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Brook 12A bass response?

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Posted on March 12, 2017 at 14:35:46
Thermionic27609
Audiophile

Posts: 326
Joined: March 29, 2009
I'm working on a Brook 12A 2A3 amp for a customer. This one is in rough condition, not a museum-piece, so there is not a lot of concern about absolute originality. It's been previously modified and monkeyed with.

In strategizing the project, I'm noticing some high-pass filters, one at the input (0.01uF into 470k) and a 1000pF mica capacitor into the bootstrapped 6SN7 cathodyne phase inverter. Now, I know that a bootstrapped cathodyne PI has a very high input impedance, but even calculating it at 10Meg, you get a 16Hz high-pass node.

Not having heard a Brook 12A before, was this amp purposefully bandwidth-limited on the low end for the speakers and musical signals of the 1940s?

On another site, someone working on a 12A clone omitted the input capacitor and bumped the 1000pF coupling cap (C9) to the PI to 0.1uF. And, in fact, on this unit, someone changed the input wiring to bypass the 0.01uF capacitor to the first stage, making it DC coupled at the input.

I was thinking of omitting the input capacitor and perhaps increasing the PI input capacitor to 0.01uF or 0.022uF. Or is it the case that this amp doesn't need the bandwidth extended for late 20th/21st century recordings?

 

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RE: Brook 12A bass response?, posted on March 12, 2017 at 18:55:50
briney
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Posts: 160
Location: Niles, Mi.
Joined: June 5, 2004
Contributor
  Since:
December 0, 0000
According to the Brook propaganda the 12a had a bandwith of 20hz to 30k.
My first thought is, how old are the coupling caps? You say the amp was monkeyed with, what changes?

 

RE: Brook 12A bass response?, posted on March 12, 2017 at 21:36:22
Thermionic27609
Audiophile

Posts: 326
Joined: March 29, 2009
The 1000pF coupling capacitors are original, square mica.

The tapped driver plate choke was missing, and someone appears to have started to modify the circuit to get around its absence, but gave up and left things disconnected. I've sourced a replacement plate choke from Heyboer to put it back to to a more original configuration. The wax-dipped paper caps have to go, obviously, unless someone just wants to put it on an altar and worship it.

That said, I'm not sure how that's relevant to the question. It came to me in non-functional condition. I've tested the power and output transformers for faults, and I'm mapping out the restoration. I can calculate the high-pass filter formed by a capacitor and grid-leak resistor, and it puzzles me that the original circuit appears to have two high-pass points that would seem to me to limit the low frequency response.

I also work on guitar amps, and I've seen guitar amps from the 40s and 50s that have the same sort of bandwidth-limiting high-pass filters. In that application, it's often a good idea.

 

RE: Brook 12A bass response?, posted on March 13, 2017 at 04:01:23
vinnie2
Audiophile

Posts: 2520
Joined: September 28, 2013
Why not just get the schematic for it (it is still around) and put everything back the way it was, replacing caps and other parts as needed? At least then you would havce a baseline to work from.

 

RE: Brook 12A bass response? 1953 update, posted on March 13, 2017 at 06:05:00
Iain42
Audiophile

Posts: 785
Location: Arcansaw
Joined: February 10, 2004
A 1953 original brook schematic showed the following updates.

R14 R15 changed from 220K to 68K
C12/C13 C14/C15 changed from two .1 to .5





High sensitivity, wide dynamic range, low distortion, and smooth frequency response. Pwk

http://www.itishifi.com

 

RE: Brook 12A bass response? 1953 update, posted on March 13, 2017 at 07:10:51
Thermionic27609
Audiophile

Posts: 326
Joined: March 29, 2009
Iain42,

Where is this updated Brook schematic available? I've found the 1950 SAMS schematic and thought that that was the only one -- with a few errors.

Are you saying that they increased the RC input to the 2A3s to 1uF/68k? Or 0.5uF/68k?

Edit: I found the Brook 22A schematic, which appears to be virtually the same circuit as the 12A implemented with 12AU7s and 6C4s instead of 6SN7s and 6J5s. It doesn't have the same high-pass points as the 12A. The input to the PI is DC coupled from the preceding 12AU7 plate, and the input capacitor has been increased 5x. The driver is coupled to the 2A3s with 0.5uF/100k, plus 1k grid stoppers.

 

RE: Brook 12A bass response? 1953 update, posted on March 13, 2017 at 12:16:12
Iain42
Audiophile

Posts: 785
Location: Arcansaw
Joined: February 10, 2004
I took notes from a Brook 12A 1953 schematic I saw in the Klipsch Museum Of Audio History. It was listed as updates. I am in the middle of a 12a clone build so I am going with the updated version.


C12 C13 to one .5
C14 C15 to one .5

R14 68K
R15 68K



High sensitivity, wide dynamic range, low distortion, and smooth frequency response. Pwk

http://www.itishifi.com

 

RE: Brook 12A bass response? 1953 update, posted on March 13, 2017 at 13:30:48
Thermionic27609
Audiophile

Posts: 326
Joined: March 29, 2009
Ah, so the 1953 Brook 12A revised schematic is a physical document that no one has scanned?

 

RE: Brook 12A bass response? 1953 update, posted on March 13, 2017 at 13:44:01
Iain42
Audiophile

Posts: 785
Location: Arcansaw
Joined: February 10, 2004
True.
It may be released at a later date but out of my control. It turns out these schematics were shipped with the actual Brook amplifiers.








High sensitivity, wide dynamic range, low distortion, and smooth frequency response. Pwk

http://www.itishifi.com

 

RE: Brook 12A bass response?, posted on March 13, 2017 at 14:16:10
briney
Audiophile

Posts: 160
Location: Niles, Mi.
Joined: June 5, 2004
Contributor
  Since:
December 0, 0000
For some reason I thought you had a working 12a. Good luck with your rebuild.

 

RE: Brook 12A bass response?, posted on March 13, 2017 at 14:37:49
Thermionic27609
Audiophile

Posts: 326
Joined: March 29, 2009
Thanks. It should work when I'm done. I tested the OPT in a few ways including running a signal into the secondary and checking the primary for any distorted or unbalanced waveforms, plus a HV leakage test using a condenser checker as the supply. With amps this old, you never want to get to the end of a repair only to turn it on and find that the iron is bad.

With point-to-point circuits, I find that it's best to think through any possible mods or updates before you start soldering components to terminal strips.

 

Brook 12A bass response is very capable..., posted on March 13, 2017 at 16:04:05
Interstage Tranny
Audiophile

Posts: 2986
Location: Eastern
Joined: October 4, 2006
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...

 

RE: Brook 12A bass response? 1953 update, posted on March 14, 2017 at 04:45:18
vinnie2
Audiophile

Posts: 2520
Joined: September 28, 2013
If you would be willing to post your final schem for the 12A clone (along with pics of course) I would really appreciate it. I have often thought of doing exactly the same thing, and I am casting about for a new project.
Thanks!

 

RE: Brook 12A bass response is very capable..., posted on March 14, 2017 at 16:51:19
Thermionic27609
Audiophile

Posts: 326
Joined: March 29, 2009
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.

 

Brook 12A response is very capable..., posted on March 14, 2017 at 17:39:49
Interstage Tranny
Audiophile

Posts: 2986
Location: Eastern
Joined: October 4, 2006
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...

 

RE: Brook 12A bass response is very capable..., posted on March 16, 2017 at 11:09:19
Iain42
Audiophile

Posts: 785
Location: Arcansaw
Joined: February 10, 2004
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

http://www.itishifi.com

 

RE: Brook 12A bass response? 1953 update, posted on March 19, 2017 at 11:12:39
Iain42
Audiophile

Posts: 785
Location: Arcansaw
Joined: February 10, 2004
I am building from the sams schematic with the changes I saw from the 1953 update. I may make some of the changes mentioned here. I'll try to track changes.



High sensitivity, wide dynamic range, low distortion, and smooth frequency response. Pwk

http://www.itishifi.com

 

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