Brook 12A Mono blocks

[michaelsamra]

Very Nice

I heard a pair of Brook 12a amps and they were quite nice but very expensive.
I like the Heyboer and Elecktraprint iron and I'm actually having Phil wind me a set of Peerless P-271s again only this time I want to source some Nickel for the center like Peerless used in the original P-271s as well as some of their other trafos but he said he would put it in if I can find it.
I see the Lundahl chokes underneath or is that an innerstage of some kind?

 

[soulmerchant]

LL660S ?

 

[unique1]

LL1667, tube anode split chokes, supplying B+ to the 6J5's.

 

[sunil]

Hi,

Sorry to dig this old thread. I'm keen to try this design but don't have the original coupling transformer (T2) and am looking around.

I found one from One-Electron (PRC-3) , but the specs are kind of different, OR, I'm reading it wrong. Can I get somebody to please check this link and help with an answer. I'm happy to start a separate thread, let me know. Thanks.

Choke - One Electron, Audio, PRC-3, 225H for Preamp Output | Antique Electronic Supply

 

[JohnAtwood]

The One Electron PRC-3 is an air-gapped choke intended as a single-ended driver or line output stage load. It is identical to the chokes used in the Artemis Labs line amp and phono premaps. For the Brook amplifier driver stage, you would need a center-tapped or dual-winding choke, and an air gap would not be necessary.

BTW, I owned a Brook 12A and preamp years ago, and it was an excellent sounding set-up.

- John Atwood
One Electron

 

[sunil]

Thanks John. That was great help.

Would you be able to wind a pair of the Brook choke ?

 

[unique1]

I have a spare pair wound for me by Heyboer. The plan was for another set of monoblocks, didn't happen. I would be happy to sell them for a reasonable price.

 

[sunil]

Hi unique1, I will PM you, so we don't derail the original thread.

 

[unique1]

Hello Sunil, please email me at bgill1@telus.net. I can supply you with pictures and further information. Thank-you. Bill Gill

 

[sunil]

Hi unique1, which schematic did you follow - Lilienthal or the Original ?

 

[multi]

Ff you use unique1 circuit you can use a push pull Transformer with center tap

http://www.ampslab.com/SCHEMATICS/Brook12A.gif

 

[sunil]

I've got the Brook going. There are some strange things happening. The NFB is at 1K.

It runs fine with great HF, ok mids and very little bass till about 30-45m. Then the sound balances out nicely.

After about an hour-and-half, the mains trafo gets uncomfortably hot and the bass from the speakers starts popping. You can hear a constant popping noise on bass from the music. It is not too evident at low volumes.

I don't want to hijack this thread and am happy to start a new one. Just thought I'd keep the Brook info in one location. Any help would be great. Thanks.

 

[trobbins]

Are you using the same power supply configuration and part values as in post #9 schematic?

 

[sunil]

I've got a choke (200ma|5H) between C1 (50uF) and C2-A (50uF). Caps C2-B C2-C are 32 and 10uF.

 

[sunil]

Here's a scribbled image of the values. Forgot to mention, these are not monoblocks. The PS is one BIG chunk of IRON for both channels in a single chassis.

 

[trobbins]

That amp uses back-bias. How are you confirming the bias power in to each 2A3?

It's quite possible that some part (most likely a valve) is starting to break down. How new are all the coupling caps, and magnetics? It's very easy nowadays to add diode protection to the valve diode, and a secondary side fuse, to avoid collateral damage if something is failing.

 

[sunil]

The negative bias voltage is with the 700ohm R, like in the original design.

All caps are new F&Ts.

 

[trobbins]

Does that mean you haven't checked the idle dissipation power in each 2A3, and you haven't checked the magnetics for insulation resistance, and you have no concerns about the rectifier diode going gassy or that the popping may end up taking out some of your parts?

 

[sunil]

The tubes are from a reliable source and test fine. The trafos and rectifier were pulled from a working system.

I'd like to understand the problem before going the diode route.

 

[trobbins]

How are you going to fault-find? Substitution of other parts? Scope with one-shot trigger looking for drop in voltage rail? Shorting of input signal at different stages to exclude previous stages?

Without voltage and current measurements of each output stage tube there is a lack of confidence that those valves are not being treated appropriately.