New Amp Build: BYOC Brit 45 with JTM50 modification

[duhvoodooman***]

Just finished this new amp build last week. I started with the BYOC "Brit 45" kit (Marshall JTM45 clone), but modified it to a JTM50, albeit one with a tube rectifier and a variable voltage regulator (VVR) on the power supply circuitry. You can see the VVR module mounted to the back of the chassis between the AC socket and the output impedance rotary switch; the dual-gang pot that controls the voltage levels is next to the black output jack. The VVR enables this non-MV amp to achieve tube overdrive at much more reasonable volume levels, thereby preserving my hearing and my marriage. It runs a pair of EL34 output tubes, rather than the KT66's classically used in the JTM45. At the bottom of the photos, I've posted a marked up Brit 45 schematic that shows all the modifications that were made in red.

It sounds, well, just like you'd expect a 50W Marshall to sound. I jumper the Normal and High Treble channels together and blend in the amount of treble I want with the two volume controls. It takes pedals extremely well, though you obviously have to keep the overall gain reigned in a bit when using time-shift effects (chorus, delay, reverb, etc.). The crunch tone with a Tube Screamer type overdrive is to die for.

And it's PURPLE, my favorite color!

[piebaldpython***]

WOW......nice work. Should really sing. Congrats!

[marnold***]

You know what would go perfectly with that amph?

[duhvoodooman***]

Nah, I could hurt myself (or others) with that thing. I'll go with one of these:

[sunvalleylaw]

Looks great! I bet it sounds great too!

[Robert****]

I want that amph!

[kentuckytukker]

Hi duvoodooman,

Where did you dig up the info concerning the differences between the JTM45 and the JTM50? I've build a JTM45 clone using the BYOC schematic which I like best compaired to the diagram which was accompanied with the kit. Concerning the JTM50 schematic it is very hard to find any info on the web. It also seems that there are many different versions made by Marshall at that time. I've been asked by the owner of the JTM45 clone to modify it to a JTM50 but like to go shure what is realy req'd. Why did you relocate the 2 1M resistors at the input circuit and why did you add the 100nF cap's?

[Bloozcat]

DVM doesn't check in as much as he used to, but hopefully he'll see your post an give you an answer. Knowing him I'm sure there was a good reason for doing what he did!

[tjcurtin1]

Bloozcat - just saw your signature line !

[duhvoodooman***]

Kentuckytukker and I exchanged PMs and some emails and I provided him with all the info I had on this modification. The short answer to his question is that the main changes are EL34 output tubes, a different OT to match up with them, some additional DC filtering, a different cathode resistor and bypass cap on the High Treble input triode, and some associated resistor value changes related to these mods. A bunch of the changes shown in red are associated with the addition of a variable voltage regulator, including the three 100 nf capacitor/1 Mohm resistor pairs and have nothing to do with the JTM50 conversion, per se. (As it turned out, the VVR was NOT a good addition to this amp; the one I used was a prototype I got from Keith at BYOC, and it turned out to lack enough heat dissipation to keep it from burning up after a fairly short amount of use, necessitating quite a bit of repair work.)

I'm not sure exactly when Marshall switched from a tube rectifier to solid state rectification on the JTM amps, as I've seen JTM50 schematics both ways. By the time the 50W "Plexi" came out, they were all SS rectification.