The Marshall JTM100 Black Flag is one the Holy Grail amplifiers, the maybe the holiest! Its a close tie with the JTM45/100. This amps is in amazing shape and all original. Just about Impossible and definitely unique to be in the shape it's in. It's a tone master. This amp has rock star status for sure. The model was a favorite of Jimi Hendrix, Clapton, and many of the guitars who grew up listening to.
One thing that makes it so rare is production totals. This amp was only made in 1967 for about 6 months.
The so-called Black Flag amps are easily recognizable by having a reverse logo “J.T.M.” faceplate. These models are sometimes referred to as “JTM100s” as opposed to “JTM45/100s”. These are the first 100W amps known to feature a steel chassis. The new chassis made it possible to reduce cost while increasing strength. However, the transformers were placed very close together, something that can lead to noise. Except for the chassis and faceplate these amps are identical to the last JTM45s. The output transformer was typically a 1202-119, but the very Black Flags appear to use the Drake 1202-132 (introduced June 1967). The 1202-132 has a 4 ohms taps instead of the 1202-119’s 100V tap. These transformers are very similar but the 132 has slightly less bass than the 119. The first Black Flag amps used 270K mixer resistors (like the JTM45s), whereas the last ones used 470K mixer resistors (like the later amps), resulting in slightly more gain as well as more low midrange on the bright channel. Both Piher and Iskra carbon film resistors are used, although Iskras became predominant in the late 1960s. Carbon composite resistors are used less frequently than on the early amps (but some Black Flags have a 1W carbon composite 10k tail resistor). The voltages on these amps are recorded to be around 480V, slightly lower than earlier amps. Headcabs used both the narrow vent found on earlier amps and the wide vent used on later amps. All these amps had serial numbers in the 10.000 range.
Pictures show Hendrix with a Black Flag the fall of 1967 and the spring of 1968. This amp can be seen the first time June 18 1967 at Monterey and appears to have been used extensively for dirty tones on Axis: Bold as Love. It can probably be heard on BBC Sessions from the fall of 1967 and on “Catfish Blues” on Blues. Tonal similarities suggest Hendrix used it also on “Voodoo Child Blues” (Blues), “Voodoo Child (Outtake)” (Electric Ladyland & Beyond), and “Voodoo Child” (Electric Ladyland). Another Black Flag user is Paul Kossoff, as shown by pictures from Free album sessions (found in the sleeve of Tons of Sobs). Finally, recent pictures and videos show Eric Johnson with a Black Flag, probably used for Hendrix inspired dirty rhythm tone.
Partly as a result of this, there has been renewed interest in Black Flags amps lately. The circuit has been recreated by builders on the Metroamp forum and by Germino amps (the early Fillmore Classic, no longer made). Some users of the original amps (as well as users of recreations) have reported problems with ghost notes at loud volumes. Some originals and clones are modified with increased filtering that minimizes ghosting, but this gives a harder tone and feel to the amps. Others have changed the grounding scheme (especially the ground loop at the potentiometers) in order to deal with ghosting. How these amps differ from the earlier and later EL34 amps tonally is still not perfectly clear. Some find these amps to sound like the later 1967 amps, while others find them to have a unique dynamic response due to the dual rectifier power supply. Needless to say, these amps are capable of great blues and rock tones when properly maintained and set up.