Note: Determining a Series I from a Series II Single Rectifier.
Series I amplifiers have a two-position mode switch for Channel 2 labeled "Vintage/Modern".
Series II amplifiers have a three-position mode switch labeled "Raw/Vintage/Modern".
If your amp is a Series II Solo 50 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, order the FC.SOLO50.
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, order the FC.SOLO50-II. If your amp has the 1/4" jack on the front panel beneath the input, order the FC.CHSO.