just tie the wires together and shrink wrap it, or install another terminal.

If it was grounded to the case, and the case was grounded to the chassis, then you need to ground all the pin 7 wires.

I would run it at 600 ohms.

The 150 might mean that these were wired into a console or something that wanted a lower impedance.

With all the modern inputs at 10K today, 600 is the way to go.
More output.
But check tone, maybe you will like 150 better.

Those private UTC numbers are a way of showing what was going on in the biz back then.

They sure are a pain, no?

I bet all the old company docs are in the shredder by now.

That guy for Magnetica might have the numbers written down somewhere.

Those private UTC numbers are a way of showing what was going on in the biz back then.

They sure are a pain, no?

I bet all the old company docs are in the shredder by now.

That guy for Magnetica might have the numbers written down somewhere.

Well it's UTC wound with unontainium so the part numbers have to be a secret. :roll: I was hoping Doug might chime in with the other schematic, the one with the 9 pin input transformer. I guess I could easily rig up an experiment to determine polarity and impedance, but I'm always looking for an easier way than thinking. Maybe someday the RCA Master Index (MI) will magically appear on the internets.

I did test the existing cap last night. Neither one will hold 5 volts but I was able to determine that pin 7 goes nowhere.

The 600 ohm question was kinda dumb but hey, look who's asking.....

-Richard

Your pin #s sound right for 150 input connection with CT and one side of sec grounded. (EDIT: SEE NEXT POST) I'd leave it there; remember this is an unloaded secondary and 150 references the suggested drive Z from the mic, not the actual load presented to the mic. Listen both ways if curious; it will depend on the mic which you like best probably. Input is connected as a single winding with center tap (probably grounded) covering roughly 30/150/600. That makes 9 pins counting 2 for secondary. Without having opportunity to look at my reference materials ATM that would suggest faraday shield tied internally to case. Polarity should always be checked on any of these old modules. Usually the manuals give reference, but field hacks abound.

It gets even trickier. There are model #s, Master Index #s, drawing #s, and replacement part #s. Any single RCA transformer may have an RCA drawing #, a manufacturer # (ADC, Hollytran, Kenyon, Langevin, Stancor, Thordarson, UTC, etc), and a replacement part #. The replacement #s are only found in the manual parts lists, and do not reference the MI #s, so you have to have several pieces of evidence before you to compare and ID. The MI #s for whole units are available in the catalog price lists (if still found in the back pocket), though you'd have to find one from about every 3 years for 45 years, and they seem to sell in the $50-$400 range. The required investment makes free revelation of hard-earned data unlikely. I've only seen one replacement parts list and it was from about 1972, and it was not fully revelatory either. I would seriously doubt any company has held on to 40-60 year old custom part #s as companies have changed hands.

found it:

BA-11A 2nd version with 30/150/600 ohm input xfrmr

Schweet!

Thanks!
-Richard

emrr said:

found it:

BA-11A 2nd version with 30/150/600 ohm input xfrmr

Click to expand...

thank you doug !