I have a 2003 DSL 50 that I bought used a few years ago. The Ultra Gain channel has developed an unnatural loud hum that appears to get louder as the session continues. The Classic Gain channel does not have a hum. The Ultra Gain hum becomes noticeable only when the standby switch is on and a guitar/cable is plugged in. The volume affects the level of hum noise. I also own a reissue 1987x and I have owned a DSL100 before and have never heard a hum like this before. I recently had a local tube amp repair man with a good reputation, and who I've used before, look it over and after 20 hours of his time he couldn’t find the problem and didn’t charge me.
I installed in a Marshall JCM 2000 DSL 50. I never heard the original reverb tank that this replaced, so I can't really compare. It sounds great with the clean channel and I'm still trying to decide if I like it with the lead channel instead digital reverb.
He even suggested that it very well could be a design flaw. But, he agreed that was odd since the hum wasn’t always present with this amp.
The power tubes are new and biased properly. The filter caps among other relative components have been checked “five times over”. The output and power transformers heat up and check out in good order. There doesn’t appear to be any modifications to this amp. Any thoughts on what is missing in the diagnosis, or could it really be a design flaw that developed over time? Thanks for the assist. Noob Posts: 5 Joined: Sat Sep 20, 2014 12:10 am.
There isn't much difference in the signal path between the lead and crunch channels. 1) In the 20 hours of his time, did your tech try replacing V1?
V1a is part of the additional gain stage for the lead channel. 2) When you get the hum, is that ONLY with the guitar cable plugged in? 3) Do you get the same hum when you plug directly into the power amp? If he did replace V1 with a good tube and the tubes are all 100% tested and good, I would use a simple debugging technique, which is basically following the signal path and grounding at at various points in the signal path until you find the source of the problem.
Its easy and simple and after 1 hour, 2 hours max, the problem should be located to only a few components. Which tube board revision do you have? It is printed on the board and you can find a explanation as to how to find it on this page, under Marshall DSL Schematics and board info. Thanks for the reply. I have replaced the preamp tubes one by one with a working set from a plexi and the loud hum is still there.
The hum is ONLY present with the guitar cable plugged in and standby 'on'. There is also a slight 'pop' sound when the channel switch is pressed in or out. Not sure about 'plug directly into the power amp,' I am plugging the instrument cable into the 1/4' input on the face of the amp and the power source cord in the back?
Board ID is JCM2-61-00, so I believe the revision number would be '00'. Someone in another forum mention a possible bad 'hum balance pot'. I plan on running another set of preamp tubes to be sure about the tubes diagnosis.
Noob Posts: 5 Joined: Sat Sep 20, 2014 12:10 am. Plugging into the power amp is as easy as plugging into the effect return jack. The fact that there is no hum when the guitar cable is NOT plugged in leads me to believe that the problem may not be with the amp itself. Could the environment in which you play the amp have changed? Maybe a guitar cord isn't shielded properly, dirty connections through effects pedals? Dirty input jack on the amp or the guitar? The problem happen when you plug straight in, no effect pedals, nothing.
Just guitar and amp. Plugging the guitar directly into the effects return jack alleviated the obnoxious noisy hum I have been receiving from plugging the guitar through the input jack on the front of the amp.
![Marshall Jcm 2000 Dsl 50 Schematic Design Are Marshall Jcm 2000 Dsl 50 Schematic Design Are](http://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JCM2000-DSL50-Schematic.jpg)
But, the environment the amp is in hasn't changed, there are other amps there too and they perform normally. I have tried numerous instrument and speaker cables, guitars, speaker cabs, no effects in the signal chain, but I get the same results only with this DLS50. Also, when the amp was with the amp serviceman he was getting the same issues. The only other possible link in the chain you mention is the input jack itself, which I will look at next. Much appreciated.
![Marshall jcm 2000 dsl 50 schematic design area Marshall jcm 2000 dsl 50 schematic design area](http://cdn.tonegeek.com/wp-content/uploads/P1020358.jpg)
Noob Posts: 5 Joined: Sat Sep 20, 2014 12:10 am. It looks like the V4 socket may be bad, everything up to that point checked out ok, and when that tube was removed, there was no hum. Also, of the four preamp tubes, only V4 is actually touching the surface of the board. Part is on order, will follow up when repaired. On another note, swapped out the stock output transformer and installed a choke, and besides the hum when idle, the amp never sounded better. Standby ON (how the switch reads), meaning the amplifier is fully ON and the guitar is playing through the amp and speakers.
Thanks for the suggestions, taking notes. Noob Posts: 5 Joined: Sat Sep 20, 2014 12:10 am.
Hi all, I came across a great deal on an older Marshall JCM 2000 DSL 50 head and am trying to make the best of it. It's yet to display the runaway bias drift issue, and other than having a noisy reverb circuit it sounds pretty good, especially with NOS Mullard, Telefunken, and RCA preamp tubes.
I removed the reverb and that took care of that issue.amp is quiet and feels pretty good. However, I'd like to make this amp the best it can be within reason, and have used Sozo caps in past builds with great success, observing they provide a very clear, clean, 'right' sound. So, I'm looking for help in choosing which coupling caps in the JCM 2000 DSL 50 I should replace for most bang-for-the-buck improvement in tone and feel. The.022uf caps in the tone stack seem like obvious ones, as well as the.022uf coupling caps in between preamp stages as well as after the phase inverter. Maybe a cathode cap or two as well? I'm not too up on the gainier side of that circuit and have not used Sozo's in high-gain circuits, so I'm open to suggestions/input.
Would one of you be willing to take a look at the JCM 2000 DSL 50 schematics out there and let me know what caps you'd suggest swapping with Sozos? It'd be much appreciated. Thanks in advance, David. I managed to fit it in.put some clear, PVC tubing on the end close to the switch, and angled the cap a bit. The rest fit just fine with one or two needing an altitude change to avoid hitting the other stuff (I hate PCB construction.sorry.).
But yes, it fired up and worked without incident!!!! When does that ever happen (to me)???
Interesting difference, though.with the 'stock' setup, the amp was more modern sounding, versatile, even-keeled. After installing a 3hy choke (Classic Tone) and replacing the major coupling caps with Sozo NextGen caps, the amp went way toward the vintage realm. Very interesting.I'm not yet sold on the Green Channel after the swap, but oddly enough, the Red Channel has been improved. Odd, as I preferred the Green Channel by far before the mods. To sum up the Green Channel tone change post-choke-and-Sozos, it's like the amp went from Green Day to 'It's a Long Way to the Top if (You Wanna Rock and Roll)', as the Green Channel seems to magically want to sit, tonally, in that mid-'70s, scooped, (JMP?) tone range. Not sure JMP is the best way to describe it, but I'm tired and that's the first thing that comes to my gut instinct.
Anyway, I'll give it a go and let the caps break in some more before giving a final judgement. One thing is for sure, there is LESS hum now, clearer bass, and more articulate top end present than before. It's not necessarily, qualitatively 'better' to my ears at this point, but it sure isn't worse.
Very interesting. Phase Two of mods has begun. Removed C12 on the 'front board', which served as a bright-cap across the Red Channel's Gain pot. Not a fan of it installed, prefer it somewhat now that it's removed, yet the Red Channel still sounds 'congested' for lack of a better word. I've seen the following mod suggestions: and HOWEVER, before I start tinkering further, I figured I'd post here as you guys, in my experience, are some of the best on the web. My goal is to make the amp more open, clear, organic, and less gainy. That is, the Green Side ain't too bad.I feel no particular compulsion to change things on that side unless it's something special.
However, the Red Channel just isn't my cup 'o tea, though I do like that Modern Metal thing from time to time and it's better now that the bright cap is out. I don't usually play with super high gain, and if given the choice I'd always opt for organic clarity over gain for gain's sake. That said, any suggestions as to how I could tweak the Red Channel to further my goal, to open it up and give it more life and breath? Might like tightening up the low end on that channel as well. Thanks in advance! Yes, you are correct that most amps, even beyond Marshalls, have a 12AX7 PI. I see it as a homogenization of topology, in that, everyone does it because it works and always has.
I look at mods and custom amp builds from a stance of 'how can I make an amp that will fulfill this particular players needs'. I made this recommendation based on what I read in your posts and what I would personally try if you and this amp were in front of me, without regard for any 'Marshall thing'. Sometimes, as modders, we look for complicated answers to simple problems. This is a simple plug and play attempt at getting you closer to what you want to hear from your Marshall. I have had some success with the JAN AT7s that you can get for under $20. Please let me know if you try it and what your impression is. Will do, thanks.
I have a JAN '80s 12AT7 and an older RCA cleartop 12AU7 ready to try in the PI, once I put my amp back together. I'm trying an altered version of the Joey Mods.pretty much identical, save for I'm going to run a.68uf cathode cap w/o the 1000pf plate cap on the first gain stage (he recommends a 1uf cathode cap with a 1000pf from plate to cathode, whereas I don't see that on your typical Marshall circuit so I'm rolling the.68uf alone to see if I can get good, true, lower-gain tones out of this thing.not so interested in high gain stuff). Alright, here's where I am with this: Main Board: Removed C9 entirely (100pf from the first gain stage plate to the cathode) and replaced C12 (the first gain stage Ck, a 4uf electrolytic) with a.68uf Sozo and lowered the same stage's Ra (R13) from 220k to 100k while raising Rk (R14) from 1k8 to 2k7. I did all that to get that first gain stage closer to a JCM 800 and/or 1959 lead schematic. Front Board: I installed a 1M to ground on the wiper for VR3 (Red Channel gain knob), lowered C10 to.0022uf (no idea what that does where it is), removed R20 and jumpered (again, not exactly sure what that does), and removed C12 (bright cap across VR3.was a 470pf, might put in a.0047uf cap as that was nice when I tried it). I like the 'Clean' side on the Green Channel A LOT better now. It's nearly perfect, ESPECIALLY through a straight 4x14 with Heritage Greenbacks.
The 'Crunch' side of the Green Channel is alright, but seems to have more midrange now and much leaner bass, maybe too lean. I'll play it more to see how it grows on me, but it might be okay.if not, it's at least close. If I could get more low end here it'd be fine, but for now I'll see if engaging the 'Deep' switch will do it for me. The Red Channel is the odd one.it's better in that it's gain is more useable and the sweep of the gain knob is more even, but there is a distinct midrangey-ness to both Lead 1 and Lead 2 on the Red Channel now that I don't care for. It's like the 'R' sound is more pronounced.a snarly thing in the midrange that is hard to dial out.
If I could get it EQ'd more like the Green Channel I think I'd be set.but the network of stuff going on in the Red Channel is beyond my comprehension/skill level at this point. Oh, and running a 12AT7 in the PI wasn't too different from the 12AX7, but I DO like a 12AU7 in the PI spot.cleans up the amp a bit, yet still has plenty of gain.
Anyway, recommendations for how I could flatten out the midrange on the Red Channel a bit and get it EQ'd more like the Green side? It's getting close.better, but not quite finished.