Original UA console - 100D preamp, EQ, 101D program amp

[systemtruck]

Does a 12BH7 also work as a drop in replacement (for the 12AU7 pinout ) in the BA-2C circuit?
I just have to start building now.
I also have to just try a Cinemag 30k:600 CM27101 output for the BA-2C to really know how it does. I don’t know the inductance yet, and i have inquired about it, since i hear that the original output in 200H.
The Cinemag 15k:600 9600T might be able to take the DC current. It’s physically much beefier than the 30k:600 so maybe it has a better inductance.

Anyways, splitting hairs ahead of the game.

But getting back to the console project overall, I have a question about summing/program inputs from resistor network… in relation to NFB preamps.

https://www.tubecad.com/april_may2001/page22.html.

In this article, particularly on the 2nd and 3rd page ( pg 23+24 ), they go into detail about needing feedback and suggest a negative rail among other things in the first stage, for a good mix input after the resistor network. I think it’s mostly noise they’re addressing.

But the whole trend of “passive summing boxes” on the market is squarely focused on letting you use any two preamps you want, as the boost. Just slap them on and roll.

RCA consoles and Putnam consoles both have NFB in their early stages of preamps and/or program amps. Basically whatever the summing network hits. But this RCA BA-2C build has no NFB. So does that eliminate it from contention as a (colorful) summing amp?

 

[emrr]

There’s nothing different in any of the old style passive 600 ohm mix buses, the amp topology never changed that.

 

[systemtruck]

I’m just wondering if there is any reason why these BA-2C’s couldn’t be used as summing amps, due to their non-NFB topology.
From reading a couple your own descriptions in other threads about this BA-2 preamp, and it’s inherent compression capabilities and ability to take serious levels smoothly, it sounds to me like a shining candidate for Left and Right summing boosters.

 

[ruffrecords]

I’m just wondering if there is any reason why these BA-2C’s couldn’t be used as summing amps, due to their non-NFB topology.

None at all. Almost any amplifier with sufficient gain can be used as a passive mix gain make up amplifier (what RCA calls a program amp).

Virtual earth mixers are a different thing entirely and that is what the latter part of the TubeCad article refers to. Virtual earth mixers rely on NFB to create the virtual earth.

Cheers

Ian

 

[systemtruck]

If you want zero loop NFB, build a BA-2. Use triode connected 5879’s if octal and grid caps are a concern. You could even do it with a 12AU7. All that strikes me as more reasonable than building an esoterically designed feedback based amp w/o the feedback.

I’ve started the build test station setup! Lots to do still, and am always thinking ahead in terms of ordering parts together to save costs…

Ironically I’ll keep pursuing RCA within this Putnam thread because RCA was just discussed here and maybe someone else will want to find this later as one train of thought.
@emrr, with your well known expertise in RCA tube equipment, do you have any thoughts about implementing the BA-3 as a program amp in a passive mixer setup? After learning from your BA-2 preamp suggestion, and realizing that i was set up for now two different kinds of single ended pieces of the puzzle, i was then hoping to find a single ended program amp design to have a unique mixer sound. I searched far and wide and finally came full circle back to discover the BA-3 by complete luck. There aren’t a lot of people mentioning this amp. And it’s right in the generation of the BA-2. Hell, i could omit the 610 if the BA-2 sound great, and go full RCA single ended console.

One truly shocking number in the service manual is that the overall gain when using the 600R input is 65.5dB, where as when using the 20k bridging input it totals 27.5dB. That indicates a whopping 38dB of gain from just using the 600 primary taps as opposed to the 20k primary taps? That doesn’t really add up when doing some transformer math, so it gives me questions about the transformer situation.

But thinking of the passive mixer setup, it seems one could just bypass the input transformer and hit the high impedence input straight away. I believe this will help the passive mixing system avoid a big loss and then big makeup gain after. This also removes the need for finding an input transformer.

Anyways the BA-3 uses two stages of 1620, then a 1622 power output to single ended transformer. It has NFB like every other program amp I’ve found. I already have a pair of output transformers that seem to kind of perfect, which is some 10 Watt Stancor A-3841. Takes a bunch of DC, and has multi taps in primary in the right range (2.5k, 4k, 5k, 6k, 7k : 600 ) so I believe can be configured to provide a good load for 1622, as well as some kind of usable “tertiary” by using available taps.

The 1620 and 1622 seem fairly easy to find. The 6J7 and 6L6 are of course even easier to find. Test quality will be important and i have a couple of folks who i buy from that run real tests not eBay “tests great” kinds of tests.

Any thoughts about tube choice in this circuit?

I will likely power these separately from the 8 preamps, so wiring of heaters isn’t really a complication.

 

[emrr]

The bridging input is a U pad, not transformer windings. The output transformer is a problem. I use mine as a mic preamp with the first pot section removed. Honestly any SE PGM level output transformer is a problem, very few choices out there. If you've got orig amps, sure. PP is far easier to do today.

 

[systemtruck]

The bridging input is a U pad, not transformer windings. The output transformer is a problem. I use mine as a mic preamp with the first pot section removed. Honestly any SE PGM level output transformer is a problem, very few choices out there. If you've got orig amps, sure. PP is far easier to do today.

Ah, a pad makes a lot more sense for a nearly 40 dB difference, haha.
Yeah i don’t think I’ve dug up any other SE program amp design other than this one alone.
Is it maybe just that SE was too difficult to satisfy the distortion specs demanded for proper broadcast standards? Maybe they tried it just once with the BA-3 and moved on to Push Pull.
But i have these Stancors that can handle 10 Watts each, and enough DC I believe. I’m not going for distortion specs, I’m just going for musicality and color. I may as well give it a shot and try adjusting the primary winding hookups and NFB values to see if something works. I’ll never recreate a BA-3 but maybe the topology leads to something very musical.

But back to the preamps… It may sound crazy but I’m thinking of going Octal all the way. I have seen some posts from you @emrr mentioning a love for Octal sound. I have the same affinity. I don’t know if it’s a placebo effect from the big glowing tubes or not, but all of my favorite sounding equipment has been Octal tubes, from 50’s/60’s guitar amps to 50’s/60’s compressors and preamps. The only exception is the 610. But would it be improved even more using Octal? I don’t know. So I did some homework and found that for the UA 610 i could get away with using a SL7 for the AX7, and a SN7 for the AY7. Pretty close, might lose 4-5dB over all but maybe could adjust. For the BA-2 i could get away with the J7 for the 1620’s. As for heaters…. I want to try one build each, using 12’s not 6’s. So… the 610 would be 12SL7 + 12SN7, and the BA-2 would be 12J7 x 2. The costs of these tubes are dramatically lower than the 6V versions, due to demand/popularity. I also hear now and again that 12V heaters are quieter than 6V heaters, and that this is one reason for going with 12AX7’s, etc.

If i did the BA-3 program amp, it would again be 12J7 x 2, but also i would have to go with a 6L6G for the output since the 12L6 is not an equivalent.

Anyways, fun stuff. I also like the top grid caps.

 

[emrr]

it's harder to get level with SE than PP. Historically you see max +24 out of SE and +30 out of PP.

RCA was all PP PGM until the BA-3 / BC-2B / BA-13, then to PPP for the BC-5/6 and BA-24 using small signal tubes instead of power tubes.

Gates SA-134 is about the same as the PGM amps in the consoles, less NFB for more gain. 25mA SE output trans, a problem today.

 

[systemtruck]

it's harder to get level with SE than PP. Historically you see max +24 out of SE and +30 out of PP.

RCA was all PP PGM until the BA-3 / BC-2B / BA-13, then to PPP for the BC-5/6 and BA-24 using small signal tubes instead of power tubes.

Gates SA-134 is about the same as the PGM amps in the consoles, less NFB for more gain. 25mA SE output trans, a problem today.

Looks like the SA-134 is a nice option that could match the BA-2 sonics quite well as a pair of summing amps, not to mention being mic preamps themselves if hooked up accordingly.

The schematic for the SA-134 can be found (posted by you I believe) but there aren’t any expected DC values anywhere. Do you know the ballpark value of where the 1st DC rail should be? Like what hits the transformer primary? I have an RCA stereo console (SHC-2/4) power setup that should work well but not sure what to tune the main rail output down to... 280DC maybe?

Also i found a quote from you on Gearspace from almost a decade ago.. “ I have a couple of Gates types I regularly use for vocals, though they are all console modules that require racking and power supply. After meeting noise requirements, the one you like will also depend on what sort of overload quality you want, one that's tight and clean but farts badly when it clips, or one that is euphonic and edges into overload transparently with growing thickness and edge. “

The thread here was comparing a handful of Gates amplifiers that someone had posted about. Are you referring to less-NFB and octal tubes… VS… more-NFB and smaller tubes? Assuming the former would be the smoother breakup (akin to the BA-2 probably), and the latter being the harsher sudden clipping. That’s been my experience with more-NFB.. a sudden clipping that’s not gradual or subtle.

 

[emrr]

300ish.

Many cases in which I'd rather use early discrete SS than late stage high feedback tube gear, considering the overload characteristics.

 

[systemtruck]

Found this catalog which has a number of other single ended amps and push pull amps.
https://bh.hallikainen.org/uploads/GatesSa.pdf
Looks like the predecessor to the SA-134 is the SA-20 which is pretty interesting. Same topology and tube choice, but what appears to be a heavier more standard amount of feedback, and a tertiary winding on output primary for it. Different values of everything throughout though.
This schematic shows voltages in circuit though. I’m assuming the 320V isn’t far off from the SA-134 main rail.
Page 16, but here it is copied below.
Fun to read the component choices in things in that link above.

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