in tube amps? apart from big fenders...
vintage Hiwatt... post 72, they were a little dirtier in the early years... power of a marshall, tons of headroom and clarity, a little fender in the mix, but lower harmonic distortion... very fast response but with just enough tube harmonics and compression to give the sound some color and nice sustain.... the 'golden age' hiwatt DR heads are damned clean most of the way up the dial but have a wide 'sweet spot'... the finished design is a really unique design too, not like anything else
AMPEG! They made more than bass amps, guys (and even the bass amps are so well designed that they can easily be dialed in for wonderful clean guitar tones.... great preamps on these guys).... Everett Hull was a jazzer who HATED rock 'n roll and thought tube instrument amps should be distortion free up to 9 or 10! other than the jet and reverboroket (both have killer clean but are light on headroom compared to their siblings, the jet because its tiny, the rockets because some of the engineers slipped a more rock n roll design past 'ol Everett) the rest of the line has fabulous headroom.... my favorites of the Hull era are definitely the Gemini series, especially the Gemini II with the 15".... after Hull sold the company the V series is fabulous though a bit hard to service and prone to failure after 40 years of abuse, people think drive from these amps because the stones endorsed them and they are favored by Josh Homme these days, but in actuality even the VT4/V2 and B25, the smallest ones, have way more headroom than you will need for most gigs and the cleans are sweet and highly tailorable due to the most sophisticated tone shaping and well tuned preamp circuits of the era... at a time when fender was ruining their amp line Ampeg was taking their old aesthetic and applying it to the rock market and while fender's innovations were poorly implemented and fairly superfluous after leo sold, after Hull sold ampeg the engineers really ahd a field day implementing more and more useful and unique features like the active, semi parametric inductor-driven midrange on the Vs.... if they weren't such a bear to keep u with I would still have one.... the reissue V4B gets pretty close to the old V sound and reliability won't be a problem due to age
while the headroom isn't there in a loud situation, vox ac30s have a very nice, highly compressed and colored clean tone when set moderately... depending on the year the response will be slightly different and the headroom will vary, but its a great sound.... and they slide into breakup more easily from a nice clean sound more easily than any other amp
Matchless DC30, EF86 channel.... very similar to an ac30 but the firmer output section makes it punchier even when powerfully overdriven, but the hot EF86 provides some fantastic wide bandwidth clean like nothing you have ever heard... again, you need to be careful setting it up for clean because the amp WANTS TO BREAK UP, but it can be done at band levels in full power mode with the stock rectifier and its so sweet, very unique sound and feel
Sunn... apart from the model T, vintage Sunn amps have huge clean tones that sit between an old fender twin and a hiwatt... they have a very blackface preamp coupled to a hifi output section (basically a Dynaco ST70 power amp design... the early ones actually used Dynakit monoblock hifi amps as their power section, but later they started scratch building them with little circuit revision)
another unheralded knockout gem for clean is the 50s Gibson Rhythm King, it has a mellow, jazzy voice and huge headroom for the era, when its starts to breakup its subtle and you will will still sound pretty clean well up the dial.... its a great tone... the Gibson GA50T is their second cleanet 50s amp and its pretty sweet too.... also ahs a phenomenal trem circuit like the GA40 les paul... but yeah, the old gibbies are mainly low output and have less headroom than their fender counterparts, but the bigger models have a fantastic tweed voice with MORE headroom than their fender coutnerparts.... they are aren't all arounders, but what they do is like nothing else before or since
old magnatones have KILLER clean.... the big ones, the little guys have like ZERO headroom and the big ones are maybe a little more open than an ac30, but man do they sound good... the new ones sound pretty good but sound more modern to my ear... they capture some of the spirit but really crossover into other signature clean tones and have less of the vintage vibe (no pun intended, get it? magnatone invented true stereo vibrato)
in solid state amps the JC120 (asTel_Nobody mentioned) is a benchmark of the solid stte boom, but Yamaha made a bunch of amps in the alte 70s that look kinda like Fenders and share the same feature set and layout as a twin that are some fantastic clean amp that are less sterile than a Jazz Chorus with its chorus disengaged... I'm always tempted to buy one
some rarer amps that really kill for clean are the Vox ac50 and 100, listen to any beatles record between 'For Sale' and 'Help!' to hear the glorious, gooey cleanness! the preamp is like a low gain ac30 into a very obust power section that operates in a hybrid way giving it a mix of ac30 and marshall feel, clean or dirty
the rarest great clean amps are without a doubt the Vox UL series.... another beatles amp as well as being a Zep amp and being the mainstay of Bernie Sumner (joy division, new order) all through his early career, they are a hybrid amp sorta like a musicman, solid state preamp (cribbed from the US Thomas organ made solid state voxes like the super beatle) into a tube power amp, but they are glorious for clean to my ears... sadly few were made and they've become quite collectible often commanding more money than early ac30s
honorable mention to the music man HD hybrid amps I was just talking about, very clean though not my favorite clean sound.... a lot of people really like them though and they are both affordable and indestructible
a lot of these amps are things you only know to seek out and try if you are a total amp nutter like me, but in the search for my sound I tried everything when I departed from fender in the late 90s.... I left out a lot of boutique stuff that's just refined fender cleans (dumble type amps and early mesas come to mind) and another response covered 2 of the most popular modern varieties of that idealized fender sound anyway...