1967 Marshall JTM50 Black Flag 50W #1987 EL34 GZ34 transition model

Early GZ34 Tube rectifier transition model with 2 x EL34's (currently 6CA7 tubes installed to give it a more JTM sound).
 
It has the original Drake 784-128 Output Transformer, Drake 1202-118 lay down Power Transformer, Drake 354-114 choke, Mustard caps, pots, inputs, cab, tolex, handle, knobs etc etc. The filter caps & other electrolytic's have been changed and the preamp filtering should be a 32-32uF cap with two 10K/1w B+ dropping resistors to make it perfectly original. Previously owned by Sander van Herk (Het Goede Doel, Alderliefste) who added a line out and a ground switch in the back which are disabled during service. Overall this is a very nice original amp considering its age.

And now you want to know how it sounds? Check out Johan Segeborn on Youtube. This thing sounds so good it will bring you to tears.... The sonically wild out of controlness of a Black Flag Plexi is just crazy. Your mind says: "Ooooh boy... she's gonna blow!!!" It's the sound of danger coming your way. And that.... my friends... is when you feel ALIVE!!! And the 50watts make it just a tad more controllable. It's the sound of the Allman Brothers Band - Live at the Fillmore East in 1970. Not too shabby!

(info with thanks to Solo Dallas:)
The Black Flag JTM50 is a rather rare and coveted amplifier made by Marshal between 1966 and 1967. It was one of the transitional periods between the JTM45 amps and the plexi panel JMPs and later metal faces. During this short period of time the JTM50 saw a few important spec changes that would be a precedent for further development of Marshall guitar amplifiers.

These spec changes and the switch to EL34 power tubes made these early 50w amps brighter and more aggressive than the early JTM45 models, but the GZ34 rectifier tube still made them retain some of the softer response. By mid 1967 the JTM50/JTM45 and MKII markings on the front panel were replaced with a characteristic black screen-printed JTM logo on the left side, next to the switches. This was to be known as the “Black Flag” Marshal.

During the development of the 100w amps, Marshall decided that the GZ34 rectifier tube wasn’t necessary. The first 100w heads were already made with solid state rectifier and this practice was to be adopted in the 50w amps later on as well. During this period, a Black Flag JTM50 could either have the GZ34 tube or be SS rectified. It wasn’t until later in 1967 that all the amps were SS rectified.