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The enigmatic BROOK 12A-K1

https://www.americanradiohistory.com/Archive-High-Fidelity/50s/High-Fidelity-1951-Summer.pdf

Page 10
I find interesting how well people like Paul K loved the BROOK amplifiers.
He goes on and on about the the BROOK especially the 12A

Now you have to read carefully but below is a snippet from the article above.

Paul W Klipsch writes:

" However, for the benefit of those readers who still insist on knowing the author's personal preference for an amplifier to use with the Klipschorn, here is the answer: the Brook. Not that this is a better amplifier than the others mentioned by name, but for the following reasons: 1) it is a good, low- distortion wide -range amplifier, z) it has a rather definite upper power limit which prevents damage to the delicate high- frequency speaker driver used in current Klipschorn systems, 3) it has a very highly refined preamplifier with proper equalization provisions, and 4) it is amenable to a very slight revision for full bass extension applicable to Klipschorn (referred to as K -1 re- equalization). The specific designation of this amplifier is the Brook 12A3 -K -1 and is rated at 10 watts output. Significant is the fact that it has been found to deliver 9.5 watts of clean output at 3o cycles. This is more than adequate power for driving high efficiency, horn -type corner speakers. Probably, for home use, peak amplifier output of 1 watt would suffice to feed a Klipschorn. ""

I've seen this modification mentioned before.

Joe Roberts in Sound Practices wrote a little blurb about it. Read it in the corner of the schematic below.
Joe is referencing the Paul WK article.

The 12A could be slightly modified to be a 12A3-K1 this mod was specifically for K horns.

The picture below is from the magazine Sound Practices
They mention the K1 mod in the corner.

Brook12A.gif


So here is my question
Has anyone here ever seen or read about this K1 mod for BROOK 12A amplifiers?
I have an idea of what could easily have been done.
But I'm curious if anyone has seen one in the wild???
 
My building partner and I have recently worked on PWK's personal Brook, and he has some notes about it on his blog: retro vintage modern hi-fi: Brook 12A Sings Again

We're currently trying to decide what to do with PWK's own homebrew from about 1946. It's too dangerous to actually run at the museum, but too historical to modify (at all). Quite the conundrum. I'll see if I can get Iain to comment about the Brook mods. IIRC he can answer your questions.

All good fortune,
Chris
 
Wow !!!!
Well NO sleep for me.
This would be fantastic to hear about.

I have a guess that it has to do with the picofarad caps in the AF amp circuit to the Phase inverter.
The SPICE models show the amp naturally rolls off on the lows.
I've read in a couple different places that amplifier engineers built in low freq attenuation as sort of a safety mechanism.
1 picofarad really don't like those lows. So I had assumed they increased its value.
But who knows???

I built a quad 300B amplifier with the BROOK 12A circuit. It turned out spectacular but yes it attenuated all the lower frequencies. NOT that it sounded bad mind you. The only place you really noticed it was when measurements were made.
The audio to the human ear was out of this world.
 
Until Iain responds, I can confidently say get some sleep. No major changes are involved, just the HP filtering you've described, IIRC. I could easily be wrong.

Paul's own amp design, which predates the Brook, is very interesting and advanced, but too dangerous in its original form (output transformers located in the speaker, high voltage in between) for DIYaudio. The concept though is very DIY friendly and could be explored.

All good fortune,
Chris
 
I think to Chris's chagrin I tried to keep as close to the original Brooks as I could. I wanted to listen to them as close to original as I could. PWK modified the Brook amp but even more so the preamp. When we looked at PWKs amp it had been worked on by Richard Modafferi's capable hands. This was for the 1996 CES show. The Brook we looked at was it PWK mods or Modafferi mods? Pretty good either way. I thought the original Brook sounded better than mine until I got the mods in place but even then it sounded even better with the brook 10c preamp. Don't tell Chris I'm thinking of a stereo 10C preamp lol...... Do multiple wrongs equal a right?

Chris what else did we change? Input cap? Didn't we lower gain? I'm just the solder monkey.

PWK mods or Modafferi mods
IMG_7622.jpeg
 
You may be thinking of your homebrews. We didn't change PWK's amp at all. But the OP was asking specifically about Paul's own mod(s). Do you remember the details, or would Jim H. know?


Much grass,
Chris


We showed Jim the changes. What we have posted is all that is known on PWK's Brook. The changes we posted I think are all that was different with the amp. The preamp is another story.
 
Well it may end up being a perpetual mystery but it will be interesting to know what
The Brook factory mod was???

The answer is out there. Buried with Jimmy Hoffa in the Bermuda Triangle.

Chris and I went through the circuit and these were the only changes we found. I also believe the input cap wasn't bass limiting. Chris do you remember? In the archives of the Klipsch museum there are lots of letters between PWK and Lincoln / Walsh discuss everything from planes to detailed mods for their other products. Apparently PWK was used by some vendors of the day to vet their products.
 
Well the way Paul wrote about that K-1 mod. It was a Mod offered by the BROOK factory.
It's a pretty esoteric little piece of history.
How he knew about it would be interesting to know. Was there an add? Was he in contact with Brook people?
That's the real mystery here.

What I am trying to say is YES PWK was in constant contact with Lincoln and Walsh. They were good friends. There are dozens of letters between them in the PWK Museum Of Audio History. I have seen these letters first hand. There are corrected schematics between PWK and Brook where PWK has scribbled on these schematics recommended changes. PWK made recommendations on the 12A, 10C and 3C.Which is why we wanted to examine PWKs personal Brook. It is curious that there were more mods of the preamps than the amps. Could these changes equate the 12A KI? The Brook was really one of the first commercial HiFi companies of the day and PWK always kept his in working condition.
 
What I am trying to say is YES PWK was in constant contact with Lincoln and Walsh. They were good friends. There are dozens of letters between them in the PWK Museum Of Audio History. I have seen these letters first hand. There are corrected schematics between PWK and Brook where PWK has scribbled on these schematics recommended changes. PWK made recommendations on the 12A, 10C and 3C.Which is why we wanted to examine PWKs personal Brook. It is curious that there were more mods of the preamps than the amps. Could these changes equate the 12A KI? The Brook was really one of the first commercial HiFi companies of the day and PWK always kept his in working condition.

I apologize I didn't see your previous post.
Also Lincoln Walsh was one hell of an engineer. I've always wondered what all he was into. He definitely had great insights.
 
I apologize I didn't see your previous post.
Also Lincoln Walsh was one hell of an engineer. I've always wondered what all he was into. He definitely had great insights.

I've seen a piece of paper a Brook document with K-I on it.. I can't post it as it is part of the archives and I don't have permission... One day though it would be nice to see the PWK / Brook documents public as a story as it is really interesting.. I will say the mods we found are likely legit but I feel the K-I refers to changes in the preamp not so much the 12A.
 

sunil

Member
Paid Member
2003-05-15 7:56 am
Bangalore, INDIA
I know a year has gone by but thought should share my experience. I had a pair built as monoblocks with Lundahl 1623 OPTs. The mains were local made by an industrial manufacturer and have great specs.

They power my Altec515b bins. With a little DSP they can sound awesome with absolutely no distortion at 'nice' levels. I run them [email protected] crossover and 'bump' at 160hz (2 -6dB), depending on music. The Brooks don't sweat at all.

Here's a pic.
 
Last edited:

sunil

Member
Paid Member
2003-05-15 7:56 am
Bangalore, INDIA
For the centre tapped Anode choke I had to use two separate 0-1500R, so that change is remaining. There is a local outfit that is building me one ~ 1500-0-1500 | 270H | 15ma.

The bass right now without DSP is surely not something the amp is capable of pushing. I will do the change and see how different it is.
 
Are you using Altec A5, A7 style bass cabinets? These aren't really very flat by modern standards, and do need equalization. Maybe someday in the future you could include their EQ into a dedicated amplifier feedback network. A simple shelf does most of the work, but not everything that a real DSP can do, of course.


All good fortune,
Chris