Monday, August 8, 2016

Brook 12A Datasheet

I ordered the original photo fact off of ebay as I had never seen all the data. Since I am building it this year I should have it. My version will be larger due to parts selection. The first build will be pretty much stock. I am curious about what tweaks would you guys make to the 12a? Not wanting to get too far from original design.


 Original schematic with error of C3 being reversed. Any other errors?

24 comments:

  1. As I'm pretty ignernt, that doesn't look like a simple push pull. What would the layout be? or am I overthinking again.

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  2. I'm totally amazed that you are undertaking this project. It's totally over my head how you would even start! For example do you contract out the building of the metal chassis? I'll be following any on going
    pictures and info. Regards Gord up in Canada.

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    1. It will be my second scratch build and actually IMO the hardest part is the metal chasis hole punched. Fortunately my friend has a huge greenlee set. I'll post pics as we go.

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  3. I found another error in the schematic... There's no return to ground for the standing current in the 2A3s. I'm guessing there should be a center tap connection on the H winding that connects to the ground connection on the M winding....

    Roscoe

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    1. Your correct I have that in my notes. I am thinking of doing some sort of center tap. That would be one tweak. Any other advice?

      Thanks!

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    2. Be sure the cathodes are grounded to the power supply ground, not true ground. The bias is obtained by lifting the PS ground 60 volts or so, so that the 6B4 grids are negative in relation to the PS. This way they didn't have to add a negative supply.

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    3. ...at least I think that's how it works. :-)

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  4. This comment has been removed by the author.

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  5. Well, obviously the C12/C13 & C14/C15 pairs can be replaced by a single .2 or .22 cap. R9 & R10 should be closely matched. There's really no reason to use 6J5s instead of another 6SN7... The cathode bypass caps are small enough you could probably use polys instead of electrolytics...

    Roscoe

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  6. Well, obviously the C12/C13 & C14/C15 pairs can be replaced by a single .2 or .22 cap. R9 & R10 should be closely matched. There's really no reason to use 6J5s instead of another 6SN7... The cathode bypass caps are small enough you could probably use polys instead of electrolytics...

    Roscoe

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    1. Thanks for the advice.

      I guess .2 was not available when they built this brook.

      Some caps I had to go with what was available. I avoided lytics where I could. I'm using motor runs in the power supply and lots of those nos russian caps everywhere else.

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  7. An Admirable Project.
    Have you selected your OPT's yet Iain
    What is T2 - is it just a ct filter choke for the driver B+?

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    1. The transformers have already been specd by cloners before me from Heyboer. I'm not sure how many amps use the center tapped choke like this I am still a beginner and learning.

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    2. T2 is a center tapped plate load choke.

      Roscoe

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  8. Two 6J5s will sound nicer than a 6SN7. :-) And you can get better matching, which is critical here, with the choke plate load. Just my .02. :-)

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    1. I'm going to build a 6J5 preamp for my digital sources when this is all done.

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    2. Use a 76, the loveliest tube in all the land. :-)

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  9. A proper scan in .pdf format of the images would have been nice. Be aware that Howard Sams is still in business and may ask you to take these down...or may not.

    The original layout was the result of at least some forethought and probably not the first or second iteration, so duplicating the layout exactly might make a good deal of sense. Especially if you do not have experience and a full command of the theory in dealing with any stability issues, which is where "audio" people without formal instruction or a good background building RF stuff have problems.

    The center tapped driver choke is used in a fair number of amplifiers, including the Altec 1570B and several DIY designs from this era. It was and is a good design. These chokes have been unobtanium for quite a while.

    The Brook was the brainchild of Lincoln Walsh, who died in the early 1970s, and was one of the first real practitioners of "high fidelity", having come out of the EH Scott high end radio console world. He had published designs going back to the thirties. Brook was undercapitalized and fell by the wayside because its watts-to-cost ratio was not as good as competitors, and its units looked like crude OEM chassis, unlike the prestigious looking McIntosh, Fisher, Marantz, and other units. They did measure well and people thought they sounded better than other brands of the day.

    However, I believe Walsh would have been appalled by today's so called "ultra-fi" movement with single ended open loop amplifiers and huge horn speakers with no pretense of time alignment. Walsh's last work was the Ohm line of speakers, which are the antithesis of the horn ethos in many ways.

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    1. Didn't realize that but I'll remove it if I need to.

      I don't have issues with layout. If I do make a goof I have a tech to look over my shoulder at my work to make sure I don't do anything too stupid.

      It wonder what Brook as a company would have been like with a proper backing.

      I've heard large horn systems sound great and terrible but there could be many reasons for this. I have some PWK jubilee prototypes that are two way and I've listened to time aligned and not. The results were not jaw dropping. Perhaps with 4-5 crossover points it makes a bigger difference.

      Thanks

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    2. PWK did some beautiful work with some very inexpensive drivers, but his crossover designs have some serious issues, and he handicapped himself with his fairly light midrange horns and, more than anything else, the crummy T-35 Avedon treble driver. There are a lot of Klipsch improvers out there and most have figured out most of the issues at this point. A suitably modded "Klassic Klipsch" -K-horn, La Scala, or Belle Klipsch-can provide very good results, better than commercial high end systems at ten times the price in many instances.

      Still; there are things that these horns do not do as well as the better high end box speakers from the high end vendors. Things like Altec 604s and Tannoy Dual Concentrics come closer, the latter being almost flawless with the right crossovers and in the right boxes. PWK certainly had access to the Altecs and Tannoys and the various electrostats and what not, and was not in the least inclined to concede anything to them in his opinion.

      Why? I think part of the reason was that PWK was, to put it bluntly, hard of hearing. His high frequency hearing in particular was largely gone at a fairly early age.

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    3. For me the genus of Klipsch is in the bass horns. The Jubilee being my favorite.

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    4. Completely agree. The guys who sell mods wisely leave that alone. If you get a good crossover and improve the treble and integrate the midrange horn better you get about the best horn speaker there is. You can spend more but it does little good.

      The Japanese make beautiful laminated wood mid horns but you won't find them sold over here. The Japanese have less than zero interest in selling their stuff over here. It's amazing.

      Me, I have Altec 604s because my listening room does not work with K-horns although the LS would do fine.

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    5. Very good BROOK 12A schematic at "www.lilienthalengineering.com" 100 amplifiers part 2.

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