The 5U4 Rectifier

The 5U4. The 5U4G, an octal based direct replacement for the 5Z3. These earlier G tubes are still in the "coke bottle" envelopes. These are great tubes, they sound good and are well built. First is an RCA built JAN (Joint Army Navy) tube followed by an Emerson labeled tube, probably by Sylvania, next is a Sylvania and last is a modern tube, a Sovtek 5U4G. The new Sovtek is quite a bit more robust looking. Spec sheets for the 5U4 can be found here: here.

The 5U4. Here are two modern 5U4G tubes, a Sovtek and a Ruby.

The 5U4. The workhorse 5U4, the straight bottled tube of the 1950's onward. Some of these are bad-ass tubes. Here are a Sylvania and an RCA. Typically, the Sylvanias tend to be taller with narrower plates, while the RCA manufacture were stouter with wide plates. You’ll find these tubes with every label under the sun.

The 5U4. 5U4 and their packaging from some of the biggest names in tubes, circa 1940's-1960's.