Difference between Altec diaphragms

Hello All,

Could someone, or several of you, please explain in layman's terms the difference between the Pascalite, and aluminum diaphragms for the 800/900 series drivers?

I have recently acquired a pair of GPA 26420 diaphragms with the intent of putting them in 802-8G drivers. As i look at the chart on the GPA site, it tells me that the correct diaphragm is 34647.

Am I making a grave error by installing the 26420's?

As a side note i should mention that one of the drivers has a 26420 with the maestro and number 909 inked on the dome. I have no idea if it's original to the driver or not, it's coil tests open although i cannot visibly see any defects.

Will there be a significant difference in detail or sensitivity between the 34647 and 26420? or am I making a mountain out of a molehill?

One of the driver bodies i'm going to use began it's life as an 808-8B, it's my understanding that the only difference between the 802 and 808 of the same vintage is in the diaphragm and loading cap. However, as i look at the cut sheet for the 808-8B, it doesn't show either a loading cap or a Symbiotik diaphragm. Further confusing is the 909-8/16A driver cut sheet as it lists the 26420 as a 16 ohm coil and the 26421 as an 8 ohm coil. The GPA 26420's both measure a DC resistance of 6.1 ohms. It's my assumption that GPA kept all of the corresponding part numbers the same. So, part 5 of my original question is, were there really that many errors/discrepancies in the original Altec literature?

I greatly appreciate your responses as well as the wealth of valueable information dispensed in this forum.

In addition to the actual construction, some of it is a bit subjective.

The Pascalite technically is an aluminum diaphragm made of a different alloy.

I have heard that the Pascalites have the same HF response.

I have also heard, and agree with, that they have slightly less.

It isn't a huge difference. I would use aluminum for home use and Pascalite for Pro use.

If you already have one diaphragm I would match it with the other.

As I say, some can't tell the difference. So it's not a serious problem either way. Both sound good.

As always, your mileage may vary.

First off, the 26420 is the 8-Ohm and the 26421 is the 16-Ohm.

The Pascalite series retain that aluminum sound that the symbiotik kinda muffled. The 808 (and 908) definately had the symbiotik diaphragms.

Next, you definately are not making any BIG mistake by putting the 26420 in your 802. Nothing will be damaged and your power handling will double.

I've done more listening on the 288 side if things and the aluminums definately have a smoother sound to me and you can show on a frequency response chart where the aluminums have better and FLATTER HF response. In particular, things get a little nasty around the 12K-13K range. Some horns do worse with this than others. If the aluminums have the power handing for the job, I go with the aluminums...if I need the extra power, I don't hesitate and switch to the Pascalite.

On the 800/900 series, I would suspect the HF response thing is far less critical...the mass of the diaphragm is notably smaller than the 288 series and will have a better HF response, inherently. That said, my 604s have an will retain their aluminum diaphragms.

My best advise is to simply listen to them...let your ears be your judge and not what people tell you sounds better/worse. You are the one that has to listen to them.

Many thanks Gentlemen,
Now i have a real meat and potatoes question. I'm at this moment getting ready to load up a pair of 802 bodies with the aforementioned diaphragms. The bodies i've chosen are originally 808-8B's. Upon removing the original diaphragms, i've discovered on both drivers, a paper shim/gasket. I've never encountered these before, i have no means of measuring the thickness but an educated guess is .003"-.005" . Should these be left in place? were they part of the symbiotik power handling by allowing more travel before the diaphragm impacts the phase plug?

The gaps are squeaky clean and ready for assembly, but i need the confidence, and a little more knowledge that i'll gain from the replies on this board.

Thanks to all

Bowtie, I suspect those were for the Symbiotiks. However to make sure I would call Bill at GPA.

Personally I don't think you'll need those gaskets. But Bill's opinion in this case is worth two of mine.

Thanks much Old Guy, I have much faith in your opinion and experience.

I have also converted a pair of 808 8Bs to 802 8G status. Removed the loading caps, installed GPA regular aluminum diaphrams and do not remember seeing any sort of spacers/gaskets that you mention. They work fine. I assume the diaphrams you are using are pascalite then? My understanding is these were supposed to handle power like the symbiotic, but have more high frequency energy. Siode note to the 808s, only the 808 8B has the tangerine phase plug like the 802 8G, the 808 8A does not, but retains the standard 802 phase plug.

Russellc

One other thing on the conversion, when you remove the loading caps the felts must be installed in the back covers.

Russellc

On the paper shims, Bill at GPA said that they were used to adjust for the stack-up of manufacturing tolerances. They had a measurement jig at the factory to measure the final tolerance in the relationship of the mounting flange to the phase plug.
In practice I believe he now installs the diaphram without them, cranks them up and listens for any interference. I didn't ask him on this, but I would assume that this is just as good a test as any, because even if the magnet structure has been built and measured, you still have a tolerance to the diaphram assembly as well.
Another way of saying this would be, if it plays full power without hitting, don't add a shim, and if it hits, it's going to need a shim whether or not it measured a certain way.

I measured the shims, they're both .003". I've removed them thinking that since i'm currently running them XO'd at 800 2nd order that i'll likely have to significantly overpower them before any physical interaction occurs even with everything at minimum tolerance. When i start playing with an active XO or DSP with them at lower than 800, i will give serious consideration to re-installing the shims. Doing the rated power test just plain scares me.

I greatly appreciate everyone's input on this one. I was too humbled and reluctant to bother Bill H. with a phone call. It's my understanding that he's a very busy man, and what he does is of great importance to us all. I just figure i'll wait and take some of his time when i order my 604-8H's................ someday.

I also need to give a thanks to the poster that made the suggestion of using a boot liner for a source of damping felt for the driver covers........... PERFECT!