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Old 15th December 2008, 12:54 AM   #1
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Default Brook 22A restoration, HELP!!

I acquired one of these, cheap, from a thrift store, I bought it just to pick up the 2A3's even though different brands. But before I ebay'd it, I thought I would just take a risk and see if it worked. I know, use a Variac or a series light bulb, I did neither
Well, it sounded incredible, even though I had to run CD player through one of 6 phono equalizer networks. It's the first mono amp I ever heard, that didn't make me wish I had stereo. It rocked the garage playing Dire Straights. Lots of bass! So I let it run for awhile, then I heard a couple POPS and shut er down. (It was still playing).
Took it back apart (not easy). looks like a 40uF cap opened up, that is downstream from a 47k resistor, and the solder connections to the series 20W, 700 ohm resistor were loose. I tagged the wiring, took everything loose off the terminal strips, because they looked sad. Replacing the cap, looked like it was going to be no problem, as I had some NOS Mallory 50uF/200VDC caps in my spares. Looking on line though there is at http://www.stormkingny.com/Downloads/index.htm
a drawing from 1953 that showed this cap rated at 450VDC. I don't have one handy. Can anyone see if this was just being overspec'd by a draftsman, or does it need to be that high a voltage. I believe it supplies the negative bias, so I can't think why it would need to be this high. Thanks, and does anyone actually have one of these? Oh the reason it was still playing was because someone had put a 15A fuse in!
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Old 15th December 2008, 02:32 AM   #2
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Sparky,

I think you are talking about C200, which is intimately associated with the B+ circuitry. Damned right you need a replacement part rated no less than 450 WVDC. $12.07 buys a nice Vishay/Sprague "Atom" from Mouser. Mouser's stock # is 75-TVA1712.

BTW, the late Paul Klipsch liked Brook amps. Maybe you need to hunt a 2nd specimen down.
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Old 16th December 2008, 02:32 AM   #3
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Can you help clarify for me why the 700 ohm 20W resistor is in the CT leg of the power supply? Is it setting a negative supply voltage relative to ground "downstream". The schematics for the Brook add in things inplaces that make it hard for me to understand what's going on. The schematic doesn't seem to show what is actually at ground potential. I'm speaking of the schematic shown on my previous message, not the ones floating around on the internet that aren't complete and don't match thea ctual amp very well.
Thanks and pardon my ignorance.
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Old 16th December 2008, 05:23 PM   #4
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You are correct that this resistor does set the bias for the output tubes. The way it does it is that all grounds for the entire amp run through this resistor including the CT of the output tubes filament transformer. The exception is the grid of the output tubes. Notice it's ground reference is the CT of the power transformer secondary. Therefor, the ground potential of the entire amp is at the same voltage as the bias for the OP tubes.

This is a common way to establish a bias without having a separate bias supply.

One could learn a lot from studying this schematic, thanks for finding it.
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Old 17th December 2008, 12:34 AM   #5
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So to your understanding, would there be any reason for a 450 volt rating on that capacitor, like the original drawing shows, over one having only about twice the bias voltage? I think the 160v 40uF cap was an original. I'm thinking or replacing all 3 wirewound resistors with Dale heatsink resistors, mounted to an aluminum plate held by 2 screws where the original solder terminals were mounted. The next big fun thing will be replacing the 3 element FP can capacitors. That will be a pain due to the number of wires tied to them and the can grounds. Needless to say, star grounding wasn't used. This mod wouldn't be readily visible, unless one removed the bottom plate. The original 700 ohm wirewound resistor bubbled off some of its vitreous enamel in one spot but still measures 700 ohm. I feel like extra caution is okay due to it being bias supply. Thanks
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Old 17th December 2008, 03:32 AM   #6
SY is offline SY  United States
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Don't skimp on the voltage rating. First, line voltages tend to run higher now than they used to. Second, the high temperatures present in a tube amp are deleterious to capacitor reliability; a healthy de-rating should be the rule. Third, voltages can swing around a bit during warmup when tubes aren't drawing current.

450V caps are cheap and easy. Why risk booms and sparks? I'd even go higher.
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Old 17th December 2008, 04:32 AM   #7
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I'm not against putting in a higher voltage cap, I even have some Xicon which I could use temporarily. It's just that I'm tried to understand the circuit better. What voltage should I expect to see from the hot side of the 700ohm to ground. The negative bias voltage I'm thinking. About -45 volts in other words. Thanks
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Old 17th December 2008, 05:22 AM   #8
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If you are talking about C200, there is at least 300V across that capacitor when the amplifier is operating and drawing its normal idle current, during warm up it will be higher. (C200 is across the supply from cathode to center tap and sees the full supply voltage at all times.) A 450V capacitor is advisable.

I would expect about -45V to -55V across that 700 ohm resistor, and yes it is effectively the grid bias.

You're talking a fairly valuable and collectible amplifier, might as well do it right the first time.
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Old 17th December 2008, 02:01 PM   #9
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Actually I think I am referring to C207, which on that dwg is rated 150V. Sorry for the mixup, hard to see that diagram clearly, haven't had time to download the hires image to see if it's clearer. I will take my time and do it right. Truly, thanks....
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Old 22nd December 2008, 01:43 PM   #10
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Hi again..
Can someone recommend a good spray flux remover or other process to remove some of the build up of 56 years. (I don't think I want to try the dishwasher, over treatment) This thing looks frightful in some ways. Really tight construction. And the flux residue makes it hard to see the solder joints clearly. I don't want to remove lettering or finishes. Are there some flux removers that are a real no=no on vintage equipment? I'm half tempted to part with this thing, as i'd want to replace the leads from the xfmrs and get rid of all the preamp components and tone controls. Or just bypass them to one of the phono plugs. Any thoughts, it did sound really good, for a short while, but I don't see it as anything other than a historically valuable piece. But I don't have much in it yet. Once I start replacing things it's hard to see where to stop!
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