by Norm Wilson N6JV – Visit the museum at N6JV.com
152T
In 1940, EIMAC tried using four 75T tubes in parallel and designated it the 304T. The 304T had plates with caps and external grid connections. A cup shield that was concave down was added later (2nd example). I believe the shield is used to protect the glass seals from overheating due to the proximity to the extremely hot anodes and grids. Experiments were made to make the grids connected internally (3rd example).
By 1941 the 304TL (VT-129) with a mu of 12 and the 304TH (VT-254) with a mu of 20 were being produced. They had an external grid ring and the cup shield was now concave up. The late 1942 production of 304TLs was made with their plates open at the top (4th example). In 1943 the caps returned to the covered type and with the external grid ring for the first 3 months. In May of 1943, the grids were all internally connected (5th example). The 304TL was made in great numbers especially in Salt Lake City. In 1941 alone, a total of 544,473 TLs were made. The TLs were used primarily as pulse modulators most commonly in the Army SCR-584 RADAR. With the great demand for these tubes, the type became copied by competitors especially Heinz & Kaufman. The last (6th) example shows a HK-304L which had the same Army VT designation in spite of the fact that was a taller tube and may not fit in all applications. The 304L had no internal shield as there were four separate wire seals that were a distance away from the anode/grid heat sources.
