DOWNLOAD A COMPLETE 13pp 1940 RCA SOUND SYSTEM PROPOSAL:  1940_RCA_PA_Proposal

Somethings, tho, are just too weird and interesting to ignore.  I found this jacketed proposal package for a 1940 church PA system.  Just the sort of audio historical flotsam that probably no one else ever bothered to preserve.  I can’t stand to see this stuff lost.  So here ya go.  If you ever wondered what those gigantic RCA amps and weird 77 variants cost new, who bought em, and how much those UrSoundBros got paid to install it… ANSWERS WITHIN.  enjoy. CR

SK has been so kind as to provide the schematic for this obscure device, as well as some background information.  I am posting the schematic full-size, so you can control-click it and download it for detailed viewing.

The photos in this post come from this Japanese auction website; the device pictured here recently sold for just Y30,000 ($300 USD).  And in working condition. 

Here’s what SK has to say about the Ikegami-Tushin Limiter:

‘(It is) very much inspired by the RCA BA6A for sure.  It even looks a bit like it.  The tube format is very similar:  just change 6sk7 to 6ba6, 6j7 to 6au6 those goes into 6v6 PP and transformers between 6au6.  6ba6 were popular and cheap in japan because we made those a lot in japan in the 1950s and 60s.

6sk7 and 6j7 were never made in japan.  This unit also has an extra gain stage before first stage, which is pretty neat.  The components seem very high-end and some are custom made for this.  When i was gathering info about the ba6a in old tube shop, an older ham radio guy told me that he DIY’d one of these a long time ago…’*************

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SK also recently built his own BA6A from scratch.  It is depicted above, and you can hear audio samples of it at his soundcloud page.  SK has also scratch-built the Federal AM864 tube limiter, and he has this to say about the projects:

“The sound you will hear in soundcloud is a good comparison with the fed864.   The fed has good high open but compressed sound,  the ba6a has low mid, ton of low mid. I love them both.  The first time I used them was at a studio in Chicago back in 90s when I was living in US.  At that time I was using LA-2A mainly, but that studio had a Fed864 and BA6A.  They blew my mind, and since then I wanted them so bad!’

SK also provided some build-notes on his BA6A project; if you are planning on building you own BA6A, you might find these useful:  S_Komiya_RCA_Ba6a_DIY_notes

Click below to DL the article by one George Singer.

DOWNLOAD: RCA-Mixer-AE-April53

DOWNLOAD: Compressors1963

UPDATE: T. Fine was so kind as to provide the entire 3-part article as a compact PDF.  click here to download it: BrdctEngnrgAudioLeveling_1963

The RCA OP-6 ‘Portable Amplifier’ is one of a handful of truly visually-iconic vintage mic preamps.   The OP-6 was designed as a “remote,” as-in, ‘on-location’ single-channel mic preamp for radio station broadcasts.  It uses three 1620 (6J7) tubes  – most mic pre designs of the period use two.  Furthermore, the 1620s are in pentode operation rather than triode.  The result: a ton of gain.  95db, apparently.  This is almost twice as much gain as the classic RCA BA1 and BA2 mic pres.  OP-6s are in high demand – click here for a seller asking $2900.   Further indication: Blackbird Rentals in Nashville has thirteen in stock as rental units.  The first time I heard an OP-6 in use was at Blackbird; I was producing/directing a live-in-the-studio performance for Martina McBride and John McBride was engineering; he was using the OP-6 for something… I can’t remember what exactly. Anyway, it caught my eye and we talked about it a bit; later I learned that he has an especially strong appreciation for these units.  A strong endorsement coming from the man who likely owns more vintage pro audio gear than anyone who ever has or ever will live.

The OP-6 schematic is pictured above (this file is readily available in high-res on the internet).  So anyhow, if we consider how in-demand the OP-6 is, it seems striking that no one offers a modern equivalent for sale, even on a small-scale level.  If you take a close look at the circuit, the reasons become apparent.  There are two big obstacles to re-creating even a semblance of an OP-6.  First is the input attenuator:

Sure, it’s a voltage divider; probably constant impedance; but what exactly are the values?  And what about that value of that feedback path issuing from the attenuator back to the input stage?  If I could get my hands on an original OP-6 and open up the attenuator, sure I could maybe sort it out.  But I imagine that re-creating that part on custom order could cost hundreds of dollars; frankly I have no idea.

The second obstacle to re-creating the OP-6 is the output stage choke.

The choke is designated L-1 in the schematic.  Curiously enough, it’s actually physically part of the output transformer.  Based on this fact, there is zero chance that this was an off-the-shelf choke, say a UTC for example, that we could track down.  OK – but the crucial value of a choke in a circuit like this is the inductance.  So long as the voltage an current handling values are sufficient, any choke of same inductance should give a similar result.  Now again, if I had an original OP-6, I could measure the inductance and maybe a current off-the-shelf part exists to satisfy the requirements.  But… I don’t think anyone out there is gonna send me their prized OP-6 to open up.  And $2900 is a pretty stiff R+D cost.  So what do we do?  Well, in the absence of any actual electrical engineering training, I looked for some good advice and then I guessed.

The very friendly+talented John Atwood sent me the diagram above; in response to what I can’t recall. This diagram explains why the choke is necessary in order to get the best possible performance from a single-ended vacuum tube line output stage.   Looks pretty similar to our OP-6 output stage, right?  Based on this… I made a wild guess.  I ordered the very inexpensive Hammond 156C choke.  150mh inductance with 8ma current capacity.  8ma might be a little low, but I have found that Hammond really undersells the specs of their transformers, so I’m not worried.

Alright so now we’ve got a choke that might work.  What about that input attenuator?  The best course of action would probably be to get a used 100k Daven T-pad, but without implementing the feedback path that the stock OP-6 attenuator has, it seems like this is slightly pointless vis-a-vis maintaining originality.  So instead: I used the input stage from my favorite, yes yet again, the RCA BA-2.

The BA-2 schematic is pictured above. The input stage uses a 1620 tube, wired as a triode, with a 100k ohm pot following it.  So I just took this input stage, up to the pot output, and wired it in front of the second two OP-6 stages (starting at the grid of stage #2).  When I did this, the whole system worked fine except when the volume pot was a zero (IE., when the grid of tube #2 is shorted to ground).  This caused weird noise and a little humming.  Not sure if this is due to the negative feedback in that stage, or if this is simply a general characteristic of 1620s when they are run in pentode – but it sounded awful.  The easy solution?  I added a 1M resistor from the grid to ground, and isolated this from the pot with a 1K grid-stopper resistor.  Done and done.  The BA2/OP6 Hybrid is born.

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The finished piece is shown above.  I used a 6X5 rectifier tube like the OP-6 uses; in fact, with the exception of using DC filament voltage, I kept the power supply the same as the OP-6.

The audio portion of the circuit is shown above.  What with the three pentode stages and feedback paths, this is extremely complicated for a mic preamp.   A lot of parts.  On the left you can see my usual Jensen 115 input transformer.  Audio caps are a mix of Solen, sprague, and some random Russian ones.  Basically whatever would fit.  Resistors are random as well; now that I have confirmed that the piece works well, I will probably replace the carbon-comp resistors in the B+ chain with some quieter modern resistors.  On the right is the output transformer.  I used an edcor 15K: 600; $10.22!

Even though this edcor is unshielded, there is zero hum following the volume pot.  And as for performance?  I did a frequency sweep through the entire unit; response is absolutely flat from 15hz to 10k hz.  At 10k there is a rise of about 1db up to 22khz, at which point response begins to fall of pretty rapidly.  This is really excellent performance considering the inexpensive transformers.  I did not measure the gain but there is a lot of.  The output level can get extremely hot.

Anyhow.  That’s it for now.  As soon as I have a minute I will provide some audio test examples; I’ll post some A/B examples of this unit versus an API 512, hopefully with both acoustic gtr and drums.

Thanks to DW at EMRR for helpful suggestions regarding this project.

***UPDATE*** Listening test has been done and results are posted here.

‘Orthacoustic’ response was a pre-RIAA record compensation curve necessitated by the the intrinsic flaws of record-lacquer material and turntable-mechanics of the 1930s.  Confused yet?  Read this very informative Wiki article on the history of the (still in use in 2010) RIAA compensation curve and all will be made clear (maybe).

Here are the manuals…

RCA_MI-11901

RCA_MI-4916-A

Of all the cults and sub-cults of audio-equipment collecting, few are more rarefied and costly than collecting antique movie-theatre equipment; especially equipment made by the Western Electric Company (hf. WE).  I won’t go into WE; the company had such a complicated history filled with intense government regulation, so tightly intertwined were they with the communication industries in American life; check out wikipedia for the details.  Suffice to say that, along with RCA, WE was a main manufacturer of the equipment used to playback sound in movie theatres at the dawn of the sound-film era (late 1920s).   Since the equipment was designed for such purpose, quality and reliability was very high.    Also massive.

(from “Recording Sound For Motion Pictures,” McGraw-Hill, 1931)

Here’s RCA’s theater system from that era:

(from ‘Audels New Electric Library,’ Audel+ Co, 1931-1958)

Mr. Yashima had quite a collection of this stuff.

(scanned from “Makoto Yashima Collection,” Seibundo, Japan)

It’s hard for me to say what the value of these WE components is, but i can easily imagine single pieces trading in the 5 figures.

Getting back down to earth, WE stopped making theatre-sound equipment in the late 1940s due to anti-trust regulations (complicated, right?), but RCA kept on building it.

This brings us into the realm of more accessible (even downright cheap!) devices.  Even though this later hardware may be inexpensive nowadays, we are still dealing with equipment that is designed for ultimate reliability, and really very good fidelity.  After all, tens of thousands of people sat in these theaters every year, paying a good fee in order to watch and listen to the latest films…  this is a case where quality really matters.

I  picked up this circa 1960 RCA 9362 booster amp for…  maybe… $70?  on eBay a while back.  I had no idea what it was, but it looked like it might be useful in the studio.  And here is where it gets technical….

I could not find any technical information on the web about this device.  The only info i did find was from www.widescreenmuseum.com:  apparently these 9362s were used after the photocell amp, but before a 4-ganged volume pot, in RCA’s “four channel magnetic track motion picture film of the “CinemaScope Type”.

I did see one other fellow on eBay who had built a couple of them into a little line-preamp….

(this is the unit someone else had built and sold on eBay)

…but a studio microphone pre-amp is more practical for me.   It seemed pretty clear to me that the shielded cable on the top of the unit was likely a hi-Z input (it was), so the only work i had to do was decipher the pin-out of the Jones connector on the bottom of the unit (1, 2= heater, 3= ground, 4= B+, 5, 6= balanced output).  Well, that and build the thing.

Since it was pretty clear from the circuit type (phase inverter- push-pull output amp) that the 9362 would not have enough gain (level boost) to use as a mic pre-amp, i took the opportunity to build this little RCA 5879-tube mic preamp from their manual RC-25.

I had never built a 5879-based mic preamp, and i have to say… it’s really pretty great sounding!  I used a Jensen 115 mic input transformer, which are great btw, and the whole unit sounds really great.

The horizontal switch / ¼” jack in the front of the unit allows the user to select either the balanced input-5879 preamp-500k pot chain OR simply go hi-Z directly into the RCA 9362 amp.

Does anyone out there use old WE theater equipment for tracking/recording?

Is there any de-accessioned theater sound equipment from the 70s/80s/90s that we should be digging into?

The BE-100 is a plug-in equalizer module that was made for use with the RCA BC-100 mixing console.  I have never seen a BC-100 in the flesh, nor am i aware of any albums that were recorded with a BC-100.  Since it was a product aimed at the broadcast market, and fairly rare, it’s possible that no albums ever were made on one.  From what i can gather from the forums, the BC-100 was apparently a custom-built product, and the pre-amps in these consoles were apparently built by API for RCA (no word on RCA’s involvement with the BE100).  A few years ago i was at a local flea market and some guys had a box truck with the contents of a storage-unit forfeiture sale.  I bought about 25 lbs of electronic parts, mic parts, etc., for a few bucks.  I also got some very interesting AES journals (interesting insofar as who had owned them) which i’ll post soon.  Anyhow, one of the parts i got was this BE-100.  just the raw module.   I bought a copy of the BC-100 manual from a dude who had been selling other parts from these RCA consoles on eBay.  He was not in the business of selling manuals, but agreed to sell me one for $25.  a very fair price.  he even spiral-bound it!  Here’s the schematic for the BE-100:

I added an old NOS BUD case, +/- 15v powersupply, a NOS UTC line-to-transistor input transformer from the local electronics warehouse, and an output transformer i pulled from an RCA broadcast phono pre of the same period.

I was too lazy to look up the specs on the transistors used in the BE100 to confirm that i was using transformers with the correct impedances, but my ‘good guess’ must have been close enough:  it works just fine.

The unit sounds…  grungy.  Aggressive.  not subtle, and not hi-fi.  It is quiet (IE no hum, white noise, etc).  I have not measured the freq response, but it sounds like it’s pretty much full-range…  i think the transformers i used are pretty decent.  The most interesting feature is the 40hz low boost.  it sounds awesome.  this will definitely find some use in the studio.

Has anyone else used one of these?  any tips or suggestions?

Does anyone know any records/studios that used these?