Amplivox Model 81

This old hearing tester was in a sorry state when I got it at a car boot sale. I suppose I got it for its parts but because it's fairly rare I decided to keep it in one piece and even to paint the case which had been left in a damp atmoshere and lost its varnish. Below, sanded down and ready to paint.

It seems it was designed for AC or DC mains and probably dates from around 1955. It uses three valves, a UCF85, a UCC85 and a B7G rectifier, at least I think its a rectifier but it may be a regulator valve. I thought this was missing but hearing a rattle I discovered it wedged inside. The valve is painted black which is why I can't say immediately what it is. Scroll down the page and there's a picture of it.

Sadly, no ancillaries came with the audiometer, just a mains lead to fit the Belling Lee mains socket.

Clearly there's a need to minimise hum so lots of smoothing condensers are employed. A green ballast resistor allows for 110 volt mains and there's a note impying that DC mains leads may have to be swapped around. This is because a diode is in circuit to rectify the AC and to protect the box from reverse connection of DC mains.

Inside the chassis there's a whopping attenuator fitted with scores of resistors and a rather large frequency selection switch. Below is the outer case. Chassis removal posed a puzzle. A piece of hardboard needed to be unscrewed and under this were three large mounting bolts. The cover was provided to isolate the operator from contact with a potentially live chassis. All exposed screws also need to be insulated from the chassis.

Below is the holder for the loose valve and in the picture below this you can see the the can fopr one of two valves used for oscillation and audio amplification.

Below are labels carrying basic information for the operator.

Above left the strange valve extracted from the wiring. Above the warning label necessary because of the live chassis.   Below, after applying a wood stain to the case.

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