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In Reply to: RE: Brook 12A bass response? posted by Thermionic27609 on March 12, 2017 at 14:35:46
Back in the eighties, Brook amps were my reference. I restored, upgraded and modified more than a few. As soon as I had a working pair of 22s, my dozens and dozens of more powerful Williamson, Ultralinear and Pentode amps were traded away. I had already enjoyed 12As and fixed a few 10Cs, but never owned a pair of 10s(30Watters). These amps are all very wide bandwidth, even with their input caps and 1000 pf couplers. BTW, even their 30W amps had .01u input caps.
Obviously, with today's very reliable preamp output capacitors, you can bypass the input cap. It was probably factory installed for safety. Back then, circa late 1940s, early 1950s, not only were wax coated paper caps not the most reliable, but many turntables suffered from rumble and subsonic aberrations (like bearing noises).
Now, concerning your math with the .01u across 10Meg, that time constant R x C = 10,000,000 x .01 = 100,000 uS. 159,155/100,000 = 1.592 Hz, not 16 Hz. You must have left out a zero....The input cap .01u across 470K input R = 4700 uS = -3db@34 Hz ! The 1000 pf coupling cap definitely yields a bass cut and can easily be increased to .01, .1 or even .25u as Danby Audio in Phila, PA brewed. Brook seems to have intentionally limited the lowest low frequencies in their lower power amps. Even the 22 had .005 couplers across 470K Ohms between the 12AU7s and 6C4s, before the center tapped plate choke. 470K x .005 = 2350 uS = -3 db at 67.73 Hz ! Even more filtering existed with a 4700 pf plate bypass cap (for HF cut), not shown in Sams, or a factory scheme I cannot find yet. When I hot-rodded my 22s, I lowered that 4700 to 1200 pf and increased the .005uf couplers to .1 uf. With my Altec VOT, bass and highs were lovely.
So, why did the lower power amps get bass limited ? How about their uncanny low distortion rating specs as a good reason? Why push the low power amps when the high power amps cost almost twice as much ? Another great "excuse" for them was that their five-knob preamps had significant bass boost abilities, with their switchable bass control as well as their many choices of phono EQ. With my 22, I could not increase the bass boost above two steps, out of eight steps available. Their five knob preamps could boost 30 Hz by 26 db ! They surely hoped you would buy their all triode system, including their very versatile preamps. Their last preamp, the model 7 is highly prized by now lucky owners. The 22 had a similar preamp section.
It actually now bothers me that I did not keep the 22s. I "graduated" from feedback amps as I built NFB amps from scratch. Buying interstage trannies, opt trannies and triode opt tubes needed funding back then. In the last ten years, I now concentrate on phono preamps and period correct phono EQ for the old mono discs. Those Brook 22 or 7 phono stages would sure have come in handily for some R&D...
Go ahead and increase the Brook's bandwidth. What do you or your customer have to lose, some speakers ? LOL...
BTW, the 10Watter CT plate choke was 3K to 3.6K OHMs DCR, center-tapped, over 100 Henries overall. A proper replacement, other than having one made, could be an old plate to line opt, but finding 100H in the primary will not be easy. Best of Luck on the resto...
Thanks for the great response and the tips. I picked up iron from Heyboer and here are the part numbers HTS-10087 HTS-10083 HTS-10104. I'm stuck on the chassis work but should start on my clones soon. I appreciate your feedback.
High sensitivity, wide dynamic range, low distortion, and smooth frequency response. Pwk
Thanks, I.T., I was hoping you'd chime in. I like looking at design decisions in historical context. It can tell you a lot.
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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