RCA BC-6B Console Project

[MidnightArrakis]

@MidnightArrakis To keep clutter down, below here are two quotes from my previous power supply posts. (Click “view attachment”) It’ll be double this schematic, but instead of having two transformers I’ll have two 5R4GY’s feeding off of one toroidal transformer which is this Antek AS-4T360 AS-4T360 - 400VA 360V Transformer
And each 5R4GY will have it’s own 5V filament supply from this Antek AS-0505 toroidal which provides two supplies: AS-0505 - 50VA 5V Transformer
Each toroidal will get covered in 1/8” thick steel.

[here are two quotes from my previous power supply posts] -- I apologize as coming across as being repetitive, but so many of these threads go through vast amounts of updates of which not all of them are always posted or sometimes even mentioned!!! So, I need to ask again just to make sure that whatever is the most recent update is known. THANKS!!!

[It’ll be double this schematic] -- GOT IT!!!

[It’ll be double this schematic] -- Will you still be using the 24V output? Like for console relays, bulbs and the like? And, is the "Hum Adjust" still being used?

[It’ll be double this schematic] -- There is a handwritten note at the top of the schematic stating -- 7-Watt --. What does this reference to? And, are all of the resistors 1/2-Watt, except those called-out as being 1-Watt? Detailed schematics are always way more useful, ya know???

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[MidnightArrakis]

FYI:

https://www.tubesandmore.com/products/capacitor-ce-mfg-525-v-404040-f

https://www.studioelectronics.biz/sunshop/index.php?l=product_detail&p=1577

https://www.amazon.com/Cermant-Elec...ds=40uf+capacitor&qid=1706862589&sr=8-48&th=1

https://reverb.com/item/27737475-capacitor-ce-40-40-40-40uf-525vdc-electrolytic-pc-mount

https://www.caryelectronic.com/home...-axial-electrolytic-capacitor-audio-amps.html

https://www.justradios.com/cart.html

At least these are a start!!! Which one do you like?


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[MidnightArrakis]

BEST VALUE:

https://www.aliexpress.us/item/2251832534469687.html?gatewayAdapt=glo2usa4itemAdapt
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[MidnightArrakis]

[i need lead wires] -- And.....speaking of "wires" and with connecting your power-supply schematic to, I am assuming the RCA BC-6B console.....how are you planning on connecting these two items together? Are you planning on, A) Bringing in a multi-wire cable-harness from the console and connecting directly to the various points of the power-supply circuitry? Or, B) Bringing up a cable-assembly with a multi-pin connector and simply connecting the cable-assembly to a mating connector on the power-supply enclosure? If so, what type of multi-pin connector? MOLEX??? Amphenol??? Phoenix Contact??? JST??? TE Connectivity??? What???

And, since there will be two separate rectifier circuits, I am also assuming that there will be two separate multi-wire power-supply connector assemblies and cables???

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[MidnightArrakis]

Very interesting, inspiring, i almost forgot about the party you guys have here

[Very interesting, inspiring, i almost forgot about the party you guys have here] -- It looks like I must have ripped a smelly fart or I must have told a really bad joke because it seems as though that I am the only one here having any "fun" at this party!!! HELLO??? BEULLER? BEULLER???..... (with "Twist & Shout" by The Beatles playing in the background)!!!

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[My3gger]

OP might be calculating costs and time needed for labor intensive project like this. We already have simpler EZ tube mixer, replacing amps with RCAs should not be hard.

 

[systemtruck]

Have a bunch of related parts and devices on the way already, including a regulated high voltage supply/system so that I can start testing tube circuits without building the custom (unregulated) supply first. Taking a pause on custom power component conclusions for now. Did order the Hammond 518-0920 though. Am assuming it is enough space for heat dissipation, and will drill holes for venting. One thing I’ll need to decide is whether or not the rails will all be created in external supply or in the main unit. Advantage of them being created in main unit are… Simple high voltage cable from supply, as opposed to like 10 rail leads/wires. Caps all in main unit, probably best to have them local to circuits? Lots of open space in power supply.

 

[Xida]

I am currently restoring a BC 5A! This is a very good thread.

If you want to group buy anything or fab some PCBs…

 

[beatnik]

Do these have the same circuit in the program amp ? (single ended 6V6 with feedback from the output transformer tertiary)

It would be great to know the specs of that output transformer (RCA p/n 46098)

 

[systemtruck]

I’m getting back to this project and am settling on the mixer bus details….

RCA has a feature that I’d like to eliminate, which is the option to use the console in “mono” mode. In this situation, both mixer preamps are sent to a common line via 18K resistors and this common line feeds both PGRM amps.
I’ve interpolated the RCA BC-6A schematic into a more clear arrangement here below. This is shown in mono mode, and the 3-part switch changes it to dual (aka stereo)

I am not that great at evaluating the gain of tube circuits still, or adjusting gain for them without messing up bias. So I wanted to get some advice here.

Using an online calculator i calculated each tube stage gain as is.
I could be wrong, but if we’re to convert this mixer setup into a typical stereo mixer, and not use this “mono” mode common bus and gain loss accordingly, wouldn’t it be desirable for signal/noise ratio to kill the extra gain in the PGRM amps and also kill the voltage divider before the PGRM amps? I believe this voltage divider was put in place during dual mode in order to keep the gain the same (down a bunch) as mono bus mode. And then, the PGRM amp has extra make-up gain that isn’t really necessary if there isn’t any gain loss from a bus/divider before it….

If all of this is true, how much gain would make sense to lose in the PGRM design, and what values would be good?

X

X

 

[emrr]

Let’s back up and ask - what do you gain eliminating this option? The console is never stereo, it’s dual mono. Either/or, no middle. The option for same to both PGM is the only way you get signal in the center of the dual output. For consideration…..

 

[systemtruck]

Let’s back up and ask - what do you gain eliminating this option? The console is never stereo, it’s dual mono. Either/or, no middle. The option for same to both PGM is the only way you get signal in the center of the dual output. For consideration…..

I should have clarified the overall project! Sorry about that..
I’m not restoring a RCA console, I’m building one that utilizes its sound/amplifiers but is not structured the same. I have 8 preamps, and they go into typical fading and panning and stereo bussing prior to these L and R mixer preamps shown on left side of image above.

My motivation is just to see if i can improve upon signal to noise ratio in the PGRM amps, by way of eliminating some unnecessary gain stages. So I’m wondering if i can send a hotter signal into them instead of having the mixer amps outputs hit voltage dividers first… and then i can reduce the gain significantly of the PGRM amps and thus increase signal to noise ratio significantly.

 

[emrr]

If you’re using the interstage amps, increase the NFB somewhat to lower gain. Same with the PGM amps. With a different mix system (and panning) you may or may not need to do that anyway. It all comes down to where you feel the gain knobs are best parked for normal operation. The BC-2B console for instance the mix and master knobs are frequently both around 9 o’clock because there’s so much gain. I would suggest the concept there is to have significant additional gain in reserve for low talkers, distant speech, etc. We don’t need that so much when outside of a speech/ribbons/broadcast environment.

There isn’t significant gain in the PGM amp, and anything before that input gain control is knocked back as needed, there’s no previous stage noise contribution because it’s attenuated. With each section gain staged correctly these are not noisy consoles. Most significant noise is always the first tube stage and associated carbon resistors. Put metal film in stage 1 plate and cathode and you will cure 95% of perceived noise problems assuming your quietest tubes are also in stage 1. Frequently the rest can be the original carbon if values are still in spec.

 

[systemtruck]

If you’re using the interstage amps, increase the NFB somewhat to lower gain. Same with the PGM amps. With a different mix system (and panning) you may or may not need to do that anyway. It all comes down to where you feel the gain knobs are best parked for normal operation. The BC-2B console for instance the mix and master knobs are frequently both around 9 o’clock because there’s so much gain. I would suggest the concept there is to have significant additional gain in reserve for low talkers, distant speech, etc. We don’t need that so much when outside of a speech/ribbons/broadcast environment.

Got it. I forgot about negative feedback existing in these. Thanks.
So for the first stage mix amps.. it’s the 1R5 160K, at the bottom, correct?
And for the PGRM amps, it’s the 2R12 18K coming from the tertiary winding of transformer, correct?

As for deleting the voltage divider between these two amps… is it ok to just lose the 18K feed/divider resistor outright? Just run the mix amp output into the 15K Master pot, and that’s it? It’ll hit the PGRM amp much hotter, so adjusting the NFB will be necessary I think.

 

[ruffrecords]

The way you have drawn your schematic, the 18K resistors are bus resistors. You cannot eliminate them. They are essential for the working of the passive mixing.

Edit: What we really need to see is a complete schematic showing you panning and bus arrangement.

Cheers

Ian

 

[emrr]

Got it. I forgot about negative feedback existing in these. Thanks.
So for the first stage mix amps.. it’s the 1R5 160K, at the bottom, correct?
And for the PGRM amps, it’s the 2R12 18K coming from the tertiary winding of transformer, correct?

Yes, and with 1R5 the associated cap may need to be larger to keep low end flat.

As to the 18k, note in the one position the 18k/15k is roughly a 6db loss with gain at max, 2 things going on here, lighter load on pre (more gain and less distortion) and some isolation between the stages. Depending on the build it may be less stable without the isolation. Note the orig console probably has a 6db output pad for isolation from unknown external loads, typical design item.

 

[systemtruck]

I’ll be able to post a more complete schematic later today when i have some time to complete the rough drawing. You will just see 8 channels of preamps to the left of that block diagram, all with panned outs L and R that feed those first two L/R mixer amps shown above on left.

For now, below is a reference point, which is from the BC-6A schematic. My drawing above is a clearer diagram of the rotary switch in the middle of this, using a 2P3T toggle instead.

Yes the 18K are bus resistors, but only during mono mode. Then when you select “dual channel” mode, the old RCA mixer design separates the two mixer amp outs to no longer be bussed and instead be like any normal modern mixer… each mixer amp goes right into its own PGRM amp. But, it includes a new voltage divider that repurposes that 18K bus resistor and gives the signal a 8200R to ground. I theorize that the point of this is to retain the same signal level as when there was a bus resistor setup, so that when you switch between Dual or Single mode, the level is about the same. That’s just my theory. I haven’t built and tested yet, and I’m a total amateur tinkerer haha.

So I’m talking about losing that 18k/8200 voltage divider and its significant gain loss, and then also reducing the gain of the PGRM amp, since it won’t be needed, and thus hopefully getting a significantly better signal to noise ratio in the PGRM amps.

 

[ruffrecords]

Ah, I just realised, the amp on the left is not a channel amp it s a booster amp. I suspect you can eliminate that completely.

Cheers

Ian

 

[systemtruck]

Ah, I just realised, the amp on the left is not a channel amp it s a booster amp. I suspect you can eliminate that completely.

Cheers

Ian

By eliminate “that” completely, you mean the whole booster amp stage?
I kind of like the fact that it nicely sets up the opportunity for a bus insert jack for external equipment or just internal patching if I add an EQ down the road.
Of are you just meaning I can eliminate the voltage divider in question?
And if so, you think I can then bring down some gain of the PGRM amps?

 

[ruffrecords]

By eliminate “that” completely, you mean the whole booster amp stage?
I kind of like the fact that it nicely sets up the opportunity for a bus insert jack for external equipment or just internal patching if I add an EQ down the road.
Of are you just meaning I can eliminate the voltage divider in question?
And if so, you think I can then bring down some gain of the PGRM amps?

Lots of questions there. I think you can eliminate them completely. I see no reason why a bus insert should not be post bus fader in which case tour balanced bus output is your bus insert point. There is far more gain than you really need but you do want balanced outs so eliminating the booster amp seems a logical way forward. Most of the other RCA mixers did not have booster amps. In most cases the PGM amps are the bus amps.

Cheers

Ian