JMP 2203/2204

ICBM

CHrisP86 said:

Interestingly, after looking at it in a bit more detail, the tech said it's actually a JMP/JCM800 hybrid and seems to have been built that way from the factory.  Almost as if they finished building it with the parts coming in for the 800's.  Unless anyone knows if the 2204 circuit was changed around 1979?

That would explain why the amp sounds different to my other JMP!  

Yes, it was. The power transformer spec was changed and the later one - which is the same as the JCM800s - provides much higher HT voltage.

JCM800s didn't actually start until 1981.

Stratavarious

Mmmm…. Thinking I might take my ‘79 JMP out on Friday.  Got a big place to fill with rock. and it is the perfect amp.  And nice easy access to load it in.  Not had a run since pandemic started.

I usually take a lighter channel switching DSL… 

JMP is def. one of the best amps ever.   Mine has a push pull I added for changing the bright cap value and a resonance mod.  Get good tones at any volume. It can be bright.. which I don’t mind.

CHrisP86

Thanks @ICBM.  That’s helpful to know.  I haven’t had the full rundown from him yet but I’m sure he said a few other bits were more like an 800.

I think they were the same values but a difference I noticed the other day was the coupling capacitors look different.

On one they are a mustard colour ‘drop’ and white box shape on the other.

The 800’s I could see online tend to have red box shape.

I’m sure it’s all marginal but would those caps make much difference to the sound of the amp, if they are the same value?

These JMP’s are pretty much all I need amp wise.

I’m now slightly down the speaker rabbit hole... I think the M Blackbacks are my favourite so far.

ICBM

CHrisP86 said:

I’m sure it’s all marginal but would those caps make much difference to the sound of the amp, if they are the same value?

Yes, cap types can make a difference to the sound of an amp in some circuit positions. This is because - unlike in a guitar where they don't - there are high DC and signal voltages applied to them, so the 'self distortion' characteristics become important. In most cases the signal is also passing fully through the cap rather than it being used to drain high frequencies to ground - this also seems to make a bigger difference.

In most cases you aren't going to hear a big difference from just one cap - although there are a very few circuit positions where you can, the treble cap in the tone stack is a common one - but when you change the type of all the coupling caps in the signal path it becomes a lot more obvious. It's still secondary to the overall circuit design and the component values, but it can make quite a noticeable difference.

CHrisP86

I wonder if that is partly why people hear differences between the JMP and 800’s then, even though the circuits are almost identical on paper?

Gigged the 2204 again last night.  It really is the perfect rock tone, especially through greenbacks.  I think they are becoming my favourite speaker. 

ICBM

CHrisP86 said:

I wonder if that is partly why people hear differences between the JMP and 800’s then, even though the circuits are almost identical on paper?

With the 50s probably, but not with the 100s which always had the same HT voltage. The 800s still sound brighter to me though, so I'd probably put that down to caps.

noisepolluter

Stratavarious said:

Mine has a push pull I added for changing the bright cap value 

I could see myself getting something similar added to my SC20H once it’s out of warranty. 

Reverend

ICBM said:

Try the master at 8 . They're actually the *loudest* guitar amp I know of, even though not the most powerful, on paper - even if you go by the usual true output power which can be substantially more than 100W. There's something about the aggression and cutting high frequencies in the sound that just makes them screamingly loud. The rare JCM800 version of the 1959 Super Lead - the 'John Sykes amp' - is up there as well, although I think the 2203 might still have the edge - but I've never directly A/B'd them.

Definitely perceptively louder than the 200W Major I had, which was much softer-sounding. Although I've never tried a Hiwatt 400 with guitar...

Gaz Jennings from Cathedral used both a 2203 and a 1959 800 with 2 full stacks. Stupidly loud. They played the smaller stage at Damnation about 10 years back. It was too loud so he just used 2 half stacks instead. 124db on stage.

spirit7

Back in major gas land for an early 70s JMP. Been loving my 83 2203 recently and am so curious TK try a four holer!