RCA BC-6B Console Project

[emrr]

Those hand written impedances are surprising. 375, 380, 49 on the primary, 8.8, 8.8 on the secondary? That seems kinds like a speaker output transformer where as this is for going to line out. Or maybe it’s a multi purpose type of transformer?

Start reading PRR's posts on tubes and transformers. General rule, DCR frequently about 1/10th design Z. 8K:180, 500 tertiary winding. 180 is probably close to the actual NFB adjusted output Z, even though designed for 600 load. and fine with 1K or 10K load. DCR varies with wire gauge. Don't overthink it. Lots of shit comes out in the wash with tube loading, higher ratio, more step-down, but more gain from tube because load is lighter = net zero perceived difference, even though THD profile is probably different. You're not using original iron, so why get granular about it? The Cinemag 9660 will be fine. PPP versus PP simply /2. Load could be x1.5, x2, x4, lots of variation to be found out there amongst different amps. 6W = +36dBm, 1W = +30dBm. 10W = +40dBm. 1/2W = +27dBm. 11/4W = +24dBm. There are charts for this.

 

[systemtruck]

Start reading PRR's posts on tubes and transformers. General rule, DCR frequently about 1/10th design Z. 8K:180, 500 tertiary winding. 180 is probably close to the actual NFB adjusted output Z, even though designed for 600 load. and fine with 1K or 10K load. DCR varies with wire gauge. Don't overthink it. Lots of shit comes out in the wash with tube loading, higher ratio, more step-down, but more gain from tube because load is lighter = net zero perceived difference, even though THD profile is probably different. You're not using original iron, so why get granular about it? The Cinemag 9660 will be fine. PPP versus PP simply /2. Load could be x1.5, x2, x4, lots of variation to be found out there amongst different amps. 6W = +36dBm, 1W = +30dBm. 10W = +40dBm. 1/2W = +27dBm. 11/4W = +24dBm. There are charts for this.

That clarifies a lot, thank you for all the help here!

 

[systemtruck]

I’m in touch with Cinemag. David there is very helpful. I’m going to order a pair of the CM9600T’s. They seem to match up well, including the tertiary winding, and are a good price for being as strong as they are.

By the way… i didn’t notice this before, but David pointed out that in the BA-25A schematic, the tertiary winding is not connected to anything. This is different than the BA-23 schematic, which has the tertiary windings connected just like these BC-6 consoles have it connected. Anyways thought that was worth noting here. They use the same output transformer. Maybe they did without the connection for the BA-25.

Moving on to settle on some mic preamp input transformers…

Anybody have any sonic / opinion / color thoughts about replacing the M-8366 (37.5/150/600 : 50K) with UTC vs Cinemag vs Sowter vs, etc? A few of the M-8366’s can be had, but not many and they’re too pricey.

I have a lead on some UTC “Ouncer” O-1’s for a somewhat decent price. I know those were used in the original UA 610 inputs. They are 50/200-250/500-600 : 50K. Similar overall gain options.

The Cinemag CMQEE-3440A is appealing and they are the most affordable option I could find, and they’re new which is nice. It has the exact set of input impedances as the original RCA inputs .. 37.5, 150, 600 : 50K. Maybe that’s not very crucial, but it’s fun from the recreation standpoint. Apparently this transformer is famous for being in Quad Eight preamps. I haven’t used a quad eight so am not familiar with the sound. This transformer has a bit less headroom than the UTC Ouncer, with a stated -5dBu spec, vs the Ouncer’s +8dBm. Not sure that even matters though.

Is a UTC step up transformer just simply the way to go whenever possible? Maybe they impart a vintage character that something modern just can’t? Or is UTC just superior due to quaity of build?

 

[ruffrecords]

INput transformers operate at low signal levels so you are unlikely to be operating in the region where saturation will occur. At low signal level the Barkhausen effect is more likely to make an audible difference and this will vary with the core material. Modern mu metals are likely to perform better than classic mic input transformers because of their smaller core size and more regular orientation of the magnetic domains.

Cheers

Ian

 

[systemtruck]

INput transformers operate at low signal levels so you are unlikely to be operating in the region where saturation will occur. At low signal level the Barkhausen effect is more likely to make an audible difference and this will vary with the core material. Modern mu metals are likely to perform better than classic mic input transformers because of their smaller core size and more regular orientation of the magnetic domains.

Cheers

Ian

Thanks! Yeah I think I’ll go with the Cinemag CMQEE-3440A for all 8 preamps. I’m unsure how this rumoured “equivalent“ 4722 Aletc / QuadEight transformer’s “sound” will turn out in the context of these RCA circuits, but I like the idea of having modern technology helping to eliminate noise/hum as much as possible. The UTC “Ouncer” series also offered some optional shielding cans, which can be bought but for exorbitant prices. I’m not sure what the shielding spec is on the Ouncer series without those optional cans, but the CMQEE-3440A shielding metal states >30dB of magnetic noise attenuation as-is. I suppose I could add to that by installing extra emf protection around each one.

 

[ruffrecords]

Thanks! Yeah I think I’ll go with the Cinemag CMQEE-3440A for all 8 preamps. I’m unsure how this rumoured “equivalent“ 4722 Aletc / QuadEight transformer’s “sound” will turn out in the context of these RCA circuits, but I like the idea of having modern technology helping to eliminate noise/hum as much as possible. The UTC “Ouncer” series also offered some optional shielding cans, which can be bought but for exorbitant prices. I’m not sure what the shielding spec is on the Ouncer series without those optional cans, but the CMQEE-3440A shielding metal states >30dB of magnetic noise attenuation as-is. I suppose I could add to that by installing extra emf protection around each one.

That Cinemag should be fin for your project. The biggest interference culprit for high ratio mic transformers is mains transformers, especially those with EI laminations. Keep them well away from mic input transformers, preferably in a separate steel box outside the mixer.

Cheers

Ian

 

[systemtruck]

That Cinemag should be fin for your project. The biggest interference culprit for high ratio mic transformers is mains transformers, especially those with EI laminations. Keep them well away from mic input transformers, preferably in a separate steel box outside the mixer.

Cheers

Ian

I’m using a torroidal. Think that helps?
If i keep it “in” the unit, the closest it will be to any mic input transformer will be about 1 foot and there will be about 1/8” of aluminum between the power area and all audio circuits.
If i put it in an external unit, the sky is the limit for distance but i worry about the length of the high voltage leads having consequences.

 

[emrr]

RCA used to advise at least 3 feet between PT's and input transformers. Some of the early '40's amps have external PT's, including filament PT's, for that reason. The further the better, within reason (wire losses). Their PT's were shielded potted units too.

 

[a soBer Newt]

I am a big fan of the CM3440 you can do some fun things with the impedance switching as well. I have this setup so I get four different options. And on the secondary I am also spitting so I can get different levels.

 

[systemtruck]

Thought I’d put these drawings here for fun. Check out how packed everything is. Got power supplies practically touching some preamps. I couldn’t find any layout evidence / indication of where the 9 mic input transformers are though. Maybe they are buried and shielded under the preamp boards.

In any case it would be a relief to keep power out of the console.

I am wondering how much internal air space I need to leave for my dual power supply, and if heat sinks are possible.
The tallest components should be the two 5R4GY’s. The widest is the toroidal at 5” diameter. This one might work.. fully steel, some vents. Would like more vents, not looking forward to drilling steel though.
https://www.mouser.com/ProductDetail/Hammond-Manufacturing/518-0920?qs=8xMK%2BwDsXhfjJvL4Ul8x2w==

 

[ruffrecords]

I’m using a torroidal. Think that helps?

Yes, it will have a lower extraneous field then an EI but it can still couple to a mic transformer. I know, I have seen it happen in my own builds.

If i keep it “in” the unit, the closest it will be to any mic input transformer will be about 1 foot and there will be about 1/8” of aluminum between the power area and all audio circuits.

A foot is pretty close. The aluminium will do nothing to stop the magnetic field. 1/8th inch of steel would be much better

If i put it in an external unit, the sky is the limit for distance but i worry about the length of the high voltage leads having consequences.

You need to take care of wire gauge for heaters and wire insulation for HT but both are well within the capabilities of modern cables and connectors. 3ft separation is what I usually aim for although I have done 6ft in one case with no problems. At Neve back in the 70s we did many amps at 24V over much longer distances - that was challenging

Cheers

Ian

 

[ruffrecords]

Thought I’d put these drawings here for fun. Check out how packed everything is. Got power supplies practically touching some preamps. I couldn’t find any layout evidence / indication of where the 9 mic input transformers are though. Maybe they are buried and shielded under the preamp boards.

Notice how all the big transformers are fully shrouded. Definitely steel at least and maybe mumetal.

Cheers

Ian

 

[systemtruck]

@ruffrecords Thank you !
Neve in the 70’s… WOW.

The toroidal company makes steel covers for their transformers.
https://www.antekinc.com/content/CA-xxx.pdf
They are all 1/8” thick steel except for the smallest one which is designed for my 5V 8A filament transformer and for that they use 1/16” thick. I can upgrade to next dimensional size but am unsure about center post / mounting fitment. Does a 5V 8A filament supply require as much thickness of shielding as the high voltage supply?

With these both enclosed, I suppose I can ease off on the thickness of the total supply enclosure. Most of those boxes are 1/32” steel at best, some thinner than that.

 

[MidnightArrakis]

This one might work.. fully steel, some vents. Would like more vents, not looking forward to drilling steel though.
https://www.mouser.com/ProductDetail/Hammond-Manufacturing/518-0920?qs=8xMK%2BwDsXhfjJvL4Ul8x2w==

[This one might work] -- Here's the HAMMOND 3D CAD-model of the enclosure you have selected.




[Would like more vents] -- From what I have seen on their website, they will add some additional vents for you. At what cost? I don't know. Below is my "Swiss-Cheese" version of the HAMMOND enclosure with more vents than are necessary!!! I just quickly threw something into the CAD-model just to show you what can be done. And.....adding-in features like this within the 3D CAD-domain is no big deal. I could create a modified CAD-model that would include all of the cutouts and whatnot that you would like to have and send the completed file to you. Then, all you gotta do is send the CAD-file to HAMMOND and they will punch-out an enclosure accordingly. Of course, doing things this way is nowhere nearly as much fun as standing at a drill-press or using a hand-drill to do all of this work while dulling your expensive drill-bits!!! But, it is an alternative.



/

 

[MidnightArrakis]

Thanks! Yeah I think I’ll go with the Cinemag CMQEE-3440A for all 8 preamps. the CMQEE-3440A shielding metal states >30dB of magnetic noise attenuation as-is. I suppose I could add to that by installing extra emf protection around each one.

[I think I’ll go with the Cinemag CMQEE-3440A] -- Of which type of configuration???



>> I personally would tend to favor this one, but your requirements probably requires having wires:





/

 

[systemtruck]

Thanks for the help and the ideas!
The 3440A’s will have the standard config, a pair of 4-40 mount points. Yes i need lead wires.

I think for cost purposes I’ll end up drilling holes instead of contracting anyone, plus more fun you’re right.

I’ve looked around at other enclosures and so far this one still seems like the best choice.
But my backup plan is the Bud C-1892. Doesn’t have a handle and is much larger and more cumbersome, but it allows a lot of breathing room.

Back to the Hammond 518-0920.. I’ve drawn up a quick layout of the transformers and the two 5R4GY’s. The outer red perimeter is the exact dimensions of thickness of this steel enclosure.

I wonder if this is this too tight of a proximity side to side? Was thinking that this is the best spacing for the tubes so that they’re not cornered in anywhere and can dissipate heat best. Those transformers are shorter than the tubes, so there will be air space In upper half of case for tubes to dissipate heat.
However, the top of the 5R4GY’s will come within around 1/4” from the top inside. Is that a bad idea for heat? I could drill a bunch of holes above each tube.

 

[MidnightArrakis]

[But my backup plan is the Bud C-1892] -- Here is a possible "Plan C":






>> 3D CAD-Model:


>> While this enclosure is also a bit larger than the HAMMOND enclosure, not only is this a "nicer-looking" enclosure for your studio, but because of its slightly larger size it then offers you the "breathing room" that you are looking for so the vacuum-tubes aren't so close to the transformers (heat warming up the toroids, hum and noise from being so close to them). You can purchase a single-unit and also have it modified to your requirements. -- A++ -- in my book!!! NOTE: With this being a "nicer-looking" and larger enclosure, it may be possible to place the tubes next to one another on one side with a steel-plate in-between the tubes and the toroids to offer not only some protection from their combined heat, but also from the magnetic-fields from either/both the tubes and transformers. Can't hurt, right???



>> Same routine as previously mentioned.....download CAD-file (already done), modify it as required, send modified CAD-file back to vendor, vendor fabricates enclosure to your spec's, vendor ships enclosure to your front-door, then "Porch-Pirate" steals delivery and you track them down and beat them to a pulp!!! You retrieve enclosure, complete its assembly, -- HUGE SMILE -- spreads across your face as the two 5R4's gently glow and your RCA BC-6B console happily sings as pristine audio and music traverses through its circuitry!!! >> HAPPY DAYS!!! <<

>> What are the Part Numbers of your specified toroids? AS-2T350 and AS-0505? Or??? Inquiring minds need to know!!!

/

 

[MidnightArrakis]

Thanks for the help and the ideas!

And.....the final piece to your power-supply enclosure puzzle is: -- What is the schematic that you are planning on using? Could you share that as well??? Inquiring minds need to know!!!

/

 

[systemtruck]

And.....the final piece to your power-supply enclosure puzzle is: -- What is the schematic that you are planning on using? Could you share that as well??? Inquiring minds need to know!!!

/

@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.
Maybe @emrr or @ruffrecords have an opinion on proximity of these toroidals and rectifier tubes drawn in post#96 just above here? Safe to keep them relatively tight like this? I hear that having a short lead from transformer to rectifier tube is a good thing anyways.

View attachment 120873

SUPPLY #1 Filter Cascade:

*100R >>>>>>>>>>>>> **

** 750R/40uF > B2: (unused rail)

** 3000R/40uF > B1: 5AR13 CH 1 LINE AMP

** 600R/40uF > 600R/40uF > ***

*** 5600R/40uF > B8 12K/40uF > B7 : (unused rails)

*** 5600R/40uF > B6 12K/40uF > B5 : 5AR11 CH1 BOOSTER AMP (CH 1 SUB MASTER)

*** 600R/40uF > 600R/80uF > B3 > 3900R/120uF B4: 5AR1, 5AR3, 5AR5, 5AR7, 5AR9


SUPPLY #2 Filter Cascade:

*100R >>>>>>>>>>>>> **

** 750R/40uF > B2: 5AR16 PGM-TB MON AMP, 5AR15 CUE FEED MON AMP

** 3000R/40uF > B1: 5AR14 CH2 LINE AMP

** 600R/40uF > 600R/40uF > ***

*** 5600R/40uF > B8 12K/40uF > B7 : 5AR12 PGM-TB.MON PRE-BOOST

*** 5600R/40uF > B6 12K/40uF > B5 : 5AR10 CH2 BOOSTER AMP (CH 2 SUB MASTER)

*** 600R/40uF > 600R/80uF > B3 > 3900R/120uF B4: 5AR2, 5AR4, 5AR6, 5AR8

 

[My3gger]

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