I can share some observations on the SS/Cal line. I was somewhat of a gearhead in my earlier days, but not so much anymore, so there's my disclaimer. Anyhow, it's easy for anybody to conduct a Google image search to compare the evolutionary differences. Browsing online market websites can also serve as a good source for product research. And of course, there are these online forums.
Early models came with EL84 (6BQ5) power tubes operating in class A. The Studio .22 was equipped with an EL84 duet and the .50 Caliber with a quartet. The tube sockets were mounted directly to the circuit board, even for the EL84 power tubes, which is probably not ideal. A couple years or so later, the .50 Caliber was re-designated at the ".50 Caliber +" and its power tubes were switched over from the original quartet EL84 to a duet of chassis mounted 6L6-GC. Also, its VOLUME control was relabeled as GAIN and had a pull switch added to access the LEAD mode, alternatively to using the footswitch. Furthermore, with the "+" series, a second 4 ohm speaker jack was added to both the 22 and 50 watters. Plastic rocker switches eventually replaced the metal toggle switches on all models in the series. Spring reverb units (or tanks) are located INSIDE the chassis on all models that are so equipped.
The rack mounted Studio Preamp is basically the same circuit, with a few extra switches to further adjust tone and gain structure. It has 2 pairs of output jacks, with one of them being dedicated for recording.
Other minor cosmetic differences involve many several variations of panel screenprints, such as:
"Studio Series" printed on the front panel, and "Studio Series Class "A" Power" printed on the rear panel.
"Studio .22" printed on the front panel, and "Caliber Series Class "A" Power" printed on the rear panel.
"Mesa/Boogie .50 Caliber" printed on the front panel, or simply ".50 Caliber", and "Caliber Series Class "A" Power" printed on the rear panel.
Later models read ".50 Caliber +" or "Studio .22 +" on the front, and the rear reads "The Spirit of Art in Technology" which replaced the "Class "A" Power" wording, even though the 22s were still built to operate in class A, presumably.
Combo serial numbers include the prefix "SS", or "FP", with the rack preamp utilizing "SP".
The series retained the amber power indicator lamp lens of the earlier MkII series, whereas many other Mesa models such as Quad Preamp and Mark III switched over to a blue lens. Early models had metal toggle switches. Plastic (dielectric phenolic?) rocker switches eventually replaced the metal toggle switches on all models in the series.
Combos were typically equipped with either the 90 watt Celestion built speaker (Mesa model designation MC-90), or with the darker sounding 50 watt Eminence built "Vintage" speaker (Mesa model designation VS-12), which has since been discontinued.
Interestingly, the Mesa Boogie brochure mentioned 3 models- 22, 38, 50, although no .38 Caliber or .38 Special was ever produced, outside of rumors. I spoke with an amp tech who swears having a 38 Cal on his bench, so there could be a few of them out there, and if so, I speculate that it's simply a rebranded 50 Cal with the EL84s.
Both powered models were available as combo and head. I have seen a lot of 50 Cal heads, but not very many 22 heads. All 50s specifically bear the "Caliber" brand; there are no "Studio 50" models, however, the 22s can be found bearing "Studio Series" and "Caliber Series" designations.
Personally, I don't believe that any of the .50 Calibers deserve the 50 watt rating, regardless of which power tubes were in use. Their output transformers are small in comparison to the Mark series 60 watt models and the 50 watt Son. You might get measurements over the 50 watt mark, but there's a notable difference in performance to the their Mark Series brethren. That being said, both versions produce good volume and perform fairly well in live band settings.
Tonewise, I can report only on the EL84 equipped .50 Caliber head. Compared to the Mark III, the 50 Cal's Lead channel is similarly aggressive, yet a bit smoother in the gain structure, slightly leaner low end, good mids, with the highs being "sweet", extended, and gently rolled off. Plenty of usable overdrive/distortion with nice "bounce" and dynamics. Rhythm channel is very usable, with good pure string tone at moderate gain settings, "sparkle and warmth", and a plenty of "bark" when turned up. The 5 band graphic EQ is very useful in further shaping the tone and achieving a good mix with other instruments. The fx loop is there if you want it, but my Roland GP-16 didn't seem compatible when I tried it, briefly.
Overall, I deem the Mesa Boogie .50 Caliber guitar amplifier to be suitable for Metal, Rock, Blues, and very likely much more. I admit to being favorably biased towards it. It was my personal beast during my early Metal days. I cut a studio demo with it, which turned out quite well. I always use the head with a 4x12 cab, either Mesa or Marshall, which really makes a huge difference in performance.
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