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Brook 12A response is very capable...

Yes ! I was scouring through my factory lit copies and confirmed the input cap was used as a safety measure for rumbly table motors and inter-station tuner noises. It can be eliminated with a wire bypass or larger value cap as they mention.

That 1000 pf coupler after a 24K series R, then another 1000 pf to ground is an untold story, though. It's definitely an intentional low-pass/high cut filter arrangement. 24K x .001= 24 uSec. = -3db@6631 Hz just for the series RC ! My guess is this is like a tone control stage. In fact, when you see a small value cap across a cathode resistor in tone control stages of many tube integrateds, they often yield even higher time constants yielding pronounced mids, in an effort to "compensate" for actual tone control usage. The Brook 12A "series" 1000 pf deserves upgrading with much higher values and that 24K can be removed. Also, that 1000 pf to ground is not really needed.
Again, we must consider Brook's original intentions for us to buy their amp and preamp "system" which they insisted measured and sounded better than the finest competition at the time (McIntosh and Marantz).

I was studying the preamp stages on the 22A schemes I have, as well as the rare 7 preamp schematic. Back then, circa 1953-1954, phase errors did not seem too important. Brook's phono EQ uses feedback networks for bass boost/turnover and passive networks for rolloff. Each tone control, bass and treble is placed after another gain stage. In fact, bass boost and bass cut are before and after successive gain stages. While the frequency response control might be plotted easily, phase errors will definitely be heard in a "dual mono" stereo Brook preamp system. I am sure equipment synergy can be achieved, but simpler tone control stages were available and cheaper/easier to install. The Baxandall feedback tone controls date back to earlier 1950s and still sound fine today....

Thanks for encouraging me to research the Brook stuff. Compared to some no neg fdbk DHT amps, the Brook amps can excel in bandwidth extension and control. The NFB DHTs excel in dynamics, soundstage and speed. Also, the Brooks prove that overall feedback loops can not only extend bandwidth and lower distortion, but they prove feedback loops can be applied around interstage trannies....

Lastly, Chief Designer and Engineer Lincoln Walsh was a true inventor, not just an innovator. While his first "Power Drive" PP 2A3 Triode amp appeared in 1939, he later (early '70s) invented the (Walsh) Ohm A and Ohm F omnidirectional, full-range, very power hungry, speaker drivers.

Best of Luck with your resto work...


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  • Brook 12A response is very capable... - Interstage Tranny 17:39:49 03/14/17 (0)

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