German "Pram" Mine Detector, WWII Tactical and Technical Trends, No. 41, December 30, 1943 (Lone Sentry)

Any thirsty soldier who has lugged an unwieldy, long-handled mine detector over miles of dusty ground with the temperature at 100° plus, would have appreciated one of the German "Pram" (perambulator) two-wheeled mine detectors. A description of the apparatus is drawn from British sources.

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a. Designation

The German designation of this equipment has not been learned, the only marking being the figures 1138 painted on each of the component parts.

b. Description

The detector consists of a folding, rectangular, tubular metal frame-work mounted on two wheels, a search element housed in a wooden box, which is attached to one end of the metal framework for operation, and the accessory electrical equipment. This fits into a pack carried on the back of the operator.

(1) Framework of Pram

The frame is constructed of metal tubing 1 1/4 inches in diameter; one half of this tubing is fixed to the wheels and the other half is hinged to the fixed section and folds on top of it for transportation.

(2) Dimensions

Length folded 3 ft

Length extended 6 ft

Width of folding section 11 in

Width of fixed section 8 in

Wheels 14-in diameter each fitted with two 1/2-in circular-section solid rubber tires.

(3) Search Element Casing

This is a wooden box, 3 feet x 2 1/4 inches x 3 inches, carried separately and attached to the open end of the framework for use; a length of detachable cable runs from the pack to the search element which is operated by means of two small levers on the handle of the framework; these are connected to the casing by a length of cable. Near one end of the casing is a search element adjuster.

[German Pram Mine Detector]

(4) Pack

This is a wooden box, 12 x 12 x 4 inches, fitting into an ordinary canvas pack and attached to the equipment. Inside is housed the accessory electrical equipment including the oscillator, amplifier, headphones, and high voltage and low voltage supplies. The electric fittings are:

Amplifier 3 stage type with 3 Telefunken tubes, DAF 11

Oscillator 2 Telefunken tubes, DC 11

H.V. 90-volt dry battery

L.V. One and one-half volt dry cell

c. Operation

400 cps oscillator and amplifier, consisting of two tubes, feeds the iron-cored coil in the center of the search coil assembly. Located at each end of this assembly are two pick-up coils connected in opposition. The difference in the AC voltage developed across these coils is fed via a screened lead into the grid of the first stage of a three-tube amplifier employing a Telefunken tube, type DAF 11. In addition, the grid of this first stage is fed with a very small voltage direct from the oscillator stage. This voltage can be varied in amplitude and its phase can be reversed by means of a differential condenser. This voltage is used to set up the detector to give minimum output when there is no metal near the search coil. Also two balance controls are fitted on the search coil:

(1) A short-circuited turn located close to one pick-up coil, which can be rotated, and thus alters the flux in the search coil by varying the field due to eddy currents in the short-circuited turn. This is controlled by a knob on the search coil assembly.

(2) A piece of steel near the other pick-up coil varies the flux in the coil. This is controlled from the handle of the Pram by means of a flexible shaft ("Bowden") cable.

d. Control Set-up

The controls should be set up as follows:

(1) Assemble the search coil into the Pram

(2) Connect the oscillator to the search coil by means of the six-pin plug and socket

(3) Plug the headphones into the socket provided on the oscillator-amplifier and keep oscillator as far as possible away from the search coil

(4) Adjustment of the controls should be made in the following order for minimum sound:

a - Flexible-shaft cable operated control
b - Turn on search coil assembly, and
c - Differential condenser.

(5) Repeat (4) until minimum output is obtained.