The JTM50 is one of those few amps built by Marshall that is considered one of those must haves if you’re an old Marshall enthusiast.

We had this gem which sold to our good customer Thomas H. in Germany. I have only seen a few but this one was hands down the finest example I had actually had in our possession. As the story goes, the JTM logo stood for Jim and Terry Marshall. In 1967 this was the last year the JTM was used on the Gold Plexi front face panels. It was soon changed to the more familiar JMP logo which is commonly found on most of the early Marshall Plexi and metal face models. The JMP logo stands for Jim Marshall Products. According to rumor, the change occurred when Jim Marshall divorced his wife and the trade logo was changed to reflect his sole ownership of the product.
As a reference you will see both JTM and JMP 1967 50 watt Marshalls which will be identical in every respect other then the front panel logos. The JTM logo is commonly referred to as the “BLACK FLAG” Logos. These are also found on the first 100 watt steel chassis which are well know for their use by Jimi Hendrix. The irony is the JTM100 is an EL34 based 100 watt version of the JTM50 but with the solid state rectifier power supply rather then the valve rectifier which is found in the JTM50. There were also JTM/JMP50s that were built with solid state rectifiers. You will find these with power transformer that still include the 5 volt heater windings on them to support valve rectified power supply operation. Not only do these solid state JTM/JMP50s have PTs with valve rectifier windings, but they also have the chassis punch for the tube socket for the valve rectifier. There are a number of interesting features on the JTM50 that should be noted. The first in the evaluation on the Marshall amp is the change from the 6L6/KT66 power tube, to the EL34 power tube. This change required a new output transformer with lower primary impedance. The designation #s on the output transformer for an originally equipped JTM50 will be 784-128. Earlier JTM45s would have the 784-103 on the top of the output transformers. The 784-103 was the 6L6/KT66 output tube transformer and would average 6500 ohms. The 784-128 would average 3400 ohms. Later plexi Marshalls, JMP logos with EL34s would have the 784-139 number atop the output transformer. The 784-128 was the earliest JTM45/50 version EL34 50 watt output transformer. Up to the point of the JTM50, JTM45s never made 45 watts, more often they ran in the low to high 30s in terms of actually wattage made. The JTM50 was a significant jump, mainly due to the power tubes and solid state power supplies.

JTM and JMP output transformer designations:
Click on photos below to see lager detailed pictures.

784-103, 6500 Ohms, 8ohm, 16 ohm and 100 Volt taps. Used with 6L6 and KT66 power tubes. (Used on JTM45s)

784-128, 3400 Ohms, 8 ohm, 16 ohm and 100 Volt Taps. Used with EL34s (Found on very few JTM45s, Mostly used on JTM50s) The JTM50 front logo is a reverse Black Letter coined the Black Flag logo

El34 Equipped with 100 Volt Tap

JTM50 equipped with the EL34 100 Volt Tap Output transformer

784-139, 3400 ohms used with EL34s (found on JMP 4 ohm, 8 ohm, 16 ohm model amps, 4 ohm tap replaced 100V tap) EL34 Equipped with 4 Ohm Tap

JMP label with 784-139 OT

The JTM50 is a favored amp for those looking for the JTM45 voice but with that EL34 grind. The EL34s were a significant jump for Marshall towards the ultimate Marshall tone they were so well known for.