JCM 900 50 watt Dual Reverb Model 4500 - MOD TUTORIAL

[GrayFox14048]

Hello there!

I have an old JCM 900 50 watt Dual Reverb Model 4500, I'm looking for some good mod to make it a better amp like jcm800 or similar, I have found a lot of forum and conversation about that but no one complete or clear.
Can somebody give me a good tutorial to make a nice MOD for this "stinky" amp??
A video could be the top, but some pictures are appriciated too!
Thanks everybody!

 

[Purgasound]

Yikes. An 800 conversion is a full rebuild unfortunately. The 4100/4500 topology is very different from any of the tube Marshalls preceding it. The gain stages in a 2203/2204 preamp are all tube and the 4100/4500 utilizes IC's for most of the gain stages and some LED's for added distortion. There are some mods you can do which are changing the preset EQ in the boost channel. I haven't really tried to alter the circuit on these since I haven't found a way to radically change the amp voicing without redesigning the whole thing. There are some DR mods that are floating around the internet and this forum but I have not seen any tutorials.

 

[Sg-ocaster]

The jcm 900 DR has a totally differant circuit to an 800, The tone stack is further in the front of the amp and the input stage is diodes.
I made a freinds DR sound much better with a tube swap. I used an old used I63 Mullard 12ax7 in V! and a Telefunken smooth plate 12ax7 in the PI(V3) position(all though a JJ sounded fine there too) with groove tubes el34M power tubes and it was a decent sounding amp after. More compressed and fuzzy than an 800 but not a "Bad" Marshall sound.
Pick up a used I63 code Mullard 12ax7 on ebay pop it in V1 ,stick a chinese 12ax7 in V2 and a strong JJ ecc83 in V3 and get a good matched pair of el34 power tubes and have it biased. Buy your new production tubes from a reputable dealer who tests them(lots of duds on the market)
If your amp has 5881 power tubes Id suggest having a tech convert it to el34(thats the Marshall sound)

 

[Spanngitter]

Easiest Mod: Sell it and buy a 2204 (and not a 2205)

 

[Lowlife]

If I recall correctly the next easiest mod is to put a patch cable into the Loop, and tweak the loop level until you find the sweet spot.

It's odd, but it works...

 

[Lowlife]

Back again, Getting an attenuator and crank it more, is also very nice...the output stage can really roar if you let it.

 

[Pete Farrington]

If I recall correctly the next easiest mod is to put a patch cable into the Loop, and tweak the loop level until you find the sweet spot.

It's odd, but it works...

Good idea! On checking the schematic, it looks like that will allow V2a to be overdriven https://www.drtube.com/schematics/marshall/cd0189-iss7.pdf

 

[ibmorjamn]

I

The jcm 900 DR has a totally differant circuit to an 800, The tone stack is further in the front of the amp and the input stage is diodes.
I made a freinds DR sound much better with a tube swap. I used an old used I63 Mullard 12ax7 in V! and a Telefunken smooth plate 12ax7 in the PI(V3) position(all though a JJ sounded fine there too) with groove tubes el34M power tubes and it was a decent sounding amp after. More compressed and fuzzy than an 800 but not a "Bad" Marshall sound.
Pick up a used I63 code Mullard 12ax7 on ebay pop it in V1 ,stick a chinese 12ax7 in V2 and a strong JJ ecc83 in V3 and get a good matched pair of el34 power tubes and have it biased. Buy your new production tubes from a reputable dealer who tests them(lots of duds on the market)
If your amp has 5881 power tubes Id suggest having a tech convert it to el34(thats the Marshall sound)

I prefer the 5881's in the DR , however if original poster is looking for 800 tones, You night want to buy a 800.

 

[Im247frogs]

Easiest Mod: Sell it and buy a 2204 (and not a 2205)

Beat me to it.