Glass-backed Aluminum cathode

Platinum target

Aluminium rod anode

Bulbar Focus Tube dating probably to early 20^th Century.

Unknown make. Possibly made by Otto Pressler.

Aluminium rod anode. Concave aluminium cathode.

Round thin platinum target, embedded in the anti-cathode and held in place by 4 small metal claws.

No regeneration device.

Note the external connection between the anode and the anti-cathode.

A peculiar feature of this tube is the glass backing of the cathode cup, in continuity with the thin glass tubing surrounding  the stem of the cathode,  and offering a perfect fit and a stable position of the cathode cup inside the tube.

In this tube, and other focus tubes, the anti-cathode lies at a small distance beyond the focal point of the electron beam. Unlike light rays which diverge after their focal point, the  electron beam emitted by the concave cathode, after coming to focus, travels for a certain distance in a parallel, pencil-like beam, making the position of the target uncritical.