This is the Standard Retube Kit for the Mesa 50/50 Power Amp. The preamp section uses three Matched and Balanced ECC83S's. The default power tubes are the JJ Electronic 6L6GC’s. With the Customize option you can configure this kit to use the JJ KT66's, EL34II's, EL34’s, E34L's, KT77’s or Big Bottle 6CA7’s.
These amps are fixed bias with no adjustment other than the switch to go between EL34 mode and 6L6 mode in the newer 50/50's. 6L6GC's and KT66's are run in the 6L6 mode.EL34's, E34L's, KT77's and 6CA7's are run in the EL34 mode.