EMG style 2 spring conversion soundbox

[kirtley2012]

If you saw this soundbox out of context you could be forgiven for thinking it was an early EMG soundbox of the late 20’s but it’s not, it started out as a bit of a mystery exhibition type soundbox, all brass and a good base point, it was missing a rubber back and someone made a poor attempt at doing over the top type springs then I had the idea to do an EMG style conversion on it.
I turned down a meltrope style back fitting and soldered it on, made new EMG style springs and after a bit of playing about with mica diaphragms I made an aluminium diaphragm based on the EMGs, it certainly looks the part and sounds a fair bit better than it would have originally and in a quick test earlier it got amazingly close to the performance of an expert 4 spring soundbox, below are some photos , I will try and record a proper video of it in the next day or so, good fun.

[old country chemist]

Hi Alex-you clever chap! Looks the part. I look forward to the video of it playing. Can we order our new diaphragms from you now....?
As an aside Alex, where did you get the finishing papers for your gramophone horn. I now need to paper the inside of my horn, as I have enough for the outside.

[kirtley2012]

Hi Alex-you clever chap! Looks the part. I look forward to the video of it playing. Can we order our new diaphragms from you now....?
As an aside Alex, where did you get the finishing papers for your gramophone horn. I now need to paper the inside of my horn, as I have enough for the outside.

Thanks Alastair, not yet for diaphragms, some refinement needed yet, this one was a bit experimental, but for papers, I got them from the following company

https://theartycraftyplace.co.uk/produc ... ge/papers/

Plenty to choose from, I think I used 475 for the outside of my EMG horn

[Frankia]

Inspiring work Alex. The photographs are great. The whole creation is so good to look at - solid and dependable. But in addition to all that, if you can produce good EMG soundbox springs and aluminium diaphragms, you'll have a queue to your door!
Thanks for sharing all this.

[kirtley2012]

Thanks Seamus, the springs I can do now, I've already been asked to do a couple, the diaphragms I will do in time when I move the lathe to the house and make a suitable setup for spinning them, this one was shaped by hand, long and tedious but it worked and sounds excellent

[old country chemist]

Alex-thanks for the link to the "finishing papers"
I think you ought to talk with chunny about diaphragm making. He has all the old diaphragm making equipment that I gave him years ago that came from George Overstall's workshop. Chunny did say that unfortunately the equipment was a bit damaged, But I think with dressing the formers that were used might be serviceable and were spun on the lathe. I remember seeing George manufacture them years ago. The aluminium he used was unfortunately too thick, and probably a little soft, but you may be able to obtain some better material. I had a feeling that chuuny uses aluminium from drinks cans..?
I had a feeling Keith Badman in Devon used to cold stamp diaphragms, and many years ago, I was asked to test several for quality of reproduction.

[old country chemist]

Alex-thanks for the link to the "finishing papers"
I think you ought to talk with chunny about diaphragm making. He has all the old diaphragm making equipment that I gave him years ago that came from George Overstall's workshop. Chunny did say that unfortunately the equipment was a bit damaged, But I think with dressing the formers that were used might be serviceable and were spun on the lathe. I remember seeing George manufacture them years ago. The aluminium he used was unfortunately too thick, and probably a little soft, but you may be able to obtain some better material. I had a feeling that chuuny uses aluminium from drinks cans..?
I had a feeling Keith Badman in Devon used to cold stamp diaphragms, and many years ago, I was asked to test several for quality of reproduction.

[chunnybh]

Well done Alex.
Regarding George Overstall's formers. Unfortunately they are still in storage in Hong Kong and with travel being restricted, I am not sure when I will be next allowed to go there. Never mind trying to visit the UK. I will return them next time I am in the UK.

There were two formers with a different profile on each. Neither one was like any EMG or Expert profile. The formers were made from soft brass and unfortunately were scarred with a spiral. I assumed someone had tried to release the diaphragm from the former using a blade and had cut into the former. I filled the scarring with car body filler and was then able to make a few.
The Aluminum roll was very soft and the resulting diaphragms were disappointing. The best results were from soft drink cans although they did retain a slight curved memory from the shape of the can.
The process was to wrap the former in the sheet aluminum and clamp it using a clamping ring. It was them fitted to a lathe and using soft tools, plastic and wooden, push the sheet into the former profile. It was finished off with soft rags. Trying to cut the diaphragm to the correct size and release it was tricky. One out of ten was successful.
I was never happy with the final results. I always meant to make new formers and come up with a better way to cut and release the finished diaphragm. One day.

I'll try and dig out some pictures.

[chunnybh]

Here they are.

[old country chemist]

Hi chunny, many thanks for the pictures of the diaphragm making equipment.I had almost forgotten what was in the "kit"
I think the damage was done by someone who I lent the formers to, and they had to use someone's lathe to try to make the diaphragms.
I will not mention who they are, but I do not think they appear on the forum. They said that the diaphragms they made were very good, and I think they even sold some, but I am sure they did not use Overstall's aluminium rolls.
I always imagined that the aluminium drink cans would be too thick, but it seems not. Yes, the drinks cans must have that memory of the curvature, and that must stop diaphragms made from them from being excellent,-but I may be wrong.