RCA BC-6B Console Project

[systemtruck]

Gotcha. Understood! That’s funny that you say that about other RCA boards because every one I’ve looked at the schematic of has had this line booster amp. Can i ask which models don’t have it?

I was going to again mention the theory that perhaps all of this extra gain, and then voltage dividing, is solely to achieve the mono/dual switching function… like I mentioned before, so that two channel bussing loss was met with a similar loss during dual mode, just for user friendliness, when they already needed the summing resistors…

BUT… That theory is now negated because the mono BC-5B and BC-3B boards both also have the line booster amps, as attached below.
And in their post voltage dividers, their values are 18k/6200, which is only a slight difference from the 18k/8200 from BC-6A above.

So even thought they don’t need switching or summing, they kept a significant voltage division/loss between the Booster and PGRM.

So what is the deal then? RCA would not have wasted all that expense and parts for no reason. Doesn’t there have to be SOME kind of operational / sonic benefit to have a summing amplifier be separate from the PGRM output amplifier? Like maybe the 4 stages of 12AY7 is preferable for whatever reason? And the two stages of 12AX7 amplification is a better setup for the Push Pull that follows? I’m just stabbing in the dark here.

And what about that pair of 100K resistors that feed the PGRM 12AX7 inputs in the actual PGRM schematic a few posts ago? How does that relate to RCA’s overkill gain staging?

 

[ruffrecords]

Many years ago, when I was first getting into designing tube mic pres and mixers, I purchased CD containing many of the RCA tube mixer manuals so I could study their circuits.
My recollection is that most of them prior to the 6B did not include boost amplifiers. I am not sure if I have the CD or even a PC that can load it so at the moment I cannot give a concrete example. However, I just searched the forum and to my surprice I see the BC-3A uses booster amps to so maybe mt recollection was wrong.

However, it seems to me that you are designing from scratch using the 6B amplifiers as building blocks, which is an excellent idea but, instead of working backwards from what RCA did it might be more instructive to look at what you need to achieve.

Cheers

Ian

 

[systemtruck]

Many years ago, when I was first getting into designing tube mic pres and mixers, I purchased CD containing many of the RCA tube mixer manuals so I could study their circuits.
My recollection is that most of them prior to the 6B did not include boost amplifiers. I am not sure if I have the CD or even a PC that can load it so at the moment I cannot give a concrete example. However, I just searched the forum and to my surprice I see the BC-3A uses booster amps to so maybe mt recollection was wrong.

However, it seems to me that you are designing from scratch using the 6B amplifiers as building blocks, which is an excellent idea but, instead of working backwards from what RCA did it might be more instructive to look at what you need to achieve.

Cheers

Ian

It’s true! I have a pretty settled plan in place for the overalls. It’ll include some basic schematics from your pan notes and your summing notes. I thank you for that!
It will be a simple mixer though. It is not really a full console for studio needs like auxes and headphones and speakers. This unit will only be for having a bunch of preamps with direct outs but also have the ability to sum certain channels together for processing during recording. So I really just am building preamps and stereo summing.
So now im just left so curious why RCA would have a doubled up mix amp situation going. I will probably take your advice and eliminate the booster amps. However then I’ll always wonder what would it would have sounded like with them, and with keeping that voltage divider in place too. So I may still build it with the two boosters, and if there really is no benefit I will just repurpose those two boosters into being their own preamps.
Regardless, I’ll try to post a complete block diagram and schematic here soon.

 

[emrr]

Consider the interstage amps here to be both isolation and driver amps. This console skipped preamp output transformers and pgm input transformers. You also can’t put a pair of passive mix busses straight into a mono sum successfully without an iso amp before the final 2->1 mix. Every previous RCA console has full in/out transformers and lo-Z mixing. This is the final cost conscious version before SS took over, and those returned to full transformer complements and lo-Z mixing.

Interstage amps, again- the only true LCR American tube console is the Altec 250SU, with interstage/buffer/iso amps between the 3 busses and the 3->2 mixing stage.

 

[ruffrecords]

Interstage amps, again- the only true LCR American tube console is the Altec 250SU, with interstage/buffer/iso amps between the 3 busses and the 3->2 mixing stage.

What about the Putnam consoles?

Cheers

Ian

 

[emrr]

What about the Putnam consoles?

Cheers

Ian

Those were all custom one-offs. There is no factory Putnam console. Offhand I don’t recall there being much info about them beyond the amplifiers. The later 610 as far as I remember was a modular channel block, I have only seen custom builds using it.

 

[ruffrecords]

Those were all custom one-offs. There is no factory Putnam console. Offhand I don’t recall there being much info about them beyond the amplifiers. The later 610 as far as I remember was a modular channel block, I have only seen custom builds using it.

I think you are splitting hairs.They were American LCR consoles made in the Putnam factory. At Neve we made loads of custom consoles but they were still made in the Neve "factory".
The 610 modules have a LMR switch right at the top. I think I have a block diagram of the console somewhere. I will see if I can find it.

Edit: Long before the 610 there was this Putnam consoles of left, mid and right channels:

https://funkwerkes.com/web/wp-content/techdocs/MixedProAudio/Putnam-Recording-Console.pdf

Cheers

Ian

 

[systemtruck]

I think you are splitting hairs.They were American LCR consoles made in the Putnam factory. At Neve we made loads of custom consoles but they were still made in the Neve "factory".
The 610 modules have a LMR switch right at the top. I think I have a block diagram of the console somewhere. I will see if I can find it.

Edit: Long before the 610 there was this Putnam consoles of left, mid and right channels:

https://funkwerkes.com/web/wp-content/techdocs/MixedProAudio/Putnam-Recording-Console.pdf

Cheers

Ian

This thread is a goldmine.

Funny that the 610 is brought up.. I am already committed to building at least four 610 preamps for this board instead of being all RCA, so maybe half 610’s and half RCA, and in fact may make all eight preamps the 610. I have some output transformers in hand for the 610, as per David from Cinemag’s guidance. Ready to build and test these preamps.

I found some older RCA board schematics, such as this one https://www.benmook.com/tech/documents/Schematics 2/Schematics 3/RCA 76-B2.pdf, and i see, as you guys stated, that the preamps have output transformers and that the program input has an input transformer.

At this point I’d like to follow the Keep It Simple Stupid method and just make the decision based on the goal.. Let’s say i do eight 610 preamps, and those have 600 ohm output transformers. I will have panning and volume and direct outs, and no other routing whatsoever. And learning what I’ve just learned here, I WANT to use an input transformer on each PGRM amp. And skip the “booster” stage.

So what tube PGRM amp should I use? The Putnam? This RCA 76 i just linked? Or should i try to dig up the 610 summing amp schematics? I tried but couldn’t find them, not surprisingly. I think if i had those, i would have my design already.

I think i need to make this decision first, and then i can decide the resistor values of all my panning and summing.