JCM 900 loop discoveries and some questions

MKIII900

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First off, sorry for the book I'm about to post but I'm really trying to find out some solid info on the FX loop in my MKIII JCM 900 2501 combo. After scouring the manual, this forum and the internet in general, I have found all sorts of contradictory information.
However, the consensus (and the manual) seems to suggest that it is a serial loop. There is a recessed trim pot for fx level. Set to low is for stompboxes. Set to high for rack gear.

I'll post my experience using the amp to give you an idea of how I use it and the issues I've run into.

Real pedalboard:
This board is all boss pedals with the exception of an EQD ghost echo. I notice that there is a faint crackle or static as the notes "bloom". Not overwhelming but it's there and once I heard it I can't un hear it. It is less noticeable the lower the pot is set but never goes away.
I've also noticed that the tone of the amp itself sounds much better with the trim pot set from halfway up to all the way up.. More gain, more bass, fuller sound in general. With the trim pot set to low the amp sounds brittle and sterile whether the pedals are on or not.

Line 6 HD500 PRO X

I have this set up in the 4 cable method. I'm using the HD500 as a preamp/multifx. Aside from the the FX, it gives me the ability to switch in a clean preamp and a Mesa style high gain preamp. With my actual JCM 900 preamp providing the classic to hard rock sounds.

With this setup I have the loop on the JCM 900 set to high and the relevant switches at the back of the HD500 (loop and output) set to line level. I have zero issues. The amp sounds great and the faint static is gone.
I have noticed that if I set either the loop on the JCM 900 to low or the switches on the pod to "stomp" the JCM 900 preamp becomes much louder than the preamps and fx in the HD500. Seems with everything set to line its perfect.

Questions

I'd really like to know if the FX loop was really designed for rack gear like I have read on forums? Seems from my experience that it is.
Considering that it works perfectly with the HD500 and I have issues with real pedals, I can assume my loop is functioning properly and bad fx loop jack's are not the culprit.

Could the faint static or crackle I hear be the pedals overloading the fx loop?
Is there a reason that the amp sounds considerably better with the loop set to high?
Even if turning the trim pot down to low solved the static issue I still wouldn't use the real pedalboard because the amp iself sounds considerably worse in that setting.

My particular amp isnt that common but If there is anyone that can shed some light on the design of the loop in the JCM 900 line (assuming that the SL-X, DR and MKIII all have the same loop) I'd really appreciate any info I can get.
 
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MKIII900

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@anotoli
Thanks for the schematic and the info about it being a dual gang pot. I'll have to do some research about how those operate.

However I have cleaned the jacks and the amp had a full service after I recieved it. I dont think dirty jacks would be the culprit seeing as the HD500 has no issues. A dirty jack would cause static no matter what was plugged into it, right? With the rack gear at line levels I have no trouble. Maybe static was the wrong description. It kind of sounds like clipping on a mixing board but very faint.
 
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anitoli

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There's no crackle with the HD500. If the jacks were dirty wouldn't they cause static no matter whats plugged into them?
Not necessarily. A smaller signal that may be affected by dirt may disappear when the signal gets large enough. This is an older amp and these things can definitely surface. That level adjustment is supposed to allow it to be used with either rack units or regular pedals. Unfortunately some of the loop designs are not universal soldiers in regards to flexibility, some require a give and take approach. Gotta remember these amps were made when the rack/pedal companies were going nuts making all kinds of new shit and trying to get a loop to balance all this gear is not easy.
 

Kelia

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If this can help , my 1989 JCM800 2205 is very quiet with a Roland SDE3000 in the loop
and crazy noisy with a TC Electronic rackmount reverb and multi effets unit , go figure ?
 

MKIII900

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Looking at the schematic it appears to use a dual gang pot, one track on the input, one on the output.
Try cleaning this pot and the loop jacks with Deoxit then recheck for the crackle.
Not necessarily. A smaller signal that may be affected by dirt may disappear when the signal gets large enough. This is an older amp and these things can definitely surface. That level adjustment is supposed to allow it to be used with either rack units or regular pedals. Unfortunately some of the loop designs are not universal soldiers in regards to flexibility, some require a give and take approach. Gotta remember these amps were made when the rack/pedal companies were going nuts making all kinds of new shit and trying to get a loop to balance all this gear is not easy.

That's my understanding as well about the fx loop being able to work with rack gear or pedals. I'll give the jacks another cleaning for good measure.
 

MKIII900

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If this can help , my 1989 JCM800 2205 is very quiet with a Roland SDE3000 in the loop
and crazy noisy with a TC Electronic rackmount reverb and multi effets unit , go figure ?

Hearing stuff like that does help actually. Makes me think that there may be one pedal on my board causing it that's not compatible. Maybe one bad jack. Doesn't explain why the amp sounds way better with that pot set high no matter whats plugged in. But the static could I suppose be coming from any pedal or jack or cable on the pedal board.

Since the hd500 works flawlessly I may just retire the pedalboard and use that instead. The amp is a keeper so if I can't make the pedalboard work I'll just use what works with the amp.
 

MKIII900

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Im not savvy enough to tell from a schematic but is there any harm in adjusting the fx loop level when the amp is on? Would be nice if I could turn the pot and listen for a crackle.
 

MKIII900

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No, adjust away.

Thanks again for the advice. I tested the board after cleaning the jacks and messing around with the pot on the back while I could hear in real time. The amp sounds way better with the HD500 plus I get more preamps to play with on a single channel amp.
I'm just going to sell off the pedalboard. I have an Axe-fx 2 for recording and since the HD500 works so well with the JCM 900, I can't be bothered to chase a bad cable or dirty jack on a 10 or more pedal, pedalboard anymore.
 

Kelia

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Someone here once mentioned that 2205-2210's are very fussy about what you feed
the effects loop with so I guess that yours is fussy too !
 

mrjones2004x

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I have a 2501 also. I use a tc nova system in the loop with zero noise. I had crackles before but since it’s service last month it’s perfect now.
Service changed every jack and every pot including the fox loop level pot. The Marshall tech said these always cause issues whether it’s the jacks or the fox pot.

also mine sounds amazing on full whack too. If I turn it down it sounds sterile and thin. I leave it on 10 and the amp is thick and ballsy.

I’m contemplating going the single pedal route now tho as the nova has too many settings and I can’t get it how I want it so once I get a few pedal I’ll be sure to listen for noises
 

Spooky88

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Not necessarily. A smaller signal that may be affected by dirt may disappear when the signal gets large enough. This is an older amp and these things can definitely surface. That level adjustment is supposed to allow it to be used with either rack units or regular pedals. Unfortunately some of the loop designs are not universal soldiers in regards to flexibility, some require a give and take approach. Gotta remember these amps were made when the rack/pedal companies were going nuts making all kinds of new shit and trying to get a loop to balance all this gear is not easy.
Anatoli is correct. I own 2 “older” (1992 and 1993) JCM 900 amps both FX loops have/had issues. Even after I thought (and serviced them) they couldn’t possibly be the issue.
 

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