Adapting 902 and 908 drivers to 1" horns

[BillWojo]

A lot has been written about the mis-match between the 1" horns and the later 902/908 drivers with the 7/8" throat. Why Altec matched up these combinations will always remain a mystery lost in time and space.
As many of you know, I have a set of HeathKit AS101's and they come stock with the 906 drivers. I had picked up a set of 908 drivers, changed out the diaphragms to aluminum to turn them into 902's.
So what to do about the big mis-match? I was hesitant to remove the bug screens so I had no idea what the internal taper was in the driver. Ideally any adapter should carry on the exact same taper. Thought about it for awhile and decided I would just make them 3/4" thick, should be fairly close. So out of 4" aluminum barstock I made adapters 3/4" thick. I taper bored them from 7/8" to 1". I added counterbored holes so I could bolt them on using the stock Altec studs with washers and nuts. Two 1/4-20 tapped holes and some bolts would secure the adapters to the horns.
So today I was bored and decided to install them. Some of you had convinced me by now to just take out the bug screens so I did. Bolted up the adapters and boy did the tapers look close! I grabbed a 1/4 wide machinist scale and laid it inside the bore expecting to see a big mis-match in the tapers but they were almost perfect! Being a machinist I'm rather fussy, I wouldn't change a thing, the tiny amount of mis-match could hardly be seen under a strong lite, no more than a few thousands. Somehow I nailed it!
So now we have definite dimensions to build adapters that any competent machinist can knock out.
Oh, and I'm still on my first CD, listening to Dire Straits, sounds great! Will only get better as I add my scratch built Model 19 crossovers.
Non of this would have ever happened without all of the input and info on this board, for that I am grateful.
Thank you all.

BillWojo

[GM]

'Sign of the times', Altec was being gutted and folks did what they could to try n' satisfy marketing with whatever was available.

It's a conical expansion of ~7-8 degs; I measured, published the one for the 1" on one of the horn centric forums, but couldn't find it in a quick search and AFAIK the smaller one is scaled to suit, so same wall angle. If I feel up to it anytime soon I'll measure mine again if either no one does it in the meantime or has a copy of my original post.

That said, better a 'flat' expansion [cylindrical] than otherwise positive or negative unless very minor.

GM

[BillWojo]

I think that anyone that matches my dimensions would be very satisfied with the results. It's basically a straight cone from the tangerine phase plug to the horn interface. I really didn't know what to expect after I ripped the bug screens out. I was very pleasantly surprised to say the least.
I came up with the 3/4" dimension for the adapter thickness when I calculated the overall length of the 902 driver against the 806 driver. I didn't want to have to modify the back panel.
My first rule when working with vintage speakers is to do no harm. I need to pick up a few pieces of aluminum to take the place of the stock crossovers. I will attach terminal strips for both the woofer and horn and mount my crossovers externally. While inside today I measured the screw mounting layout for the stock crossovers and will use the same holes for the mounting plates. All the stock components will be packed in a box so that it can be restored to original if need be.
After listening for a bit, I find the 902 drivers quite a bit hotter than the 806 drivers. I think if I have some time I will hookup the 30904 networks that I picked up.

BillWojo

[GM]

Right, same as the 800 series, but is its wall angle the same?

GM

[Earl K]

So Bill, how would I ( not being a machinist ) duplicate your efforts ( of "taper-boring" ) with ( say ) Home-Depot type parts ?

I can buy aluminum stock here, & I don't mind buying metal working bits ( & I also don't mind using the odd router bit on softer aluminum ) and have access to a buddies drill press.

Since I have a pair of orphaned 902's, I'd love to make some aluminum adapters like yours .

Thanks <> EarlK

[bowtie427ss]

One of the best ways i've gone at horn, and adapter throats is to turn out a couple pieces of hardwood at the desired angle as arbors/mandrels, glue strips of abrasive to it, and go to work either by hand, or the lowest speed on the drill press. If you drill a 7/8 pass hole thru the adapter to begin with, you're not looking at taking away a whole lot of material. I must confess that this idea came to me when many years back we acquired an Edison Amberola that came with a similar tool for dealing with cylinders that didn't fit the machine's taper as snugly as they should.

There are thermoplastics that would probably be ideal for making adapters as they would be just soft enough to negate the need for gaskets. Lexan, Nylon, high density polyethelene, etc.....

Of course, this all requires access to a lathe. But, a simple wood lathe will do if that's all you have.

I also know fellas in the 2-cycle engine business that are so good with a die grinder or dremel, that they could pretty much scribe lines and put the taper in the workpiece freehand. The cost for their time is however fairly cost prohibitive. But, a clever fella with time, material, and a basic workshop..................

Just some thoughts for the crafty DIYer.