RCA Ba2A Pre amp qestions & answers

[emrr]

That's the BC-2B console preamp. LS-10 and LS-27 is incorrect, but nice substitutes.

 

[CJ]

OT, check out the octagon transformer on this BA 1:

http://i33.tinypic.com/6e2asl.jpg

http://i34.tinypic.com/2ykxh7m.jpg

Look at this weird thing:

http://i38.tinypic.com/34ni89f.jpg

 

[emrr]

Pretty sure the octagonal on the BA-6 is a Thordarson. Probably says on it. All the interchangeable power outputs have the same drawing #, regardless of manufacturer. Usually there's a specific manufacturer # also.

2nd one is the 41-B preamp output. Good example of the pre-war mystery manufacturer iron. Maybe in-house, maybe not. Maybe Ferranti or Amertran.

 

[emrr]

This a copy of a post I put in the Brewery, since it will probably disappear over there.

Comparison of RCA tube type input transformers used in broadcast and theater equipment from 1933-1967. Cases and sizes compared with UTC LS, HA, and A types in the background for comparison. There are of course a few others; for instance 1940's Hollywood Photophone production used Hollytran square can iron, in many cases with the same part #s as the broadcast stuff. There's also some square cased transformers pre-1933. This is effectively representative of everything they used in broadcast audio during these years. The PA/industrial line type is thrown in as another example, and seen on the lower end non-broadcast stuff, as are some novar socketed plug-in types.

Case types, left to right: 1933-41, 1937-41 (DeLuxe line), 1938-57 (2 pieces of differing sizes, program amp and preamp, representative of BA-3A and BA-1A among others), 1945-50 (OP-6), 1945-approx. 1956 (PA/industrial lines), 1954-1967 (BA-21A and BC-3/5/6 consoles). UTC LS, HA, and A in background of two pics.

The first one weighs 10 lbs, and is encased in 1/2" solid copper shielding. You can get 1/2" scale by comparing the thickness of the top on it. It goes with the output CJ showed above.

Only the last two shown are UTC, with the PA type also having been previously available in RCA winding.

 

[CJ]

Ya know, I don't think I could hack that chrome one.
Even me.

 

[craptical]

Wow, nice collection! Should be put to work ASAP. :wink:

CJ: I was planning to build from the BA-2C schematic in the begining of this thread, but with 6SJ7´s (or 6C5´s) instead of the 1620´s. Any suggestions iron-wise?

Found this in one of my docs... Quite a drop (-2dB´s) at both ends. (Diagram shows 60Hz to 15kHz. My apologies for the poor quality!)

:sad:

Best,

/Dave

 

[emrr]

I doubt you can hear 2 dB at those extremes. I use a lot of gear that's much worse. I just bought a new CD that's low passed (for some reason) and -120 dB down at 16 kHz. You can't really tell.

 

[craptical]

Well, I do like gear that has a sound, so I´ll go ahead and build the little beast! (I´ll post some pics later if you like!) Thanks for your help and opinions so far. :thumb:

By the way: What are the function(s) of the meter in this circuit?

/Dave

 

[emrr]

[quote author="craptical"]

By the way: What are the function(s) of the meter in this circuit?

/Dave[/quote]

cathode current

 

[craptical]

It´s alive! Running silently, despite the lack of hum balance control and point-to-point mess etc. (powered by a tube variable psu my father and grandfather built). Bass roll-off is ok/moderate, very defined and classy mid-highs! Sadly not much shiny treble, so I might have to change that output tx anyway... Or could it be something else? Anyway, here it is (my first on the forum!):

Insides (before 600 ohm term. switch was added):

Thanks everybody!

:sam: :sam: :sam:

/Dave

 

[emrr]

Looks nicer than the inside of a real one. Tailor the interstage filter until you get the highs back. You may have to give up some gain to get more high boost. Make some real plots to confirm. Then compare against a real one here:

http://www.groupdiy.com/index.php?topic=29509&start=0&postdays=0&postorder=asc&highlight=mccurdy

 

[craptical]

Thank you, sir! Tweaking, as in increasing resistor/decreasing cap value? The real one doesn´t seem to have too much highs either... Is there any point in trying to imply some of the BA-11A hi-frq compensation network? The Collins looks great, though! Tried to find a schemo, but no success. Well, back to tweaking then! :green:

 

[emrr]

The roll-off is certainly in the RCA iron. Increase the resistor to vary blend of hi-passed signal passing through cap and full range passing through voltage divider; decreases gain. Vary cap value to select boost point. RCA generally selected to extend LPF point or give very slight boost. BA-11 tailoring is in feedback loop, so not an option.

Collins has 25ish dB of feedback loop through a tertiary output winding.

I use the Collins and the RCA side by side frequently. The charts imply more difference than the casual user would observe, and don't necessarily suggest what sounds best either. The RCA has more character and richness. The Collins sounds like clean tube.

 

[craptical]

Thanks again, emrr! :thumb: Tried switching the 600 ohm termination out, voila!, extended (expanded?) treble (but bass suffers). No other tweaking as of yet. Total gain: 43/47.7dB @ 1kHz term./unterm.. Here comes an old school hand-made plot:

Looks as if I have slightly more treble and less bass compared to your original (when terminated). I also tested my proto "6SN7 one-bottle" with A24 output iron, it´s within tenths of a dB 20-20k :shock: But I really LOVE the sound of the RCA!

[quote author="emrr"]The roll-off is certainly in the RCA iron[/quote]
Might go for another "hifi-ish" build to try out the 6C5 option, is there any suitable iron coming to mind?

Best,

/Dave

 

[emrr]

Sounds kind of normal on term/unterm. Try terminating with 1K and see if bass stays close to best and highs improve.

Suitable iron is always a matter of budget; hard to say. Lots of choices.

 

[craptical]

Work got in the way. Had a band in and tried the preamp on guitar, very nice! Anyway: I installed a 5K pot to have some options, but treble suffers long before I get something back in the lows, so I´ll use it where bass isn´t critical. And I´ll leave the term switch in, in case you want the "older" sound... Maybe I´ll look into a parafeed (con)version of the circuit to make choice of OP TX easier.

Thanks

/Dave

 

[CJ]

OK, I found the output iron that emrr sent, full props, no wonder this thing rolls off.

Anybody know the turns ratio of the output?

10:1 WTF, over?

The windings were cooked, so no DCR or Inductance specs were taken.

looks like

Pri: 6700 Turns, split inside - outside
Sec: 690 Turns, split - side by side

Coil Structure: 1/2 Pri - Sec - 1/2 Pri

The Pri is wound back and forth, layered paper, thin wire. 3/16" margins
The Sec sits between the two secondaries and is wound as side by side pies.

36 Layers Pri 185 turns per
14 Layers Sec 50 turns per


One of the Sec pies is reverse wound for CMMR.

Core looks like 50-50 Ni

625 EI

Not gapped, but stacked in a staggered form.
7/8 inch stack

Core staggered into 4 by 4 chunks.

No E shields

Sorry it took forever, eventually everything gets done around here.
Pics if you want.

 

[emrr]

Awesome.  Glad to see this.  Now that you've seen the inside, any feeling on who wound it?  Does it bear any clues?  Pics would be cool if you have them. 

This was a preamp output from an RCA 76 series console, which would date it 1941-50 roughly.  250 ohm secondary; triode connected 1620 (6J7) driving it.  I have seen a significant number of these that were cooked, and have wondered why.  One thought was that over-voltage was easily possible in a 76 console if the monitor amp (well more than half the total current draw) was disconnected.  Tiny wire windings sounds like a good reason too. 

10:1; crazy.  I measure a lot of voltage loss through almost all of the octal era RCA preamp output transformers, and have always guessed 30K or more on the primaries.  This one is about the same as that found on the BA-1 / BA-2 except for the sec Z, and would be substitutable. 

Thanks for having a look.

 

[CJ]

OK, RCA winding print on the way...

 

[CJ]

Just a few bad shots from the ailing Cannon, too many xfmr fumes I guess.