JMP 2203/2204

CHrisP86

Here is the selector switch:



Is that the pull out/fall out kind?

CHrisP86

Pictures of the inside:







Would be grateful for any comments as to things that don't look right.

The only other thing I'm not sure of is that all but one valve socket is really loose.  To the point that I wouldn't be very confident in changing a valve without pushing the socket back though the chassis!

Is that a common problem and relatively easy fix for a tech?

ICBM

Yes. Very dodgy and risky - not least because they stick out just far enough that it's easy to knock them when carrying the amp, so even worse than the previous version which is shallower.

The mains one has already been hardwired, which is good - but check what voltage to, I have seen them done to 220/230 when it should be 240/250.

bananaman

Yes that's the 'fall out kind' of impedance selector. Where's the voltage selector plug? Also there's a melted resistor bodged across an anode resistor. You should be able to just tighten up the valve base. Be careful putting your hands in there if you don't know what you're doing.

ICBM

CHrisP86 said:

Would be grateful for any comments as to things that don't look right.

The only other thing I'm not sure of is that all but one valve socket is really loose.  To the point that I wouldn't be very confident in changing a valve without pushing the socket back though the chassis!

Is that a common problem and relatively easy fix for a tech?

The main filter caps appear to have been changed (good) - although it could just be the colour balance in the pic, they're a very dark blue to be original - likewise the power switches (not uncommon) but possibly also the power transformer, although it could have just had the wiring loom altered a lot when the voltage selector was bypassed... but it's not normally done like that.

Loose sockets are often caused by broken brass mounting bolts - they corrode where the head meets the steel chassis. It's a good idea to check all of them.

bananaman said:

Also there's a melted resistor bodged across an anode resistor.

That's a cap, to suppress parasitic oscillation that these can be prone to. Remove at your discretion! Sometimes Marshall fitted these across the valve instead, and/or glued various wires to the chassis in specific places - they're very sensitive to wiring dress.

It also looks like the input network cap and resistor (only just visible) have been changed. Everything else on the PCB looks original as far as I can see. There are no screen resistors fitted (as original), but which should be. The bias caps should also be changed, most likely - more important even than the main filter caps.

CHrisP86

@icbm thanks for that info. Much appreciated!

I asked if the filter caps had been changed and the seller said no. He also said it had Sovtek EL34’s and it actually has Mullard’s, so I’m now guessing he didn’t care/pay attention when it was last serviced!

I’ve had a look at the transformer and compared it to other ‘gut shots’ online, does look to be original.

What is the issue with not having the screen resistors?

So in summary, it at least needs the screen resistors back, replace the bias caps and hard wire the speaker selector.

On the basis that some of those changes are just unavoidable on a 40 year old amp, it sounds like it’s in reasonable enough condition.

ICBM

CHrisP86 said:

What is the issue with not having the screen resistors?

It tends to blow valves. Old Mullards were tough enough that Marshall didn’t think the resistors were needed, but they are with modern valves.

CHrisP86

Ah, I see, thanks.  Misread your previous post.

I was just playing it and also noticed a pulsing sound when the amp was fully on (no volume) but went away when I turned the amp to standby.

Is that likely to be tubes or power supply caps on the way out?

CHrisP86

Had the house to myself this morning and managed to crank the amp up a bit, although only managed to get it up to 2.5 on the master before shelves started rattling!

It's just sounds so good!  It's crazy that Marshall don't have a JMP re-issue.

tekbow

CHrisP86 said:

Had the house to myself this morning and managed to crank the amp up a bit, although only managed to get it up to 2.5 on the master before shelves started rattling!

It's just sounds so good!  It's crazy that Marshall don't have a JMP re-issue.

Well.. I mean, if they re-issue it, it would basically just be a JCM 800 2203 in a different box unless they 'premiumised' it with upmarket caps and trannies etc.

They could offer the headshell and chassis option though. That would be cool

CHrisP86

Been a while since I updated this thread.

I loved the 2204 so much that I ended up buying @shaunm 2104 combo.

Maybe not that surprising for two old amps but they both actually sound quite different, even through the same cab/speaker.

The 2204 I have is quite full sounding at any volume.  You can set the eq at 6, even the presence, and it sounds good.

The 2104 is much brighter and quite percussive sounding, which in some ways is more what I think of as a plexi sound.  You really have to dial the presence and treble back on it.  I've not had a chance to push the volume on the 2104 yet though, so that is just at low volumes.

I like the sound of both but just made me wonder what the 2204 circuit is 'meant' to sound like.  What do others get from their 2204's?

I'll probably get both serviced soon and one of the amps might be going to Dan Gower to be modded.

Emiel

There must be some differences between them. I know the early ones came with Mustards caps and the later ones didn't but I wouldn't be surprised if something changed in the transformers. The '76 2203 I had sounded much fuller than every other 2203 I've tried. Recently I played this early JCM800 2204 (horizontal input) and basically had to turn the presence and treble off, it was so sharp sounding. Nevertheless, they all start singing when the master volume goes past 3 or so...

ICBM

CHrisP86 said:

The 2204 I have is quite full sounding at any volume.  You can set the eq at 6, even the presence, and it sounds good.

The 2104 is much brighter and quite percussive sounding, which in some ways is more what I think of as a plexi sound.  You really have to dial the presence and treble back on it.  I've not had a chance to push the volume on the 2104 yet though, so that is just at low volumes.

I like the sound of both but just made me wonder what the 2204 circuit is 'meant' to sound like.

Have you removed that partly-melted cap across the plate resistor? If not that will take off a fair bit of top end.

If the amp isn’t stable without it, it’s still the wrong place and type to use really - Marshall used a 100pF ceramic cap across the V1A gain stage from plate to cathode, on the valve socket itself.

CHrisP86

To my shame I still need to take my original 2204 to a tech for a service and sort out some of the bits.

Definitely not complaining about the sound of either, just intrigued by the differences.

It sounds like my 2204 is probably darker sounding than it ‘should’ be. 

CHrisP86

Finally had both amps serviced and they are sounding great, especially with the RFT valves.

@icbm I’ve just seen an old comment of yours on a different JMP thread about THD Hotplates and Marshall’s. You suggested an impedance mismatch to get the best sound out of them.

I’ve got a 16ohm Hotplate and did accidentally have the amp set at 8ohm and then going into an 8ohm speaker cab. It sounded pretty good though.

Is that ‘safe’ under the same logic/science that it is safe to have a one step impedance mismatch going straight from an amp to a speaker cab?

ICBM

CHrisP86 said:

@icbm I’ve just seen an old comment of yours on a different JMP thread about THD Hotplates and Marshall’s. You suggested an impedance mismatch to get the best sound out of them.

I’ve got a 16ohm Hotplate and did accidentally have the amp set at 8ohm and then going into an 8ohm speaker cab. It sounded pretty good though.

Is that ‘safe’ under the same logic/science that it is safe to have a one step impedance mismatch going straight from an amp to a speaker cab?

It's actually *better* to set the amp to 8 ohms with a 16-ohm Hotplate, due to the slightly odd impedance curve the Hotplate has - but this doesn't apply to all attenuators, in fact for example the Marshall Powerbrake follows the impedance curve of a real speaker much more closely, and should be set to the same impedance as the amp.

Likewise with a real speaker cab - always set the amp to the matching impedance if possible.

CHrisP86

Amazing, thanks! 

CHrisP86

I ended up having to take one of the JMP's back to the tech for a better look over as it was making a lot of noise at higher gain settings. Just a simple coupling cap that needed to be replaced in the end.

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!  

McToot

CHrisP86 said:

I ended up having to take one of the JMP's back to the tech for a better look over as it was making a lot of noise at higher gain settings. Just a simple coupling cap that needed to be replaced in the end.

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!  

Hey Chris - was it Henk that serviced it? If it was, he knows his Marshalls so I bet he was intrigued by the JMP/JCM mash up...

I wouldn't be surprised if he'd tried to buy it off you 

CHrisP86

Yes, it was Henk.  He really knows his stuff and is such a nice guy.

He did seem quite intrigued by it.  He hasn't asked about buying it yet!  Last time I saw him his advice to me was actually to buy more Marshall's, rather than sell any!