Eitel-McCullough, Inc.; various addresses: Brand: Eimac. Since 1958 the Eimac Division is headquartered in San Carlos, California.

The company's first products were transmitting vacuum tubes which operated at low voltages, a design feature driven by the needs of the Ham radio community.

Bill Eitel, W6UF and Jack McCullough, W6CHE who founded the Eitel-McCullough, Inc. (most probably in 1934). Before they did the engineering work at Heintz & Kaufman in the early thirties for the HK354 tube which was designed to compete and surpass the RCA 852 tube.

During the development of the HK354 at H & K they had realized that the ideal transmitting tube of that time should be an extremely "hard" tube with a thoriated tungsten filament, Nonex glass bulb, cylindrical elements supported by the bulb with no internal ceramic insulators which could be a source of arc-over and plate and grid made of tantalum which could be made almost entirely free of gas so the tube need not have a "getter". The 150T was first advertised in QST in 1934 and was exhibited at the ARRL Convention 1934 in Sacramento. After that the tube was produced in series.

Shortly after the 150T followed the 50T, 300T and 500T, latet in the 30ies followed the 35T, 100T, 1500T and 2000T. The number was the plate dissipation a tube was made for. If a tube tested hat far ecxceeded the original ratings, they were numbered higher: 50T = 100T, 150T = 250T, 300T = 450T, 500T = 750T but the 35T, 100T and 2000T retained their original numbers and ratings.

Redesign in the late thirties and early forties were filament and grid structures, the open ended plate structures were given a domed top so that a single large plate lead instead of three separate leads as in the older design exits the top of the bulb. The internal glass ring, three wire grid support structure was changed to a simpler "press" design which made the tube easier to build. All this served to strengthen the tube and improve reliability.

Later these higher amplification factor types were to be labeled "TH's" and the older original lower amplification factor types were labeled as "TL's". Thus the 35T, 100T, 250T, 450T and the 750T became the 35TL, 100TL, 250TL, 450TL and 750TL.

During the late 30ies and early 40ies some other tubes were made like the 75T, 152T, 304T etc. and the UHF types UH35, UH50, UH51 and the Twin Triode were developed as well as tubes for the WW2.