RCA BC-6B Console Project

[ChuckD]

Sure just email it to me or Yousendit or you can email me and I will point you at my idisk where you can upload it!

cdickinson66 (at) hotmail (dot) com

Or click the email thingy under my name on the side here.

Thanks!

Chuck

 

[emrr]

Let's discuss the use of the two different B+ sources within a single preamp.  That I've never seen done in an antique USA preamp.  Noise improvements?  Less interaction between earlier and later stages?    Mechanical and logistical simplicity of keeping filter caps off the preamp cards, and at the PSU?

The BC-3 is the same in terms of components, simply less channels. 

 

[ruffrecords]

Let's discuss the use of the two different B+ sources within a single preamp.  That I've never seen done in an antique USA preamp. 

That has always puzzled me to. There is only a few volts difference between them; the only thing I can think is you might want more smoothing on the earlier lower current stages to minimise PSU noise.

Cheers

Ian

 

[emrr]

I think I answered my own question, once I thought twice.  The cards are small, and have no local filter caps.  More wire in the loom was cheaper than local filtering.

Close up of the parts in question.  Note the exploded bumble bee cap.

 

 

[ruffrecords]

OK, so it looks like I am getting my hands on the remains of an RCA BC-6B console.

You lucky devil. If you know of any more of these in any condition I would be interested in importing one or more into the UK.

Cheers

Ian

 

[emrr]

BC-3 preamps:  Into a 10K load, one recapped example did 73dB max gain.  Into a 15K:600 transformer that's loaded with 620R, they do 58dB max.  Low end response was better into at loaded 15K:600 transformer, and I tried both an Edcor and an old RCA.  The Edcor response was very good, you should be able to use many different equivalent transformers.  Bypassing the 2nd output cap improves low end as well. 

 

[My3gger]

Thanks for posting this. I compared BC-3C (5AR4-7) and BC-6B (5AR10-11) preamps, they are not exactly the same. NFB and biasing are different, although both have the same topology and gain. I checked several times to confirm this. So your preamp was probably 5AR4.
Is there any particular reason you used 5:1 outputs, maybe too much gain? Output impedance is low and this follower should probably drive 1:1 just fine. I remember someone used Jensen 1:1 outputs with almost exactly the same circuit, it was on the internet several years ago, don't know if info was correct.
Which Edcor and RCA did you try for the output? It would be nice to know how they compared in highs because it seems like many old American outputs didn't go as high as modern ones.

 

[emrr]

The preamp was revised somewhere along the way, the consoles were made concurrently for many years, so I expect either version to show up in any of the 3 consoles.  The one I looked at matches the one pictured above, not sure right now which that is. 

I used 15K:600 because the original console approach feeds a fairly high impedance.  Then also because I have a 15K:600 RCA that is the same iron footprint.  To the gain question, sure, I never need 73dB of gain for anything, 58 is much more ideal to me.  As well, I didn't look at harmonics, but generally a cathode follower pushing a low-Z has more upper harmonic content when it approaches the squonk line.  I frequently drive true 600 ohm loads, so the transformer step-down helps sooth my mind, if nothing more.  Looking at a 10K:600 or even 5K:600 would be informative; I didn't. 

The 1970's RCA and the 0.5W Edcor look exactly the same up top.  My converters are frequently unhappy driving mic transformer inputs at the low levels required, and I expect the upper end roll-off is related.  I see that pattern over and over again.  If I drive a 40dB line to mic pad upper roll-off is even worse, with anything you want to test.  The raw input transformer in a stand-alone test looks much better, as do both the outputs.  Anyway.....

The RCA was flattest on the bottom, the Edcor has a 1.5dB resonant bump centered around 24Hz with only one output cap are present, both hit -1dB around 10Hz.  Go to 2 output caps, and the Edcor is -1 at 24Hz, resonant bump more like 0.5dB around 50Hz.  I only kept the loaded (620r) plots, as it looked better than unloaded. 

Driving 10K directly had earlier and more pronounced low end roll off.  I didn't keep it in the plots, but remember something like -1dB at 50Hz.  The resonance effects of the transformer appear to be giving some frequency correction. 

Let's see if I can attach this......

 

[My3gger]

Card from the picture is 5AR1-5AR9 type from BC-6B. Your remark about higher harmonics with cathode follower driving low impedance is interesting. It seems like some recreations expect higher loads because they seem to have more like 600:600 output transformers. It might take some time but i will try different ratios here because it is interesting design beside BA-2 i'm working on now.
Compared to European tube gear of the same era this designs seem a bit more flexible. Datasheet for BC series consoles clearly shows how to make something similar to tube DI/preamp out of one card and it also has interstage pot. I really don't know why this classics don't cost much more. Something like V72 might be so expensive because they made 25.000 of them and quite a few people could try them. Do you maybe know approximate numbers for WE, Gates or RCA? Were numbers produced lower because they mostly put cards into consoles? I know about WE141-A and RCA OP-6, but there are many more which are more useful with signals today. I'm moding someones preamp with 48dB of gain because he is clipping converters all the time even with a bit of control over gain range...
I looked into many tests you done with American transformers and most if not all of them had pretty bad high freq. roll-off. It must be a problem with sound card if you get much better readings directly. Maybe also because most of this transformers have high ratios which don't have as good response as 1:1 and similar.
Every time you post things like this i wonder why people still insist on building R47 and V72 which don't have much control over sound. Can you try to describe differences in sound between this two and BC-3 or other known American classics with useful gain? I guess you can go from nice clean sound to some distortion with certain front end and interstage pad settings.

 

[emrr]

I have some of both types, I'll take a closer look.    I looked back at some pictures, and I had a BC-3C with the preamp type I showed previously.  Pictures of a BC-5A show the other type, so I still think it's a revision that went across all of 3/5/6 rather than particular to one console type.  The preamp schematic I have from a BC-3 manual is like the one in the BC-6B manual that Ian has.  I don't find a schematic for the other version anywhere. 

I really don't know why this classics don't cost much more. Something like V72 might be so expensive because they made 25.000 of them and quite a few people could try them. Do you maybe know approximate numbers for WE, Gates or RCA? Were numbers produced lower because they mostly put cards into consoles?
Every time you post things like this i wonder why people still insist on building R47 and V72 which don't have much control over sound.

Right or wrong, I'll editorialize a bit.  No offense intended. 

The grass is always greener, as they say, and the Americans started the vintage studio electronics craze, so 'we' wanted the European stuff.  15 years ago all the American gear was dismissed for being broadcast gear, as if that meant something.  It never did, but it was an easy way to trash talk that which was plentiful and readily available, when one was trying to sell people imported 'better' European gear.  R47 and V72; The Beatles The Beatles The Beatles.  That was what concerned Americans at the very beginning of this craze, and it still seems to dictate.  Elvis was old and square, why would we want his RCA input signal path?  We wanted the exciting imported tones of The Beatles.  Or something like that. 

Given the size of the US market versus the European market, it's safe to say there were far more RCA/WE pieces produced, and there's tons of Altec, Collins and Gates out there too.  The V series is easier to familiarize oneself with, since it was a standard driven by state broadcast requirements, here companies built whatever they wanted, so long as it met a technical spec, and competed to sell it to anyone who could find a reason to own it; broadcast, film, sound reinforcement, etc.  All the WE went to southeast Asia decades ago.  In my opinion a lot of the WE dealers in the 80's and 90's worked overtime to destroy as much RCA as they could, so it wouldn't detract from the legacy they were selling with WE.  They couldn't sell it to Asians at the time, and it could be a distraction if you actually listened to it in comparison.  RCA 76, BC-2, and BC-3/5/6 preamps only existed within consoles, and those mostly went to the dump or the scrap metal place due to size and weight.  A lot of old-timers have shelves full of transformers they robbed out of these consoles years ago, they seldom ever kept the circuits.  There are fewer BA series amps, since they generally went into larger installations that needed more than the stock console could handle.  One difference is likely that European broadcasters auctioned old equipment.  Here larger broadcast corporations had a tax problem if they generated a profit on depleted equipment, so it generally went to the landfill.

I honestly don't know what the average person considers 'the known american classics', I have attempted to try as many as I can, and it's a very broad palette.  It all depends on what sounds you are attracted to.  A lot of the 'classics' are later day advertising by dealers holding stock in those pieces, and the day before they started selling them they weren't considered any better.  I've seen a lot of this. 

 

[lassoharp]

IMO the last thing you want any useful (and that's really a relative term, right?) piece of old gear and/or it's components to become is a "Classic", unless you want to have to chuck 1/3 of your yearly income down the pipe just to have a basic front end for a small studio setup.

Beyond the Beatles it seems to be a demon that lurks in the rushes and can strike anywhere at anytime.  There's the Federal/DOC limiters, Ampexes, ever growing types of tubes and transformers, etc.  It's like watching a swarm of locusts storming the wheat fields.  Doesn't seem to take much to get it started.

I'll shut up now - just some shots for the ongoing war of attrition. 

 

[emrr]

Back to frequency response, yes, if I test with my Audio Precision test set, almost everything looks different. I tend to not use it as much as I have no way to save or print data.  Goes to show that inductive coupling changes response with varying surroundings; and we can't even measure how real world microphones interact. 

I just noticed the BC-6B manual states "the voltage gain of the amplifier is approximately 46 dB".  They can't be counting the input transformer gain of about 20dB, and even then there's 6-7dB missing from what I measured. 

 

[My3gger]

This thing was crossing my mind; i compared picture of your card to picture of 5AR1-5AR9 cards (fig.3) in BC-6B manual and they look exactly the same.
Original schematic from your link is the same as 5AR1-5AR9 (fig.2) in BC-6B manual. The only difference i can see is no input trafo shown on the top of schematic.
Values you marked are the same as on 5AR4-5AR7 booster amplifiers (fig.6) in BC-3B manual. The only difference is 1C3 marked as 0,042uF, not 0,047uF.
"3rd" type (if we consider some changes in biasing and nfb as different types) 5AR1-5AR3 from fig.4 in BC-3B is interesting because they added negative supply to avoid one more cap at the output. Static charge is probably leakage, were those caps that bad? The way input transformers are grounded also seems interesting to me. I better check it tomorrow, i'm tired...
Every time i look at this designs i like them better. They sound good, gain can be varied with ease, material is cheap and easy to put together, etc.
I'm not sure if we are looking at the same BC-6B manual because mine doesn't have input trafo on fig.2. Let's see what we have.

 

[emrr]

I put the input transformer on that schematic using Photoshop once when I was selling a bunch on ebay, so copies like that floating around came from my auction.  I scanned it originally from a BC-3C manual. 

Another piece of the puzzle is the existence of three input transformer revisions.  There's a silver cased that says 'made by RCA', then the UTC with the same part # plus UTC #, then a UTC with M-8366A marking. 

The first output cap would leak enough to cause clicks, because when a preamp is assigned to the center there's no load, to draw down leakage at all.  Many modern caps with no load also exhibit a leakage voltage. 

The BC-6B manual scan I have is 73.4MB.  From it I have the following two variations, only difference being the extra output cap.  These boards are both marked 643262-1. SUB-0. 

All the caps on the unit I measured today were replaced, so I was uncertain if any changes were made.  There are no board markings of an kind.  Now that I drew it out, I found a BC-3B manual I forgot I had, and I see 1C3 marked as 0.047 on one board, and 0.042 on the other.  I see there the more descriptive sentence regarding gain, which explains the BC-6B manual being a poor transcription/edit.

From it I have the following two variations, which I hadn't really looked at before.  I see also the PSU has 6.3 winding dedicated to a 6X4 to generate the -190V bias you mentioned.  The BC-3C manual matches the BC-6B manual regarding the PSU, and lack of bias rectifier.  I would guess the negative bias was still found unsatisfactory, or the bean counters decided the extra caps were the less expensive approach for continuing production.  Probably the expense angle. 

 

[crochambeau]

The BC-3 is the same in terms of components, simply less channels. 

http://binauralaboratories.net/lab/consolette/mainrca.htm

I'm hoping that the BC-3C manual scan that was hosted at binauralaboratories exists elsewhere or is rendered unnecessary by the BC-6B & BC-3B manuals referenced..  I let the domain name lapse long enough that a squatter scooped it up. Those were just .jpg because I didn't have the facilities to render .pdf at the time, anyway, let me know if a copy would be handy and I'll see about digging it up for a rescan. The first few google hits for BC-3C manual fold back to the dead link.  :-[

That said, this discussion is inspiring me to dig into the consoles. Keep fighting the good fight, cheers!


Quick question: will I outgrow the captchas if I post more? 3 lines of prompt to edit out a superfluous "anyway" in my post seems a bit..  excessive.

 

[emrr]

You will outgrow the captchas.  Thanks for putting that one out there long ago.  The full BC-3C is probably redundant, though I don't know that there's a scan of the BC-3B out there yet.  What would be missing is the console specific info, for anyone trying to sort out all the interconnects and patch points. 

 

[My3gger]

Crochambeau, i have BC-3C "instructions" in pdf form made from photos, if this is the same thing you wrote about. I can check my years old backups if there is anything more. Thanks from my part too, i'm sure many others enjoyed discovering all that information from one place. I just checked your new site, it confirms EMRR's claim that Americans made pro audio gear in greater numbers

 

[emrr]

That's probably from his webpage, I swear I saved it all too, but am not finding it so far. 

US versus Euro broadcast equipment:  look at the data on number of stations in different eras, that gives a very good idea. 

 

[crochambeau]

You will outgrow the captchas.  Thanks for putting that one out there long ago.  The full BC-3C is probably redundant, though I don't know that there's a scan of the BC-3B out there yet.  What would be missing is the console specific info, for anyone trying to sort out all the interconnects and patch points.

Crochambeau, i have BC-3C "instructions" in pdf form made from photos, if this is the same thing you wrote about. I can check my years old backups if there is anything more. Thanks from my part too, i'm sure many others enjoyed discovering all that information from one place. I just checked your new site, it confirms EMRR's claim that Americans made pro audio gear in greater numbers

Thanks for the kind words! Yeah, I had searched for one before posting that ten some odd years ago. It's nice how much easier raw data is to come by these days. I see the mic input is posted here, so I won't dig unless I see someone is in a bind with the I/O terminal block. Hopefully I'll be in the position to contribute something to the conversations soon. Digging this place, thanks for all the top notch work all!

 

[My3gger]

I used 15K:600 because the original console approach feeds a fairly high impedance.  Then also because I have a 15K:600 RCA that is the same iron footprint.  To the gain question, sure, I never need 73dB of gain for anything, 58 is much more ideal to me.  As well, I didn't look at harmonics, but generally a cathode follower pushing a low-Z has more upper harmonic content when it approaches the squonk line.  I frequently drive true 600 ohm loads, so the transformer step-down helps sooth my mind, if nothing more.  Looking at a 10K:600 or even 5K:600 would be informative; I didn't. 

The 1970's RCA and the 0.5W Edcor look exactly the same up top.  My converters are frequently unhappy driving mic transformer inputs at the low levels required, and I expect the upper end roll-off is related.  I see that pattern over and over again.  If I drive a 40dB line to mic pad upper roll-off is even worse, with anything you want to test.  The raw input transformer in a stand-alone test looks much better, as do both the outputs.  Anyway.....

The RCA was flattest on the bottom, the Edcor has a 1.5dB resonant bump centered around 24Hz with only one output cap are present, both hit -1dB around 10Hz.  Go to 2 output caps, and the Edcor is -1 at 24Hz, resonant bump more like 0.5dB around 50Hz.  I only kept the loaded (620r) plots, as it looked better than unloaded. 

Driving 10K directly had earlier and more pronounced low end roll off.  I didn't keep it in the plots, but remember something like -1dB at 50Hz.  The resonance effects of the transformer appear to be giving some frequency correction. 

Let's see if I can attach this......

Hey,

i finally built this one (5AR1-9)into my test bed (not powered yet), output cap is one and bigger, the rest as on schematic, except for one HT and added 22u decoupling caps each tube. Input tx is nice 1:7 Cinemag, i have another two RCA's to try also.
I have several questions about it:
1. There is EMRR's remark about (probably) lower distortion with higher ratio OT (5:1) when driving 600ohm lines. Is it because of lighter loading on CF with trafo like this? I really don't need as much gain as original have, so i'm leaning towards higher ratio for this reason. I will mostly be driving 10k loads.
2. Now i have 22u decoupling cap at each anode, except for CF. Am i doing this correctly, does CF need any decoupling and where?
3. Probably have to elevate heater for CF, does anyone know approximate voltage at cathode?
4. I know at least one well respected manufacturer with very similar CF output (gain too) and top of the line 1:1 Jensen. It would suggest that CF into 1:1 OT is not bad, at least in my trials with SA70/WE141A Mashup it sounded good. Maybe it is possible to give CF more headroom to avoid clipping, but i really don't know how with all this nfb.