Friday, December 4, 2009

BROOK Electronics 12A one of PWK's favorites

I think I've finally found the Brook 12A modification specifically for Klipschorns. Brook and PWK worked on this in the early days of Klipsch. This document comes straight from the Paul W Klipsch archivist Mr.Hunter. At first I thought I had the wrong document because it lists the 6B4G instead of the 2A3 but they are close enough it turns out. The 2A3 and 6B4G are used interchangeably in the 10C and 12A schematics. The Brook 12A 2A3 PP amp was reportedly one of PWK's favorite amplifiers. He talks about it in his interview with vacuum tube valley at the age of 94. The last time I was in Hope, AR I got a good pic of PWK's actual Brook 12A23KI and preamp. It is still in his office. Here is the pic.
* SORRY guys I seem to have screwed up on this one. This is not the PWK mod but a different schematic from the archives. Can anyone identify what critter this is from?  *

This schematic is supposed to be of just the modifications of the Brook 12A to the 12A3KI. If anyone can shed light on this and what it actually does please please comment on this post. Here it is

Here is a schematic I've found on the electroweb.

Here are some great Brook preamp and 12A pics and a stereo pair. My ears crave this setup.

A couple of links.
An interesting post on Brook click here.
A TubeCad article on the Brook by John Broskie click here.


  1. HEY how can i contact you ?
    i have an old preamp, nobody i have asked knows anything about it, i feel you could be the man
    here is my email.

  2. Wow, the picture of the stereo pair is amazing. The pre-amps are absolutely exquisite and a real inspiration for cosmetics. I'm going to have study these!

  3. The 12A3KI schematic bears hardly any resemblance to the Brook amps. For example:

    It is cathode bias, the Brook amps are fixed bias.

    The differential-pair driver stage uses plate loaded 6SJ7 pentodes, whereas the Brook amps used a choke loaded driver stage with either a 12AU7 or a 6SN7.

    While the amp may be built on a Brook chassis, and may indeed have been built by Lincoln Walsh (who knows), it looks like a fairly standard 2A3 push-pull design, but with none of the features that are associated with the other Brook designs.

    But it would still have sounded extremely good - it's very hard to beat direct heated triodes - but I have a feeling the Brooks might have had an edge

    The phase splitter is similar, but doesn't have a volt amp preceeding it like the Brook amps

  4. Thank you so much for the detailed information. It seems I'll go back to the archivist to see what I can find.

  5. C3 is backwards in the 12A schematic. I think I recall the SAMs Photofact schematic having the same error.

    1. Thank you.
      I plan on doing a modern 12A so that is very helpful.....

  6. The book "High Fidelity Techniques" by John Newitt (there are two other books of the same title by others, avoid them) has a good deal of Brook information. Brook was in business until at least 1954 and is mentioned in the then-classified W.O.Dixon Navy report of 1955 or thereabouts. The old Sound Practices zine had some Brook stuff too. Brook amplifiers were expensive and low powered as compared to competitors and sold poorly, although they had a dedicated following from the beginning. (By contrast no one in America ever thought WE ERPI anything ever sounded especially good:it's clear the Japanese fixated on them because of W. Edwards Deming.)

    1. I will look for the book. I read everything I can find regarding Brook. Thank you

  7. Iain,
    Did you find the 1955 US Navy report on commercial power amps that Mr. mcStinky mentioned?
    I may have it.

    1. I have not been able to find it but I would love to read and post it.