The VT25 directly heated transmitting triode has a large L4 base. Originally the VT25 had a carburised thoriated tungsten filament that proved fragile in airborne applications. This Australian RCA made VT25 has the shouldered envelope rather than the wide tube seen on UK made versions.
The box anode has ceramic supports and the upper support is held by the glass envelope.
The RCA name with the Radiotron brand and made in Australia.
The blackened box anode with the grid visible inside. The top mica carries a single filament tension spring for the inverted V thoriated tungsten filament. Later versions of the VT25 featured a more reliable M shaped oxide coated filament.
The valve tester specifications and results.
The anode curves for this exhibit.
This image shows how bright a thoriated tungsten filament burns.
Fabio Camorani sent the images and the valve tester results.
The classic envelope is 47 mm in diameter and, excluding the L4 base pins, is 120 mm tall.