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I am buying a 50s model 302A 1 B-199A 12" Woofer
1 B-200X Dual Tweeter
1 B-209 Midrange
1 N10102 Crossover Network
My question is what do I need to do to get them as good as they can be. If I read right they changed in 1960 are the ones before done as good as it gets and how do I tell. It's a one owner and never ever been opened up. He bought it new has all the paper work. I imagine I could change the tweeter but being its 100% original and pristine do I change it at all. It's a true survivor.
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I had Bozaks...don't remember the model, but it was a floor standing speaker with a 12 inch bass speaker and 2 small tweeter mounted across its opening. I had to send the speakers back for repair twice in the 3 years i owned them.
Sounds like a B300. I enjoyed a pair for 25 years and never had a problem.
As Lederman suggests: the electrolytics in the crossover have certainly drifted: Change them They are in the black box and are stacked inside.
The other Issues I have had with older Bozaks are several:
earliest Bozaka used a paper cone for the midrange, and then switched over to aluminum. The painted black aluminum mids have a foam rubber dampener under the dust cap.
In several units, I have found that that mid drivers suffer from limited movement because the foam has hardened. You can assess this by gently pushing the cone at the edges and then at the center and evaluating the range of movement.
If stiffened, you can cut out the dust cap, and find a cardboard blank across the VC gap, which you will also have to carefully cut out. Remove and vacuum the foam out and replace parts. You can use aftermarket dust caps with little issues.
In higher humidity areas, I have found the woofers to be slightly hydroscopic. They absorb water and become "soggy", Again you can assess the condition by pushing gently against the cone in various locations. If the cone only moves in the spot being pushed, you have issues, as it not moving in a pistonic motion.
I use Min Wax wood harderner to brush the cone. It is water like in consistency and a little goes a very long way. It will seal the cone and stiffen it significantly. A clear fluid, it dries invisible. One application or coating is enough in my experience.
Good luck
I would be very hesitant to modify the woofer.
The B199 cone is a "variable density" design, quite the breakthrough idea from Rudy, and SHOULD be very soft at the perimeter, and hard at the apex. Painting it with any material that will harden the cone removes it basic design which was to minimize and thereby remove the mechanical impedance mis-match at the cone suspension interface, thereby reducing harmonic distortion by an order of magnitude. One cannot assess one small portion of the cone as being soft and apply that "measurement" to the stiffness of the complete upper part of the cone when it is dynamically being driven. Rudy's variable density woofer design was revolutionary, and reduced bass distortions significantly. Stiffening it makes it an entirely different driver. No longer a Bozak.
Peter Ledermann/Soundsmith
Peter,
Thank you for reminding us of Mr. Bozak's genius. He was the first to figure out just about everything important in making speakers work really well for most music, with most amps, in most rooms, without tone controls so common `way back when. Hard to believe people bought JBL Hartsfields and Jubals instead!
I thought this a good place to mention to those audiophiles who are not professional engineers that a good designer will always consider all possible "impedances". Which sounds totally boring, and it can be.
There are mechanical impedances, acoustical impedances, electro-magnetic impedances. We have optical impedances for camera lenses and sunglasses. There are fluidic impedances in oil refineries and in your toilet, thermal impedances in a car's engine and your home's walls, transmission-line impedances for electric power lines and for microwaves in ovens.
When Impedances are not considered properly, the 'thing' will fail, which is why steam boilers killed so many people until after this type of math was first mastered in the 1830s.
The concept of Impedances is useful because it gives the most clear view of many problems, and is a particularly convenient way of keeping track of where and WHEN energies will flow. And still boring.
Detailed discussions of impedances require everyone to be fluent in the math, so it is rare on the internet.
Best,
Roy Johnson
Green Mountain Audio
I was formerly director of engineering for Bozak.
You can certainly change the caps; make careful check that the coils are NOT aluminum wire - which we Bozak switched over to long before I got there and did so for years. If they are - terminate the wires with a SCREW and NUT tightly, and include a lug that you can solder to. TWEETER - CAREFULLY remove it - there are NO REPLACEMENTS suitable.... and look for a concentric crack or discontinuity/fold in the cone - mid way. There should be NONE. The cone should be smooth.
Peter Ledermann/Soundsmith
I will be checking it out tmrw for sure and thanks. Must of been pretty cool working there. At least in my opinion. I'm learning to love this stuff it's getting in my blood. I can only imagine what it was like being into designing these. What's a good book to learn about the basics of speakers for the newbie??
The caps in the crossover are not electrolytics; they are wax sealed caps. That is what they had in that era. Replacing them with plain electrolytics will cause them to short, and destroy the tweeter and midrange. There ARE high quality polypropelene or polyester caps, and those are very good ( look in Parts Express). There ARE non-polar electrolytics, but they have terrible impedance characteristics at high frequencies, and are to be avoided in any high quality speaker design.
BE EXTREMELY careful removing the dust cap on the midrange - the cone is THIN aluminum, and you will destroy the cone. The dust cap DOES offer some structural integrity for the cone, so replacing it is NOT trivial on this driver. Frankly, I would NOT attempt to remove it Bozak drivers are well thought out, and it is best to leave them alone. They were designed for longevity. I would drive the midrange quietly with a swept signal (get a test CD) to see if it buzzes in its range (100Hz to 10KHz.)
The bottom line with Bozak Drivers - if it ain't broken - don't fix it.
The gentleman is correct in saying that the surround is likely hardened, but softening it with chemicals would likely damage the existing glue joints. I have looked for a chemical that softens butyl nitrile after it gets hard, and if anyone knows of such a material, I would be glad to know about it.
Peter Ledermann/Soundsmith
Maybe change the caps in the crossover to modern film types, and leave the rest alone.
" I love you now change"?
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