Power was the need, even though most people were using relatively efficient speakers and horns were at the peak of popularity. Bear in mind that these users were nearly all middle-aged men who listened primarily to classical have been desperate for louder crescendos, perhaps to drown out the wife's TV shows?

Obviously there was a demand for a tube that could do more than 50 watts, with low distortion, at a cost below that of the 6146. Mullard's EL37 could do it but was about as expensive as the 6146.

The 6550 was designed for home audio equipment, and it was designed to save money. No plate cap meant no plate cap connector, with the added advantage of less risk of electrocution to the user when replacing tubes. A large octal base with a metal ring at cathode potential was another safety feature which also improved electrical stability. Tung-Sol engineers had a distinctive envelope made for the tube, derived from the older "ST" shape. This squat, rounded "Coke bottle" appearance set the tube apart from the crowd. It might have been a marketing ploy, I suspect--it makes the tube LOOK powerful and tough.

For many years, Tung-Sol's version was seen as one of the best power tubes on the market. But, in 1957, a competitor appeared from England. The Marconi- Osram Valve Co. of Hammersmith, London, which already made and sold the KT66 and KT77 beam tubes under the Genalex name, introduced the imposing KT88. Its improved ratings made it a popular substitute in hi-fi amps. The first popular British amp to use the KT88 was the 50 watt Leak TL50+ in 1957. The first popular American amplifier was the early Dyna Mark III. Other notables were the Harman-Kardon Citation II, Heathkit W-6M and McIntosh MC275. And one of the largest guitar amps ever made, the Marshall Major of 1967, used four KT88s at 650 volts to deliver 200 watts. Although the 6550 was a tough tube, operation at above 550 volts in triode, near-triode or ultralinear connection was very hard on it. And in spite of its greater grid leakage currents, the KT88 could handle those operating conditions very easily.

By the 1960s, these tubes had set the standard for power amplification. So it was inevitable that other makers would introduce their own versions. GE came out with the straight-sided 6550A in 1971. It was called 6550A, because of the introduction of an exotic 5-ply metal plate material manufactured by Texas Instruments. This metal, produced by cladding a copper and iron core with aluminum outer layers using explosives, made for a more reliable tube with greater bias stability. The clad metal was better at dissipating plate heat, and it thus lessened plate and grid emission and reduced hot spots on the plate.

Sylvania wanted to get a piece of this action, so they introduced their own 6550A in the early 1970s. It is taller and skinnier than the GE, and has quite different mica spacers. But its plate looks exactly like the early GEs, complete with rough-looking spot welds. In fact, it looks suspiciously like a Sylvania 6L6GC or 6CA7 with a large metal-ringed base. Some experts claim that this tube was made by GE under contract, and a few are insistent that it never existed at all! Yet I recall seeing it occasionally in guitar amps during the 1980s. Sylvania apparently had such difficulty competing with GE for this market that production ceased in the early 1980s; I don't recall seeing any that were date-coded after 1984. Many were rebranded and sold by RCA and Westinghouse.