My New JMD501 Has A Strange Fizz

[technoriff]

Hey Guys - This is a great forum, and I've enjoyed browsing the many experiences and advice offered on various Marshall amps. Hoping some of you JMD guys could give some sage thoughts on an issue I'm having.

I took the plunge a few weeks ago, and bought a brand new JMD501 - my first Marshall. Out of the box, it was great getting those Live Zep sounds effortlessly and the feel of the amp seems to elevate my playing to a new level. However, after listening more closely with a critical ear, I started hearing a low-level "swirling" fizz or buzz that oscillates in and out as a chord or note is held and decays into silence. This seems to happen on any of the distorted preamps (all except the Clean ones) regardless of the Master Volume or other control settings, and is possibly more pronounced on the Overdrive/Lead ones. This occurs when plugging the amp into other broken-in speakers or headphones.

Just wondering if this is "normal" for a Marshall amp, or whether my unit might require retubing / rebiasing, etc. Again, it's a brand new amp so I didn't think any such adjustments were needed. Appreciate any thoughts anyone might have. This sound has been driving me crazy once I started hearing it, and I'm hoping it's something really simple. Thanks.

[guitarest]

Grounding? Can you swap guitars and see if it foes this with different guitars? Swap cables? Back off all effects to 0 and see if thats it? Depending on the effects used I primarily do not use any effects with the amp except reverb and maybe a slight delay; but the noise gate (which is not a effect) is the best thing going for the amp besides the different tones.

[technoriff]

Thanks, Guitarest - a good checklist of factors to eliminate. I did try all of the above, and had turned off all effects. There is still that low-level "swirling" fizz/buzz (or sizzle?) that is beneath the main sound - it seems only noticeable once the held note(s) decays to a low level. I've asked and been told by my dealer that tube amps will exhibit this to some degree, and is part of the distortion characteristic. Thing is, I just don't know what "normal" would be on this amp and whether it could be reduced by other means like retubing/rebiasing , etc. I have some JJ EL-34Ls on order (read about these on this forum's other topics) for a quick test to see if that helps.

[guitarest]

Your welcome but be careful unsure what this sound your talking about and high your master volume actually is but I have a 50watt head and 1960 cab and there is a little sizzle on the high gain stuff but thats what the noise gate is for. It zaps it 100% of the time.

[Spotted Dog]

I had a sound in my 2266 head just as you described technoriff. The amp was brand new and I owned it for about 4 weeks when I first noticed it. Turned out to be a preamp tube. When it was replaced the sound disappeared. Good luck.

[technoriff]

Guitarest - Thanks again - can you recommend your best way to set the noise gate for a particular setting?

-- Thu Jan 20, 2011 1:02 am --

Hey Spotted Dog -
Thanks for confirming with your experience. How did you choose your replacement preamp tube - and can you recommend one? I see for the JMDs, there's an ECC83 preamp tube / phase inverter that could also be aggravating this noise if it's off. I'm willing to try everything reasonable to get this amp to sound its best - love the sound.

[guitarest]

Remember if you use the gate you can't use anything else on that knob which includes the chorus, Flange; etc.... I generally set the gate for between 3/8 and 1/2 total effect. Its makes for a very quiet amp until you do something on the guitar; although using a gate will kill your notes somewhat premature but for me its well worth the using the gate vs noise.

[Hellhound]

I had a similar issue with the JMD501 that I had. A lot of it was fret buzz. The 501 seemed to amplify the buzz whereas the VM did a great job of not noticing. I switched to JJ's, raised the action and they both helped. Plugging the combo into my 1960AV helped as well. I have the JMD100 now and it is much better at not noticing the buzz. Like Guitarest, I have the noise gate on all the time and it is usually set to noon.

I also have similar problems with all my amps at night time. I have dimmer switches in all the rooms and really old electrics. This always results in a buzzing and hissing swirly sound. Perhaps it could be eletrical interference?

[technoriff]

Guitarest - Engaging the noise gate does help - thanks. Too bad that this uses up the Modulation as you pointed out. Really it was an additional independently switchable effect.

-- Sat Jan 22, 2011 2:54 am --

Hellhound - Thanks for the tips and insight. This being my first Marshall, I don't have any other model to compare it with. Sounds like the JMDs might be a new breed - being digital - that are extra sensitive to other factors. I did just get some JJ EL34Ls and intend to swap them this weekend to see if they help this. Also have pulled an 12AX7 from my Mesa to see if that would warrant getting a new preamp tube as well.

I do like my actions to be on the low side, so that could be something to ponder. So the VM is not sensitive to fret buzz? That's an alternative to consider. But I really dig the JMD's ability to store presets for instant recall - I'm getting lazier as I get older, and any help I can get to save time is appreciated.

[Hellhound]

You're welcome. It could be any number of different issues, all of the above posts are valid. I really rate the JMD because it is so versatile. The noise gate is the best I've ever used and the footwswitch is amazing! I hope you manage to sort the problem, let us know how you get on.

[technoriff]

Totally agree, Hellhound - it was a total joy finding that mega-footswitch included with the JMD rather than requiring more cash outlay.

I think swapping in the JJ EL34Ls does seem to improve the sound a lot, and smoothes out that post decay fizz. I also noticed how much larger in diameter the stock tubes are compared with the JJs - perhaps this is a contributing factor. But swapping in an old Mesa 12AX7 has resulted in inconclusive results - will now look at getting a new JJ preamp tube. Just wondering if anyone has tried and had good results with this sort of thing?

A neat trick I learned from Noise5150's topic on using the Effects Loop as a a boost seems to push the noise floor lower relative to the guitar sound. This lessens or even eliminates the need for engaging the Noise Gate that then frees up the other Modulation effects for use. I like not having to use the Gate as it somewhat affects the sound for my style of playing.

I'm really warming up to the JMD501 - it's been well worth the time to play around and work thru all the great tips from folks on this forum. Am now appreciating why this amp series is so well regarded.

[JC1]

I know that "fizz". I have it on my JCM800 2203 right now and it's been a bear trying to located the cause. Mine is a 1983 year model. It was sort of a trade between Ken Bran & I back in 1983 while I was stationed in England from 1980 to 83 with the USAF. Ken wanted a mini stereo system and I got it for him in exchange for 3 2203 heads. He also gave me a 4210 50w combo that was sitting in his foyer of his home where I picked up the heads. He said it was just returned by one of Eric Claptons people because Clapton had a contract with Musicman at that time and under contract wasn't allowed to use anyone else's equipment. He said "it's used by you can have it if you like". Ken was a great guy and a good friend. For those of you who don't know who Ken Bran is...he's the guy, the guy that started it all in a back room of one of Jim Marshall's music shops when he was a tech for PanAm. I was fooling around with building amps and Jim owner several shops. The 2 of them ended up being responsible for one of the greatest things that ever happened in music...the Marshall amp!! If you ever heard of a NARB amp..that's Bran spelled backwards.

Anyway, I modded up this amp over the years, but the past year I really modded it up. I was thinking maybe I did something to cause the fizz I'm hearing but I've taken every precation and advice from my local amp botique shop & guys like Aiken & Webber.

Now changing preamp tubes didn't fix it. I get it at low volumes. I have a Master Volume Pot pre P.I. tube. I have one extra tube installed with Reverb (Tank) & effects loop (parallel). All pre amp tubes are 12ax7 and the P.I. is a 5751. Output tubes are EL34's, biased for 55 - 60% idle plate dis.

Now, this "fizz" under the notes is much more pronounced when chorus is engaged with preamp distortion. Slide a finger up the B string to say a high D or E and you hear the fizz right after the note. It can't be crossover distortion because 1: I'm not driving the output tubes much whe I can hear it and their biased up correctly with no notch & 2: I also did the Aiken mod of a zener diode & a regular diode in series across the bias resistorss (220K's) at 2-3v above teh bias voltage, or in this case, -37vdc with a 39v zener. Don't see any crossover in the output form (oscope) when pushed into clipping.

I amp has Ken Fischer channel switch mod with a few tweaks to it to switch from clean to overdrive using just one input. Amp sounds great except for this parasitic fizz problem. I even added a balance PI B+ pot between the 82k & 100k plate resistors to balance out the audio drive to the output tubes.

Can you try a chorus effect to see if your fizz sounds even more pronounced?

[Beck-Ola]

Certainly not normal for this amp (that swirl/fizz). Technoriff, does it do it on all the preamps or just certain ones? From your description it follows the decay of the note and doesn't occur when not playing?

@JC1: Wow, that's awesome that you're friends with Ken Bran. Maybe you could call him up and reignite the friendship and get some help, lol. I've read about he and Dudley Craven in many accounts of Marshall history. BTW, there is a JCM800 section on this site where you may be able to get some responses concerning that issue. I have a 2204 myself with only some minor mods but it doesn't exhibit any of that fizz sound either.

[guitarest]

I know that "fizz". I have it on my JCM800 2203 right now and it's been a bear trying to located the cause. Mine is a 1983 year model. It was sort of a trade between Ken Bran & I back in 1983 while I was stationed in England from 1980 to 83 with the USAF.

Really now I was at RAF Lakenheath from 82 - 88 and purchased my 2203 in Nottingham and brought it and a 1960A back on the train then loaded her up in the mini when I got back to the train station. Was a fun day for sure.

[JC1]

Really now I was at RAF Lakenheath from 82 - 88 and purchased my 2203 in Nottingham and brought it and a 1960A back on the train then loaded her up in the mini when I got back to the train station. Was a fun day for sure.

Speaking of trains, a friend of mine before I met Jim & Ken wanted a Marshall so we went to London & bought a head & 4x12 cabinet. We rolled it around the streets and took a train back and loaded it up in his car too! We were stationed at RAF Croughton outside Upper Heyford. I lived off base and was only about 20 miles or so from Milton Keynes. I was there from 80 -83.

Anyway, speaking of fizz, I think I located the problem. I'm at work this morning writing this (Gov't Job lol) but last night before bed I placed a 330p cap across the plate resistor V2b (plate to audio ground) and noticed the parasitic I was seeing on my oscope when moving the treble pot around was gone. Later today I'm going to listen to it and try a small cap to see if it eliminates it completely with minimum effect to my tone. I noticed on a later JCM800 series in 1984 that there was a cap, I think 100p from plate to cathode on V1b. Perhaps they were having a similiar issue with that amp?

Anyway, if the JMD501 is having a similiar problem, don't rule out that it won't need a similiar mod to fix it. It's not unheard of for any manufacturer to have problems pop up after design & production and have many revisions to their original design. I'll try to attach a schematic to the 1984 version of a JCM800 2204 to show what I mean. Someone if their infinite wisdom left out a 10k B+ dropping resistor with a 50uf to go with it for another node in this version and IMHO sounds like awwww! The pre voltages were way too high making the amp sound brittle. I modded this amp last month for a friend and added a 10k & 50uf cap and upped another 10k after the screen filter caps to 22k which lowered the preamp & PI voltages and warmed up the tone. Now it's built like an earlier 2204 with it's voltages.

So, again, if you can't rid your fizz, it might just be a parasitic problem.

Stay tuned, I'll be back later once I experiment around with some caps.

-- Thu Feb 17, 2011 8:19 am --

Certainly not normal for this amp (that swirl/fizz). Technoriff, does it do it on all the preamps or just certain ones? From your description it follows the decay of the note and doesn't occur when not playing?

@JC1: Wow, that's awesome that you're friends with Ken Bran. Maybe you could call him up and reignite the friendship and get some help, lol. I've read about he and Dudley Craven in many accounts of Marshall history. BTW, there is a JCM800 section on this site where you may be able to get some responses concerning that issue. I have a 2204 myself with only some minor mods but it doesn't exhibit any of that fizz sound either.

I tried to reach him awhile ago by emailing the Marshall web site to see if anyone knows his whereabouts or if he's still with us since he would be getting up there in age and smoked alot back then. If anyone knows anything please respond and let me know.