Determining a Version I from a Version II Single Rectifier.
Version I amplifiers have a two-position mode switch for Channel 2 labeled "Vintage/Modern".
Version II amplifiers have a three-position mode switch labeled "Raw/Vintage/Modern".
If your amp is a Solo 50 Version II head, you will find a Bias Select switch on the back of the amp as well as a RAW mode in Channel 2 on the front panel. If your version doesn't have these features, it is a SERIES I model
There were two different footswitches used throughout production of the Version II Solo 50 amps. If your Solo 50 has the bias switch and RAW mode on the front panel, next look for a 1/4" jack under the INPUT jack labeled FTSW or a 5 pin DIN jack on the back of the amp. If your amp has the 5 pin DIN on the back, the FC.SOLO50-II is the footswitch for you. If your amp has the 1/4" jack on the front panel beneath the input, it uses the FC.SOLO50-IIA.