Many tube amplifiers use the common 12AU7 and various simular versions such as the 5814, 5963 and 6189 tubes. A lower distortion replacement is the E80CC with slightly different operating parameters and double the filament current requirements. Be careful regarding the installed height of this tube, about equal to a 6BQ5 & taller than 12AU7 tubes. Regarding the higher filament current, make sure your amplifier has the extra current capacity. Most amplifiers have plenty of reserve capacity. One way to view it is a 250va or say 250 watt power transformer will require an additional 1.9 watts per tube. That is a very small percentage increase in power demand on a 250 watt transformer. My example above with regard to power transformer additional loading is only a very rough guide. A small preamp without a power tube output section can overstress the power transformer filament with E80CC tubes. In this case, Brimar 13D5 or CBS Hytron or CBS 5814 or 5814A are your only option for best quality sonics and are far superior to the standard 12AU7. I also read Sylvania early 1950s tall black plate are of superior sonics, but never heard these tubes.

The Amperex E80CC also labeled at times as 6085. Many persons performing a search miss that 6085 only listing and buyers can get a super deal. Various labels of Amperex manufactured tubes are Valvo, Telefunken, Siemens, etc. Another brand is the Tungsram E80CC. Both are great performing tubes. The Amperex produces a warmer midrange and slightly darker high frequencies compared to the Tungsram manufacture. The Tungsram are brighter on the high frequencies and a little cooler regarding the midrange producing accurate sonics without being the least bit sterile sounding in my applications. I truely enjoy either manufacture.

Both brands of tubes are highly recommended and in most cases will greatly outperform the 12AU7 in home entertainment amplifiers. However, a few audiophiles do like the much high distortion of the 12AU7 for whatever reason. (Be advised my guide is more addressing home entertainment amplifiers vs guitar amplifiers for accurate & neutral playback sonics). The 5814 series was actually a redesigned by RCA 12AU7 due to a few complaints of the 12AU7 distortion figures when introduced in the late 1940s. The 12AU7 tube was not meeting some manufactures requirements such as Collins Radio now called Rockwell Collins.