RCA 76B2 Mixer schematics, & info

Or any info on the RCA 76B2 Mixer..

Cheers ........................Gary O.

I've never heard of an RCA 760 mixer. Tell us whatever you happen to know, link to a pic if you have one, etc. Is it tube? Solid state? About how old?

RCA usually had an "MI" part number for their professional audio products along with the model number. The more information you can give, the better.

I would guess you mean 76 rather than 760? Everything later than that (1950ish) starts with BC- or BCM-.

RCA 76 sounds familiar; I think I might actually have docs for that one. Let's hope Gary just added a zero by accident :wink:

76B ?

"O" Ive had jokes about my O before :grin: Iv done it again I meant to type the O but it doesnt exist the mixer is a 76D.... perhaps my name isnt gary O ..imight be gary nothing
Anyway I was just reading about some fella called Elvis and sun studios, was trying to find out what mics & electronics his voice went thru,that was what they had RCA76D, before a "presto" sadly no model number but was said to have warmed things up a lot.
Im looking for new project ..say mic pre from that era hi volts carbon resistors... bit like the BC2B.

Thanks guys ...............O so sorry {terrible} about my extra O ..Gary O.

I don't have anything on the 76D, but here's the 76-B2.
Have fun!

Coooooooool ! Thanks Dave 1x 1620, 2 capacitors, 2x transformers and 3 reistors plug my mic in and sound just like Elvis..... I wished.

Those mic pres are so sweet gonna get me soldering iron out and solder 1 up, could be just what I need for little pre I always have too much level.

Thanks once again Dave.


Cheers guys...........................Gary O.

Note that the resistance figures given for the transformers appear to be DC resistance, not nominal impedance. I'll see if I can find more info about them. Also note that the preamp output transformer must be a gapped type.

Not to toot my own horn, but if you want something simple with moderate gain, check out the "One-Bottle Preamp." It uses common transformer types and a single, inexpensive tube.

Thanks Dave sadly im not as technical as most here like yourself I was just in the mood to make something from the past even if its bad for more of an experiment I thought about making just the pre from the Altec 438 compressor too, iv seen your great little 1 bottle pre and toot away you have something to toot about I may have a serious go at that 1 one day too.
to use I have a huge Sowter 9040 5:1 single ended triode tran not that I totally understand about all that and a Sowter 9045 from a RCABA1A pre I messed with.

All the RCA single ended output transformers before the BA-11A are more or less the same ratio, with slightly different secondary taps depending on the usage. That output is something more than 15K, I suspect 25-30K, and is about the same as the BA-1A which has a max output of about +12dbm. The BA-11A is about the same ratios, but with +18dbm rating.

Most of the RCA inputs are 50Kish secondaries, though the pre-war inputs are sometimes 30K secondaries.

Most of the 76 series consoles are about the same; that schematic gets you about 90% there for any but the 76A, which doesn't seem to exist in anyone's collection. If you have one, I want it. Most of the B/C/D/E differences will be # of channels, changes in functions, change of metering type from VI to VU, and very little if any in actual circuit blocks.

The 76 console preamps are either four or six on a single chassis. The circuit was sold as standalone modules called the 85-B, and has about 30 db gain.

The program and monitor amps are about the same as the 84 and 82 standalone amps. Obviously, this all relates to the 86 limiter amp.

Those xfmr ratios make sense. 50K secondaries were definitely common on mic input xfmrs in that era. As for the output transformer, the triode-strapped 1620 in that circuit will have a plate resistance of about 12K, which indicates an output transformer with a 25K (more or less) primary was probably used.

So, with a stepup of about 23dB in the input xfmr (250 ohm primary as specified in the manual), about 23dB gain in the bottle, and about 16dB loss in the output xfmr:
(23 + 23) - 16
Give or take a dB here or there, it still works out to about 30dB gain--just like you said.

Dig that crazy low-frequency bootstrapping arrangement at the input!

Dave, you want to expound anymore on that input scheme? It's very common in pre-war gear. One explanation I've seen is that by having the bottom of the secondary see the cathode you get a bit more gain than having it tied to ground.

BTW, I measure 22 db loss through that output transformer in circuit.


Gary, can you edit the title of the thread? Then others can find it easily. Don't know if possible.

Not sure if its possible to rename a thread??? I cant see a way.. anyone else help here.......
I can start another thread before we go on ..could Quote all or some of the prevous posts... edit out my wrong model numbers.
Call new topic RCA 76 B-2. schematic and information...or something like that.

Cheers .....................Gary O.

Thanks Doug, So looking at the specs my transformers dont seem to be any good for this experiment then heres the specs from Sowter

9045
Primary configuration Series Parallel
Voltage ratio 1CT:5CT 1:10CT
Bandwidth (-0.5 dB points, typical) 10 Hz to 60 kHz 10 Hz to 55 kHz
Total DC resistance ref. primary 48 ohms 12.5 ohms
Max. input level at 20 Hz +10 dBu +4 dBu

9040

Ratio Z ohms (nominal) Primary inductance (large signal) I dc DC Resistance pry / secy Bandwidth
(3 dB points)

Please excuse me im really not very good with the impeadance and dc resistance thoery type stuff so be gentle...thanks

And if they no good for this what can I use the 9040 for especially it costs £76 and is huge.....any ideas
5:1 15k:600ohms 50 H 40 mA 650 / 34 ohms 20Hz-60kHz
(5000 ohm source

Cheers fellas thanks for bearing with me....................Gary O.

9735 input is more like what you want. Tons of choices from lots of manufacturers here. Your 9045 will work, but probably not the way you want unless you are using really hot mics at all times.

The 9040 will work for the output, but has way more level handling than you need. The simple answer is that you may lose some output level with the lower primary impedance, but it's somewhat counteracted by the lower step-down ratio. If it worked for a BA-1A experiment then it will work here; same basic iron design was used for both of these preamps.

Interesting my BA1A experiment worked but like you said I dont think it worked how it was intended compared to my other experiments the BA1A had low output very clean but thin sound was usable but didnt get used cos me other pres always sounded nicer.
Like I said I dont understand much theory but using 9045s input trans and various output trans as advised by the ever helpful Brian Sowter I made the following experiments
V72
RCA BA21A
Altec 1566A
Pultec MB1
Pultec EQ
Mccurdy u300 {I think}
LA2A
RCA BA6A limiter
Altec 438 mic pre only
AM864 limiter

They work and are usable are bit of a mess cos I keep trying to improve them, tho its hard to no how they sound compared to the originals having different transformers, i wondered if the 9040 could be better employed in one of these other units,
The 76B2 was gonna be next experiment but not having the right bits for the job I think il leave it for now.


cheers......................Gary O.

I like the 76 console pre for it's low gain; specifically good if you are following it with a high gain vintage type limiter, and you only need a 'buffer' preamp before hitting your limiter. That way you're not throwing away a lot of gain between units with attenuators. It sounds most like the BA-2A, which is two triode stages with a gain control in between for 50db total.

The low ratio of that Sowter input is likely not sufficient step-up to overcome tube noise concerns unless used in a line level situation. It can also be seen as dragging the input grid closer to ground than is normal for this type amp.

Yeah totally agree thats what I need theres always too much level and as it goes thru the chain it has to be attenuated doesnt seem efficient.