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.