The differences between the 1987 circuit and 2204?

[vinceB]

Besides the master volume of course. It seems like it's very minimal and I could get some good 2204 grind by adding the MV. I'm sure this has been covered here somewhere so I would settle for a link to that thread. I built a JTM45 clone before and sold it before I could really play around with it and I'm thinking of doing another but I want to go with the 1987 this time.

 

[mott555]

The first couple years of 2204's (through mid 1977) were basically a 1987 with a master volume. After that, the gain channels were cascaded so those have much more preamp distortion available than an early 2204 or any 1987.

I'm sure there are other circuit differences but the cascaded gain channels is the biggest one.

 

[vinceB]

Looking at the schematic it's very similar apart from the MV and the cascaded gain stage. Some component values are different. I guess I don't quite understand the cascaded gain stage and how that is different than a jumper across the inputs on the 1987.

 

[mott555]

Looking at the schematic it's very similar apart from the MV and the cascaded gain stage. Some component values are different. I guess I don't quite understand the cascaded gain stage and how that is different than a jumper across the inputs on the 1987.

Jumping the inputs on a 1987 is parallel gain. You take your signal, amplify it, then take the original signal and amplify it slightly differently, then mix the two. It does not result in more gain, just some different texturing of gain. Sort of like putting two microphones on a single cabinet and blending them to taste at the mixer.

But a cascaded 2204 takes the input signal, amplifies it, then takes the amplified signal and amplifies it again. That results in a lot more gain and a lot more signal clipping. Sort of like using a megaphone pointed into another megaphone.

 

[HumanJHawkins]

Another way of putting this is that component-wise they are almost identical. But the path the signal is sent through is very different in the pre-amp.

Instead of having two channels that each use half of the first valve differently for different sounds, the 2204 has one channel driven through both halves of that first valve.

 

[vinceB]

I've made a decision. I'm going to build a JTM 45 clone because it's a relatively simple build and work my up as I learn more. If I need some over the top overdrive or grind I can put my ADA MP-1 in front of it or just use my 4210 for that type of stuff.

 

[Gene Ballzz]

vinceB,
It's a fairly simple matter to convert even a JTM45 to that same style of cascaded gain stages for similar results. I have a Weber later 2203 schematic (nearly identical to the later 2204 except for power section) that is fairly clear and accurate to Marshall specs. Let me know, via PM, if you'd like me to email it to you to compare alongside your JTM schematic. The swap over is a piece of cake and when you see the 2 side by side it becomes fairly clear and obvious! Only a few extra, small, cheap components to buy and easily reversible, if you don't like it. You can even use the same pots!
HTH,
Gene