Re: Retubing Fender amp... which tubes?
The 6L6 (6 internal elements, L means tetrode, and a 6 volt heater) is the classic high power American audio tube dating back to WWII. It was originally in a metal case, but during the 50s they started to come in a glass tube, hence 6L6GT or GC. Letters were added to the end as modifications were made to the design, similar to how the military adds letters to equipment. For example, there was the F16A then the newer version became the F16B and it's now the F16C. Tubes were used by the military extensively so a lot of letters got applied to various models of the basic 6L6. JAN is for "Joint Army Navy" numbering system. So this were the terms like JAN-Phillips comes from.
5881 was a smaller bottle 6L6 based on US Navy special reqest in 1958. The four didgit number for tubes was also adopted by private industry. Late fifties and early 60's Fenders usually used the GE or Tung-Sol 5881s. The Russians copied the 5881 during the cold war, and after the end of the cold war the Sovtec 5881 became a cheap alternative to dissapearing and expensive American 6L6 models. The Russian 5881 has a softer sound, less power, and is less durable than American models.
Marshall's first amps used the American 5881, but most came with the huge KT66. The KT66 was a heavy duty British military tube to be used in place of an American 6L6 or a an EL37 (a European standard 6L6). KT stood for kinkless tetrode, meaning the tube had high head room and could take high plate voltage with a hot bias. But distort it would when push far enough, and it's distortion was wonderful. It became the original Marshall sound. They have more bass than most standard 6L6 types. Kt66's can be used in Fender amps, but changes must be made for it to be stable.
During the 70's a larger more heavy duty 6L6 called the 7581 became available. These 6L6 types were also used by Fender and Mesa but called 6L6GCxxx or in some cases (iirc) STR414. These larger 6L6 types do not distort easily. The difference among 6L6 models is really the shape of the bottle and the size of the bottle, and the location within the bottle of the internal componants. GC 's have symetrical bottles as do 5881's and 7581's. GT's have curved bottles and so forth....A larger bottle tube will usually have a harder vacuum and run cooler, being more powerfull and cleaner. The smaller bottles the opposite.
