The Talking Machine Forum — For All Antique Phonographs & Recordings

I agree, but I imagine with a "varnished" diaphragm-may have even been shellac coating, is there a chance that the non-metallic material of the diaphragm's composition may be better protected from such as atmospherics? Or even the break down of the coating could have effects on quality of reproduction?
Certainly an interesting subject. It is bad enough with aluminium diaphragms suffering with oxide degradation. I wonder if the good old mica or Bozon's patent tortoiseshell may be the only ones not badly affected by the atmosphere, but they could certainly be affected by internal stresses through use and adjustments. This is a subject I know little about, and would like to know more of.

Alastair, the closest example to the Astra diaphragm that comes to my mind is the Edison diamond disc, made of layers of shellac-coated rice paper. They all aged badly, even NOS pieces. This is demonstrated by the vast increase in quality of reproduction when they are replaced with newly-made diaphragms made by a couple of very creative enthusiasts, both forum members. It becomes then guesswork to speculate how a brand new Astra soundbox would have performed. The scarcity of extant pieces also limits the comparison among differently-aged diaphragms.

Thanks Carlos for your information. I think this is a subject that could be of interest to quite a few of us.

Re Astra diaphragm:

Because mine had a tear, I could see a web of threads inside it. I'm sure I've read somewhere these were a silk composite?

Mined didn't age well either, and came in rather deformed (dished). I used steam to try correct that to some extent, but will never know what it sounded like new. One of those days I'll fiddle about with making a new diaphragm for it

Re EMG spacer padding:

Out of curiosity, I've been playing around with small bits of standard gasket tubing squeezed between the soundbox body and platform to see what that does. Depending on where I placed them, the effect ranged from not much to dialling down treble a notch, and perhaps made the sound a bit less open. So much fun tinkering with this thing!