16-02-2025, 10:03 PM
(This post was last modified: 16-02-2025, 10:08 PM by SVLWinwick.)
Hi everyone
If you read my post about the restoration of a GEC AC5, you will realise that this post is related. Briefly, the AC5 was to be used on a regular basis to support historical social events, often with a WWII theme. This is the story of the contemporary extension speaker to be used with that radio.
It’s a W.B. device, vintage circa 1931. It has a Darwins of Sheffield four pole permanent magnet assembly.
Darwins was principally a cutlery manufacturer but in the late 1920s developed a reputation for producing particularly good permanent magnets for loudspeakers. To put this into perspective, the first patent for a moving coil, permanent magnet speaker was filed in 1921.
For any readers not familiar with the intricacies of loudspeaker design, early speakers differ from modern ones in the design of the spider (the bit that connects the speech coil and the bottom of the cone to the frame). The early design has the spider (which is shaped a bit like the Isle of Man legs) on the inside of the speech coil and bolted to the middle of the magnet pole piece. That’s what the nut is that you can see in the centre of this one.
This significantly limits the travel of the speech coil and hence the cone and causes a rapid drop-off of response at the bass end. A modern speaker has a spider that is more like a circular accordion which fits around the outside of the coil, connecting it to the speaker chassis. This allows much greater travel and more bass. The spec. that was agreed with broadcasting companies in the early thirties was that speakers should have a flat response from 300Hz to 5000Hz so that orchestral music could be reproduced at an acceptable quality. This one does reasonably well down to 200Hz but drops off sharply at 5000Hz. And it has an odd ‘dead spot’ around 2900Hz that I assume is a resonance issue. However, it sounds pretty respectable now it has been restored.
The problem it had was with the suspension, which is at the upper end of the cone. Originally, it was made from chamois leather and over time it has just decayed.
When this happens, the speech coil can touch the pole piece and it causes rattles and eventually, electrical failure. Someone had stuffed a lot of paper around the magnet and under the cone, probably to subdue the rattles.
Many people want to preserve speakers of this vintage as near to original as possible, and with care, the rotten chamois can be treated with a liquid rubber solution to restore flexibility and strength. That would be impractical for a speaker that is going to have to work for a living.
Fortunately, the cone, speech coil and spider were in good condition, so I replaced the suspension with a modern foam replacement and realigned everything.
There’s a company called North Speaker Parts who sell all sorts of bits and pieces, so it’s just a case of measuring carefully and finding the closest match.
This left me with an authentic-sounding and reliable driver. You can hear it in the cabinet by clicking here for music and here for speech.
The speech coil has an impedance of 14 ohms but the unit is intended for use with a high impedance extension speaker output like the one on the AC5 which comes straight off the output valve anode via a capacitor. If you have something similar, I strongly recommend you replace the capacitor with a modern, X-type safety capacitor. There is a Ferranti output transformer acting as a matching transformer inside the speaker cabinet. This is original.
Once the driver (speaker) had been repaired and cleaned up, the only thing left to be done was some minor attention to the cabinet. Some beading needed regluing and the surface finish was cleaned and sealed. A hole in the cloth covering one of the rear vents was repaired by gluing to fine black netting using spray-on contact adhesive.
An advertisement in Wireless Magazine from June 1931 prices it at £8:18:6 which is about £750 in today’s money. So rather like buying a high-end sound bar for your television.
If you read my post about the restoration of a GEC AC5, you will realise that this post is related. Briefly, the AC5 was to be used on a regular basis to support historical social events, often with a WWII theme. This is the story of the contemporary extension speaker to be used with that radio.
It’s a W.B. device, vintage circa 1931. It has a Darwins of Sheffield four pole permanent magnet assembly.
Darwins was principally a cutlery manufacturer but in the late 1920s developed a reputation for producing particularly good permanent magnets for loudspeakers. To put this into perspective, the first patent for a moving coil, permanent magnet speaker was filed in 1921.
For any readers not familiar with the intricacies of loudspeaker design, early speakers differ from modern ones in the design of the spider (the bit that connects the speech coil and the bottom of the cone to the frame). The early design has the spider (which is shaped a bit like the Isle of Man legs) on the inside of the speech coil and bolted to the middle of the magnet pole piece. That’s what the nut is that you can see in the centre of this one.
This significantly limits the travel of the speech coil and hence the cone and causes a rapid drop-off of response at the bass end. A modern speaker has a spider that is more like a circular accordion which fits around the outside of the coil, connecting it to the speaker chassis. This allows much greater travel and more bass. The spec. that was agreed with broadcasting companies in the early thirties was that speakers should have a flat response from 300Hz to 5000Hz so that orchestral music could be reproduced at an acceptable quality. This one does reasonably well down to 200Hz but drops off sharply at 5000Hz. And it has an odd ‘dead spot’ around 2900Hz that I assume is a resonance issue. However, it sounds pretty respectable now it has been restored.
The problem it had was with the suspension, which is at the upper end of the cone. Originally, it was made from chamois leather and over time it has just decayed.
When this happens, the speech coil can touch the pole piece and it causes rattles and eventually, electrical failure. Someone had stuffed a lot of paper around the magnet and under the cone, probably to subdue the rattles.
Many people want to preserve speakers of this vintage as near to original as possible, and with care, the rotten chamois can be treated with a liquid rubber solution to restore flexibility and strength. That would be impractical for a speaker that is going to have to work for a living.
Fortunately, the cone, speech coil and spider were in good condition, so I replaced the suspension with a modern foam replacement and realigned everything.
There’s a company called North Speaker Parts who sell all sorts of bits and pieces, so it’s just a case of measuring carefully and finding the closest match.
This left me with an authentic-sounding and reliable driver. You can hear it in the cabinet by clicking here for music and here for speech.
The speech coil has an impedance of 14 ohms but the unit is intended for use with a high impedance extension speaker output like the one on the AC5 which comes straight off the output valve anode via a capacitor. If you have something similar, I strongly recommend you replace the capacitor with a modern, X-type safety capacitor. There is a Ferranti output transformer acting as a matching transformer inside the speaker cabinet. This is original.
Once the driver (speaker) had been repaired and cleaned up, the only thing left to be done was some minor attention to the cabinet. Some beading needed regluing and the surface finish was cleaned and sealed. A hole in the cloth covering one of the rear vents was repaired by gluing to fine black netting using spray-on contact adhesive.
An advertisement in Wireless Magazine from June 1931 prices it at £8:18:6 which is about £750 in today’s money. So rather like buying a high-end sound bar for your television.
