INTRODUCTION

The Amplion Cabinett is a speaker for valve radios. It appears to be a small speaker, smaller than a horn loudspeaker, and smaller than a table top speaker. The size is 10 x 10 x 10 cms and weighs 10 kilos. It is a high impedance device, so it can replace horns, or headphones. It connects to radios in a similar way as horn loud speakers.

Front View

DESIGN

The design and construction of this is fairly simple. It is merely a horn loudspeaker in a wooden box. The horn has been made small enough to fit in a box, and it has a reflector to direct the sound out the front. It is screwed to the base of the box. There is a large rubber grommet in one end, and the driver fits into this, with a tight fit. The driver has a screw thread molded into the Bakelite neck, so that it can screw into the rubber, expanding it to make a tight fit in the small end of the horn.

Driver

The box front looks like a conventional speaker. It has a wooden fret with speaker cloth behind it. The front is made of plywood, and has a radial sun burst pattern. I have seen another box with a different design of fret work. The sides, top and bottom are made of thicker wood. On the right hand end is a door which provides access to the diver. The driver can be adjusted, by turning the screw on the end. The wires from the driver exit through a hole in the back. There are four feet on the box corners. The top is just a friction fit, and can be removed easily.

Different Front Fret Work

RESTORATION

The speaker was in poor condition when acquired. The wooden box was warped and water stained, and the screws and nails in it were all rusty. The front was falling apart as the plywood had been wet and the layers were separating. The speaker cloth was mostly in tatters. The steel horn inside was very rusty but intact. The driver was hanging down as the rubber grommet had melted with age. The driver was open circuit.

Horn with Reflector

The horn was sanded and the melted rubber grommet removed. It was then painted black. A new grommet was fitted.

The case was sanded, and then glued back together. It required clamping and re-gluing and new screws. The front was a plywood fret work. It was re-glued and clamped many times, and any missing laminated wood was replaced. It was repeatedly sanded, and as other bits separated, they were glued and sanded. It took many attempts before the fret was sound and looked correct.

Box

The whole box was stained and reassembled. Some new speaker cloth was fitted to the back of the fret work.

Clamping the Speaker Cloth

The driver was disassembled. This was tricky, as it was designed not to be opened. Two on the four screws holding it together, were a tamper proof type. When it was assembled, the two screw heads were designed to break off, when it was finally screwed tight. Thus there was no screw driver slot or any other way to turn the screw. A hand held disc cutting tool was used to carefully cut a slot in the top of these two screws. Care was taken not to mark the Bakelite housing. When removed, a better cleaner more professional slot was cut with a hacksaw.

Driver

Both driver coils were open circuit. The turns were removed and counted, but it was difficult. The wire was very fine and broke constantly. They were both rewound and tested before reassembly.

Rewinding the Driver

PERFORMANCE

The driver was tested on the bench and installed in the box with the horn. A Sound Level Meter was used to measure the audio output, placed 1 cm from the driver. An audio signal generator was used to excite the driver.

Diver Testing

The frequency response seems to start at around 500 Hz and finish at about 3500 Hz, with 2 peaks, at 600 Hz and at 2700 Hz. The horn and driver were installed in the wooden box, and the frequency response re-measured. There was little difference.

Frequency Response (horizontal in Hz) verses (vertical Level in db SPL)

CONCLUSION

This Amplion Cabinett is a neat and compact horn loudspeaker, with a high impedance. This may be an attempt to move away from large and tall horn loud speakers and transition to normal loud speakers. It has an appearance that matches the long box like radios of that era.

Original Condition

Copyright
Ray Robinson

INDEX