Plexi 2204 Question

[TheElaborateDream]

Shouldn't really be needed, as they are essentially identical pre-amps all the way to the phase inverter. The big difference is that the 2203 has 4 output valves and a big chunk of iron in the output transformer. You should be able to get pretty close to a 2203 "feel" by playing with transformer matching and impedance curve.

I have an Axe FX III, and I also have a 1980 JMP 2203 and a 1979 1960A 4 x 12. So I guess what I really ought to do some time is to see if I can tweak the Fractal 2204 model to behave exactly like my favourite old Marshall. I have some time booked off work over Christmas, so I'll try to make a project of it.

Liam

The biggest difference I noticed between the 50w vs 100w is the 50w breaks up a lot sooner, the 100w obviously has more headroom and the low end feels tighter to me on the 100w, there is also a more pronounced mid-range growl on the 100w. That could be a difference in 6550 vs El34 though. I really haven't been a fan of most 50w amps though.

 

[LiamH]

The biggest difference I noticed between the 50w vs 100w is the 50w breaks up a lot sooner, the 100w obviously has more headroom and the low end feels tighter to me on the 100w, there is also a more pronounced mid-range growl on the 100w. That could be a difference in 6550 vs El34 though. I really haven't been a fan of most 50w amps though.

I've only ever used the EL34 versions of any of these amps - it's all they were ever fitted with here in the UK. If I remember right there was at least one small change in the tone stack of the 6550 versions, and then some differences in the plate resistors of the preamp stages, as well as the bias circuit being different. I also have a 1978 50 watt "model 1986" bass head that spent some of its life with me converted to the 2204 circuit. Like you, I couldn't love it so much in that format, so it became a relatively accurate impersonation of a plexi 1987 50 watt shortly after. I think it's nicest in that configuration. (Great amp for blues and mellower classic rock.)

Agree entirely that the tight but big low end, deceptive "clean but not clean" headroom, and "punch in the chest" mid-range of the 100 watt amps makes them really special.

One thing I noticed in this thread is that Cliff believes the later JCM800 version of the 2204 had more gain stage cascading than the original JMP. I'll dig through some schematics as it's a while since I worked on a JCM800 version, but I'm pretty sure the "horizontal input" version had a slightly different layout, but was electrically and functionally identical to the JMP. Capacitors and tubes took a bit of a nose dive in quality and reliability as the 1980s took hold, but I think the circuit topology stayed very much the same, just some small changes in layout to make it a bit more PCB oriented. Now the JCM800 2205, that was definitely a different beast. Never learnt to love the 2205 or 2210, although others got great sounds from them.

Liam

 

[Budda]

The biggest difference I noticed between the 50w vs 100w is the 50w breaks up a lot sooner, the 100w obviously has more headroom and the low end feels tighter to me on the 100w, there is also a more pronounced mid-range growl on the 100w. That could be a difference in 6550 vs El34 though. I really haven't been a fan of most 50w amps though.

What you described (tighter low end, different mids) is the headroom.

 

[sumitagarwal]

Still trying to wrap my head around the differences between the master volume Marshalls...

Is a non-cascaded 2204 basically similar to plugging into the low input of cascaded 2204, since the low input bypasses a gain stage?

 

[mr_fender]

It's similar but not the same. The low input on a cascaded 2204 inserts right before the 1 Megaohm preamp volume control. That means the setting of that control will affect the input impedance presented to the guitar and the load on passive pickups. Lower preamp volume settings will load the pickup more. It also depends on the bright cap value on the preamp volume control.

On a non-cascaded 2204, you have fixed resistors on the input instead so the input impedance will not change as you adjust the preamp volume since there's a preamp tube stage between the input and the preamp volume control.

 

[Wired]

I can't get a bad sound out of this model!