250W zender Buis 4-250a

The 4-250A / 5D22 / 6156 / QB3.5/750GA is a compact, ruggedly constructed power tetrode having a maximum plate dissipation rating of 250 watts. It is intended for use as an amplifier, oscillator or modulator. The low grid-plate capacitance of this tetrode coupled with its low driving-power requirement allows considerable simplification of the associated circuit and driver stage.

The 4-250A / 5D22 / 6156 / QB3.5/750GA is cooled by radiation from the plate and by circulation of forced-air through the base, around the envelope, and over the plate seal.
Rare find
price varies from140-160€ on the  net