Can you change a JTM45 into a 2204?

[CitizenCain]

I've got a Ceriatone JTM45 that I always wished had "moar"

Is the circuit close enought to be able to mod it to a 2204 spec? The best Marshall I ever played was a '77 JMP 2204 MKII MV Lead.

[gldtp99]

Yes, it's possible----- but by the time you change to the cascaded preamp and tone stack/coupling cap changes, switch to a solid state rec, change the filtering values, and maybe change the OT (depending on what output tubes you want to run)--- you could have a lot of time and $$$ into it.
Remember that a JTM45 is a @30 watt amp----even with the cascaded preamp and other changes it won't be the same as the '77 2204 you played.
I'd sell the JTM45 clone and get a 2204 if that's what you really want..................gldtp99

[gldtp99]

To get your JTM45 up to 2204 50 watt (w/EL34's) territory you'd need to change the PT, OT, and choke to 2204 spec components to start with---the entire power supply would need to be replaced if you want a 50 watt 2204-type amp.
If you could live with 30/40-ish watts then you could your PT and use 6L6GC or KT66 output tubes with the OT you have.
But the lower wattage amp wouldn't really sound like a '77 JMP 2204...........gldtp99

I've built some lo-buck 6L6GC 2204-type amps and people like them---- but they don't have the mid-range bark that a healthy EL34 2204 has---don't tell the people using the 6L6GC amps, ok?

[Ronsonic]

I've got a Ceriatone JTM45 that I always wished had "moar"

Is the circuit close enought to be able to mod it to a 2204 spec? The best Marshall I ever played was a '77 JMP 2204 MKII MV Lead.

The preamp changes - the cascaded channel thing, that'll be pretty easy. The solid state rectifier, you can do with a couple of diodes on the rectifier tube socket. That's even simpler. The rest as others have pointed out can get pretty extensive.

If you want to mod it in that direction, no prob. If you want to create a proper pre-800 2204 that's maybe time for another amp project.

[CitizenCain]

I just threw the '77 into the discussion as an idea of what my ideal Marshall tone was. If I could just get the Ceriatone headed in that direction I'd be happy. It sounds like I could do some simple modding to at least start down that path. A juiced up JTM would be a good thing :D

[Ronsonic]

I would try the cascaded gain stages thing. It is very reversible, parts needed is like a couple resistors and some wire. Don't be afraid to mod the mod. Once you have that gain path in place, feel free to add or lose a bright cap here or there or change out a cathode bypass resistor, etc. until you've got it dialed in.

If Ceriatone offers a 2204 type amp, I would look real hard at that for stealing the layout.

[Macinbest]

Hey, I'm not sure what the end result will be like, but I am currently building an amp based on the JTM45 but with 4 different voicings, one of them being the 2204.

Actually its going to be like so: JTM45, 1986, 1987x and 2204. There's a bunch of things on the board that needs to be swapped.. but they're incremental changes.. I'll put in a few switches on the back which would activate a certain set of relays depending on the mode selected, and switch the component values to the corresponding ones..

I am using a JTM45 PT (Metroamp 1202-55), a Mercury Magnetics O45RS-L-16 OT (3.4k, 6.6k and 8k primaries & 16ohm secondary), and got a few different chokes to try... 3H, 5H, 7H, 9H.

Switching different amp voicings will always be done while the amplifier is on standby.. because it will switch a bunch of things, including rectifier (5AR4 vs diode bridge), the choke and the filtering caps, etc.

I will also make a little system to drain all of the filtering caps on demand, because those out of the circuit could still have a charge in them.

It will also have the london power scaling, a pre-PI MV and a post-PI MV, a pot to control the NFB, a gain cascade switch and a shared/split cathode switch. I guess while I'm at it I could also regulate the voltage of the B+ so match each amp.. but that won't be in the amp's first iteration. I've already built the power scaling PCB but it won't be installed in the first iteration either.

Of course with all these mods, it will not sound exactly like any of the originals, but it should be close enough to sound good across the spectrum :D Where I can, I will also make my mods such as the master volumes, NFB pot, etc bypassable. This way it should get back as close as it can to its original JTM45 specs.


Gotta love big projects !

[Ronsonic]

System to drain caps? If there isn't a resistor just going from B+ to ground in parallel with the filter caps, just add that. Use something like a 2W 390-470K and call it good your caps will drain.

[Macinbest]

Yeah, I guess something like that does it. What I usually do is power back the amplifier with the rectifier tube pulled out and wait a bit. But this strategy would not work when you have charged up caps switched out of the circuit :)