Marshall JCM 900 SL-X with mods: discussion and review

[JonPearson1994]

I have recently gotten my SL-X modded; i have had it for a while but just now got around to getting it tweaked, which I always had in the back of my mind.

Now, this is the third SL-X I've owned over the years. This is the second 5881 model. I keep coming back to these amps because they have a kind of rude, aggressive sound to them that I find missing from the current Marshall lineup, and they have a proper amount of gain for metal where the 2203/4 and 1959/87 by themselves fall a bit short.

This one came to me much as I remembered it - gainy and super fun to play, but a bit difficult to dial in. All JCM 900s are pretty bright, but the SL-X models have a problem with mud once they start getting high on the preamp volume dial (for those unfamiliar these amps have two gain controls located at different places in the circuit). You have a limited range of tweakability for this reason, where you need a lot of high end to fight mud but then you can end up with a harsh sound.

Set aside the tonal issues for a moment - I knew from past experience that these amps have issues with the rectifier for the DC heaters cooking the board. The rectifier is underspecced, so it heats up and causes all kinds of trouble. I went about rebuilding the heater supply circuit on a terminal strip and replacing the filter caps.

I got the idea that it would be cool to try some mods out, but I also don't have much space or time for experimentation like that, so once I got the heaters and filter caps sorted, I took this over to my buddy Jon at Berrier Audio Works and had him take a stab at modding.

The results are killer! I recorded a video (pardon the rambling, that's my southern kicking in) showing off a bit of the sounds:



Overall, this fixes the biggest problem I have with the amp, which is the flub when you push the gain. Now I can dial it all the way up with no issues, though I don't need to dime it to get sounds that I like.

I've considered whether a choke is worth it, but honestly it is pretty tight as is, and even with the choke I think I'll probably still like the boost for the lower stuff so it seems like it might now be worth it.

 

[Rising Farce]

I have recently gotten my SL-X modded; i have had it for a while but just now got around to getting it tweaked, which I always had in the back of my mind.

Now, this is the third SL-X I've owned over the years. This is the second 5881 model. I keep coming back to these amps because they have a kind of rude, aggressive sound to them that I find missing from the current Marshall lineup, and they have a proper amount of gain for metal where the 2203/4 and 1959/87 by themselves fall a bit short.

This one came to me much as I remembered it - gainy and super fun to play, but a bit difficult to dial in. All JCM 900s are pretty bright, but the SL-X models have a problem with mud once they start getting high on the preamp volume dial (for those unfamiliar these amps have two gain controls located at different places in the circuit). You have a limited range of tweakability for this reason, where you need a lot of high end to fight mud but then you can end up with a harsh sound.

Set aside the tonal issues for a moment - I knew from past experience that these amps have issues with the rectifier for the DC heaters cooking the board. The rectifier is underspecced, so it heats up and causes all kinds of trouble. I went about rebuilding the heater supply circuit on a terminal strip and replacing the filter caps.

I got the idea that it would be cool to try some mods out, but I also don't have much space or time for experimentation like that, so once I got the heaters and filter caps sorted, I took this over to my buddy Jon at Berrier Audio Works and had him take a stab at modding.

The results are killer! I recorded a video (pardon the rambling, that's my southern kicking in) showing off a bit of the sounds:



Overall, this fixes the biggest problem I have with the amp, which is the flub when you push the gain. Now I can dial it all the way up with no issues, though I don't need to dime it to get sounds that I like.

I've considered whether a choke is worth it, but honestly it is pretty tight as is, and even with the choke I think I'll probably still like the boost for the lower stuff so it seems like it might now be worth it.

I've played, and gotten great sounds from, all the 900 amps, including the Studio version I have currently. This one sounds excellent.

 

[JonPearson1994]

I've played, and gotten great sounds from, all the 900 amps, including the Studio version I have currently. This one sounds excellent.

Those little studios sound great! Everyone dunked on them when the came out but I think the peanut gallery piped down when the demos started coming out.

 

[Rising Farce]

Great demo, by the way. It's always nice to hear something other than one-finger chug-a-lug in drop Z.

 

[70 Mach 1]

I always liked the 900 line.
Back in 93 i bought my 4500 DR new.
It was during the tube shortage and took a few months to come it.

Then i was disappointed because it had 5881s.

After i bitched for 15 minutes He showed me an SLX that was there and had EL34s that i could have swapped for.

I decided id rather have the option of a clean channel than have gain and more gain.

Im a blues guy so the cleans made more sense to me.

I still use the head although its been giving me issues lately with cold solders and dried out traces.

I may just trade it in for something else or just sell it before it cost me more money

 

[csguy0]

I have recently gotten my SL-X modded; i have had it for a while but just now got around to getting it tweaked, which I always had in the back of my mind.

Now, this is the third SL-X I've owned over the years. This is the second 5881 model. I keep coming back to these amps because they have a kind of rude, aggressive sound to them that I find missing from the current Marshall lineup, and they have a proper amount of gain for metal where the 2203/4 and 1959/87 by themselves fall a bit short.

This one came to me much as I remembered it - gainy and super fun to play, but a bit difficult to dial in. All JCM 900s are pretty bright, but the SL-X models have a problem with mud once they start getting high on the preamp volume dial (for those unfamiliar these amps have two gain controls located at different places in the circuit). You have a limited range of tweakability for this reason, where you need a lot of high end to fight mud but then you can end up with a harsh sound.

Set aside the tonal issues for a moment - I knew from past experience that these amps have issues with the rectifier for the DC heaters cooking the board. The rectifier is underspecced, so it heats up and causes all kinds of trouble. I went about rebuilding the heater supply circuit on a terminal strip and replacing the filter caps.

I got the idea that it would be cool to try some mods out, but I also don't have much space or time for experimentation like that, so once I got the heaters and filter caps sorted, I took this over to my buddy Jon at Berrier Audio Works and had him take a stab at modding.

The results are killer! I recorded a video (pardon the rambling, that's my southern kicking in) showing off a bit of the sounds:



Overall, this fixes the biggest problem I have with the amp, which is the flub when you push the gain. Now I can dial it all the way up with no issues, though I don't need to dime it to get sounds that I like.

I've considered whether a choke is worth it, but honestly it is pretty tight as is, and even with the choke I think I'll probably still like the boost for the lower stuff so it seems like it might now be worth it.

Where did you have it modded? Local shop that does these sorts of things, or did you have to ship it off somewhere? I'd love to have my Friedman modded, but I really don't want to ship my amp from Florida to California:/

 

[JonPearson1994]

Where did you have it modded? Local shop that does these sorts of things, or did you have to ship it off somewhere? I'd love to have my Friedman modded, but I really don't want to ship my amp from Florida to California:/

Yes, a local guy, Jon Berrier. He has been doing it for a while, but recently he has been posting more and building up that side of his business more. I like Jon because he has a pretty broad taste in music and tends to understand better what I'm looking for when we talk than a lot of guys.

Unfortunately I don't know anyone in Florida who is in the amp modding world, but you may be able to ship Jon your amp up to North Carolina! If you look up Berrier Audio Works, he has his own site. He is on Instagram as well, you'll see some of his videos going through mods or repair work.

 

[csguy0]

Thank you for the reply! North Carolina wouldn't be too bad. I'll check out his site, prices pretty reasonable?

 

[JonPearson1994]

He charged me around $350 I think for what he did, which included a new set of tubes and all the components for replacing electrolytic caps (except the filter caps which I had already done), and the modifications he made. Obviously the more intrusive the mod, the more the cost would go up. For the quality of work, I think his prices are great.

 

[csguy0]

That is not bad at all, right in line with what I would have expected. Cheers!

 

[rsm]

Very pleased with my stock SL-X 2100 (100W) with 5881's. I only use standard tuning or half step down, and 6-strings, so no issues with mud, flub on the low end.

I have other EL34 Marshalls.

 

[JonPearson1994]

Very pleased with my stock SL-X 2100 (100W) with 5881's.

I have other EL34 Marshalls.

They have some merits, to be sure. The 5881s do have more bottom, from what I can tell, which can be a nice thing. Not that I would expect it, but I do think Marshall would have done these amps a favor if they had used a transformer designed for the 5881s when they went to those.

 

[ChuggNorris]

Your tuning and floppy sounding bass strings really didnt do this amp justice at all in this demo. My SLX sounds so much more percussive and define.

Youre dialing it a little off too.... you didnt hit the right combinations at all.... doesnt need a boost to be percussive and tight.

However you did a better job with the 2nd guitar. Goes to show how tuning and guitar make a difference.

You delivered it well though so its slightly difficult to hate on you overall. Appreciate the SLX love. One of my all time favorite amps.

I preferred it in the room to a herbert iii.

I used crazy settings though.... nothing like how you dialed it. LoL

 

[JonPearson1994]

Your tuning and floppy sounding bass strings really didnt do this amp justice at all in this demo. My SLX sounds so much more percussive and define.

Youre dialing it a little off too.... you didnt hit the right combinations at all.... doesnt need a boost to be percussive and tight.

However you did a better job with the 2nd guitar. Goes to show how tuning and guitar make a difference.

You delivered it well though so its slightly difficult to hate on you overall. Appreciate the SLX love. One of my all time favorite amps.

I preferred it in the room to a herbert iii.

I used crazy settings though.... nothing like how you dialed it. LoL