Glory: Brook Model 7 Mono Preamplifier, Part I
Note: Part II has been published. Substitution of the 6SJ7 results in major improvement.
The Long and Winding Road
More than 10 years ago, when I was addicted to Ebay, I purchased a pair of the legendary Brook 12A's (2A3 PP) from a gentleman in Germany. I also simultaneously from the same seller purchased a pair of the little known mono Brook 7 preamps. I could find no info on the Brook 7, and I wondered whether I paid too much.
I still remember the day they arrived at my door. The packaging was not sturdy and a hole was punctured in the carton, with an edge of one of the amps showing. Luckily, no great harm was done. Talking about in the nick of time!
I hooked up the 12As to my Klipsch La Scala (Paul Klipsch himself used them with the mighty Klipschorn, and I could understand why), and they worked but both had a pronounced hum, likely in need of capacitor replacement. But even though the listening was necessarily brief I could tell these were great amplifiers. So lively, so direct, so much presence! They impressed in an instance and left an indelible memory. It is my fault that I never found time to restore them, but that is due to change soon, I promise.
Shamefully, I had to wait till January 2012 to hook up the Brook 7 preamps (here). Sacrilege indeed! But, very similar to my reaction to the 12A, the fleshy sound immediately appealed to me. One of the preamps was faulty, and I only found time recently, two years later, to rectify the situation by having a dear friend restore them, and they are now treasured possessions that I doubt I should ever part with.
Brook 7: Basic Info
There is still very little information on the internet, and I hope my article adds a little to the database. First, I quote a comment on Audio Asylum:
"...All of the Brook stuff is very fine. Their first preamp was mounted side-saddle on their super rare first version 30W PP2A3 amp. Then, the four knobbed type 3 preamp arrived, which umbilically powered up from the 10C (30W) or 12A (10W) PP2A3 amps. The model 4 preamp came, soon after, with 5 knobs and very versatile, switchable/variable phono EQ. It also stole voltages from either amp, or, a very, very rare external power supply. All of these preamps, so far, used triode connected 6J7 and 6SJ7 tubes...The model 7 preamp is a very desireable, self-powered preamp, with the same versatile EQ switching and very useable tone controls. Utilizing some miniature triode tubes, it's topography is similar to the preamp section found on their final amp offering, the model 22 integrated amp. Into the mid-'50s, designer/engineer Lincoln Walsh was the last, stubborn hold-out for all triode circuitry, while everyone else was racing to the higher power pentodes. He knew the triodes had the "natural sound."..."
Next, my comments on the features of the Brook Model 7:
- For detailed info, please read the 2-page brochure attached at the end of this article. The detailed specifications put many modern preamps to shame.
- The tone controls are among the most unusual I have seen. Tone controls are frowned upon by purists these days but they were ubiquitous back in the 40's and 50's. They are in the circuit and unless bypassed might as well be used. Please note that, unlike most cheaper designs utilizing a carbon pot, no expense was spared, as both treble and bass controls utilize a 10-position stepped resistor-capacitor ladder (click on top right pic). While the Treble Control clearly indicates position 8 as flat, and so mostly attenuates (click on top left pic), the Bass Control does not have such an indication on the front panel. However, if you look at the middle of the 3 tables at the bottom of the brochure, position 4 is flat and so boosts more than attenuates.. Also, unlike the treble control, the bass control is not independent of volume: note the first line in the brochure (under frequency range), the measured spec's were understandably conducted with the volume control at 7 (if you look at the left pic, this is at 1 O'clock, which is usually nominally in the flatest range of a carbon potentiometer used for the volume control). The bass boost is in addition to the defeatable "automatic bass compensation", which is what we usually know as "loudness control".
- Like many preamps of the time, there are provisions for many phono equalization curves. This would be very useful for mono vinyl fanatics (also, only one mono preamp would do! :-))
- As with designs of its time, it uses tube rectification. Like most others, a 6X4.
- As noted above, all preamp tubes are triode or triode-strapped. The tubes used, 6J7 (octal pentode, 1935), 6SJ7 (octal pentode, 1938) and 6C4 (miniature 7-pin single-triode, 1942) are common for that early time, which overlaps with Western Electric era, and long before the advent of smaller noval double-triode tubes like the 12AX7. Note also the 6C4 is electrically half a 12AU7.
- The phono section uses 1 x 6J7 (V1) and 1 x 6SJ7 (V2). The line section uses 1 x 6SJ7 (V3) and 2 x 6C4 (V4, V5) and seems to be an anode follower (hence better to avoid long interconnects) . I was told the circuit is rather unusual. I would love to learn more about the design and so if you have more to say on this subject, please post a comment or email me. Thank you in advance.
First, associated equipment are as described in a recent article (System A in this link):
CD Transport: Sony BDP-S190
DAC: Arcam rDAC (also CAS through dongle)
Preamp: Kondo M7, Brook Model 7
Amp: Western Electric 124B (original)
Speakers: Yamaha NS-1000
As I have said above, even when faulty the sound appealed to me, and I was stunned after the restoration. Allow me to expand:
- First, This kind of sound is impossible to describe, so different it is to almost all other vintage and all modern preamplifiers. Although not without its faults (see below), it is on the same footing as Western Electric, and for me that is the highest accolade.
- I have only auditioned CD's through Input A. The sound is completely different from the Kondo M7. Every CD I played is a rediscovery, so different is the musical presentation. Our reactions to the WE133 amplifier can be equally applied to the Brook Model 7. To quote myself a little: "...the real WE sound is quite neutral, possessing transient speed, refinement and detail of the utmost order, fully up to par with modern gears. The WE has superior PRaT and conveys like nothing else the leading edge, phrasing and articulation of music. And all of this in a balanced whole..."
- The Kondo M7 (which also employs the 6X4 rectifier) is imho one of the very best modern preamps. It reveals more rhythmic subtleties and microdynamics than most rivals, but the Brook Model 7 trumps it in rhythmic finesse (though the Kondo is better with large-scaled music - see below).
- The Brook Model 7 is not without its Achilles' heel: with large-scaled symphonic music, it tends to loosen its control - no iron-grip of heavily regulated modern preamps here. This is no surprise to me, as this is a universal trait of vintage preamps (WE is the only exception). As my main diet is usually symphonic music, you can understand why I have always used modern preamps. I do own many famous vintage preamps (like Marantz 1 and 7, McIntosh C22 and C20, Harmon Kardon Citation I etc), have used others (like Eico HF-85) in the past, and have heard numerous others, but the Brook Model 7 is the only one that is able to make me forget its shortcomings. Again, a passage from my article on the WE133, a comment by my friend icefox, can be equally applied here: "...the WE just illustrates what is wrong with hifi...usually we spend all our energy listening very hard trying to identify what is good or bad about the system, fixating on one or two particular qualities...not so the WE experience; with WE we just listen to the music as an organic whole, relaxing and enjoying the experience...the WE goes straight to the essence of every type of music played and draws you in...it may not casually seem the most detailed at first listen, yet you find everything is there... " Note: Part II has been published. Substitution of the 6SJ7 results in major improvement.
- In my setting, the loudness control is defeated. With the Treble Control set at "Flat", there is plenty of air and space around the instruments. Simply Superb! When it comes to the Bass Control, I actually find it useful and prefer it at position 2/3 - this is likely because my Yamaha NS-1000, placed upon a wood platform (see pic above), suffers from a just little too much bass. However, even at the "flat" position 4 music is still eminently listenable. Of course, it should not be a huge effort to have the controls bypassed altogether, but personally I'd prefer to keep these rare vintage items as near original as possible.
- I wish I could evaluate the phono section and the efficacy of the various phono equalization curves, but that will have to wait.
- Tubes are what came with the unit, rather generic: metal 6J7's, metal 6SJ7's and GE 6C4's. I plan to roll in some tubes (for the line section) in the near future. I am sure performance can be taken to a even higher level. The amazing thing is, given my dislike of metal tubes, the unit still performs as miraculously as it does.
Links and Acknowledgements:
- There are some great pics at Radiomuseum.
- I thank Tom Bravis at Audiophool for the schematic.
- In addition to the PWK link for 12A above, the precious pics below of the Brook Model 7 brochure are from Iain at itishifi: