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Classic gear from yesteryear; vintage audio standing the test of time.

Brook 12A bass response is very capable...

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...


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