My New JMD501 Has A Strange Fizz

[Beck-Ola]

I don't believe this swirl/fizz noise is something endemic to the JMD:50. It must be either something in that particular specimen or maybe a noise being picked up from the locality. I have been trying to simulate the noise and there is nothing like that coming from my JMD:50 head. I went through the preamps and cranked the Gain, Volume, and MV (man, it can get LOUD :high ) and with the noise gate both on and off, using both single coils and humbucking pick-ups, and it is very quiet with no odd noise signatures. Tried chords, single notes and sustained vibratos and it sounds completely normal. Even the hum associated with single coils is pretty darn low on mine. I have mine plugged into a 2x12 loaded with G12Cs.

The JMD line doesn't have any preamp tubes so there are no plate resistors to need a snubbing cap. The preamps are modeled and fed to a tube output section on these amps. The only triode in the JMD is the Phase Inverter.

"JC1", like you say, in the JCM800 I've heard of guys adding snubbers to get rid of excessive fizziness.

[SteveD]

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.

Welcome to the forum JC1.

Ken is still alive and lives just around the corner from us. Alas, Dudley passed away some time ago.

[technoriff]

JC1 - Great to know that you've heard of this - and just as I've been getting "used" to this fizz (if we're talking about the same sound). If so, and can be easily addressed with a cap - even better. I'll need to set aside some time for checking out the sound with the chorus engaged.

Beck-ola - Yes, this sound only seems to occur when a note is played, not during a silent passage. Seems to occur for the Overdrive and Lead preamps, less so for the Crunch ones, and is not discernable for the Cleans. The sound occurs when the note is first played, and seems to maintain the same level while the note is ringing (with Noise Gate disabled) - it becomes noticeable when the note decays to a lower level.

It occurs regardless of the Master Volume setting; often for quiet playing, I'l have it at 1 and the Channel Volume near maxed. Once I noticed it, it's difficult to ignore. Also, it appears to be easier to hear with a human ear in front of the speaker; it doesn't always record very well for whatever reason. So perhaps this is not normal aggressive tube break-up?

[Beck-Ola]

JC1 - Great to know that you've heard of this - and just as I've been getting "used" to this fizz (if we're talking about the same sound). If so, and can be easily addressed with a cap - even better.

I hate to be redundant but it could hardly be the same thing or fixed the same way because he is referring to the preamp tubes in the JCM800 while the JMD doesn't have any preamp tubes or plate resistors to bypass with a cap. Howbeit most fizzy sounds do originate somewhere in a preamp stages and not the power amp. Not sure in this case. Maybe you could try a different Phase Inverter tube (the single ECC83 there next to the EL34s). Maybe that is the culprit.

It occurs regardless of the Master Volume setting; often for quiet playing, I'l have it at 1 and the Channel Volume near maxed.

So perhaps this is not normal aggressive tube break-up?

It would not be normal aggressive tube break-up until the Master volume is turned up quite a ways (i.e., past 5). If you have the Gain turned way up of course you're going to get a simulated "preamp tube break-up" but it's a modeled characteristic. Too bad we can't hear exactly what you are describing. I don't know if you're used to EL34s, which might sound fizzier in a way compared to other types but it wouldn't explain the swirl or the oscillating.

[JC1]

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.

Welcome to the forum JC1.

Ken is still alive and lives just around the corner from us. Alas, Dudley passed away some time ago.

Can you get a message to Ken for me? I'm sure he has a computer & an email address and I can give you mine and maybe you can get his for me? I'm not sure of the rules here so i won't post my email address on here unless it's ok. Just tell Ken it's JC the American serviceman from back in the earlier 1980's with the Black Tower wine bottles, cartons of Marlboro cigarettes for him & case of Michelob beer for Jim. Ask him if he still has that mini stereo system. I know he's also a ham radio operator and I'm wondering if he' still doing that or not.

I'm sorry to hear about Dudley passing away, I hope everyone else is doing fine, Mike Hill, Pete, Steve, etc.

-- Thu Feb 17, 2011 6:09 pm --

JC1 - Great to know that you've heard of this - and just as I've been getting "used" to this fizz (if we're talking about the same sound). If so, and can be easily addressed with a cap - even better. I'll need to set aside some time for checking out the sound with the chorus engaged.

Beck-ola - Yes, this sound only seems to occur when a note is played, not during a silent passage. Seems to occur for the Overdrive and Lead preamps, less so for the Crunch ones, and is not discernable for the Cleans. The sound occurs when the note is first played, and seems to maintain the same level while the note is ringing (with Noise Gate disabled) - it becomes noticeable when the note decays to a lower level.

It occurs regardless of the Master Volume setting; often for quiet playing, I'l have it at 1 and the Channel Volume near maxed. Once I noticed it, it's difficult to ignore. Also, it appears to be easier to hear with a human ear in front of the speaker; it doesn't always record very well for whatever reason. So perhaps this is not normal aggressive tube break-up?

That was a great audio description of what I'm hearing too at a lower volume level without driving the EL34's much. It has to be preamp induced background noise that rides a note and can be heard better as the note decays, like you first hear the note, then immediately the "fizz" right under it. By the way, the 330p snub cap I placed across the 100k plate resistor of V1a (not (V1b like i said before, my mistake), it doesn't get rid of the 'fizz" but it did clean up my 1000hz clean sign wave signal coming off the treble pot. It looked like a slight parasitic riding the sign wave without the snub cap.

Now my preamps are all DC filamnet biased & the rest of the amp is AC filament (PI & output tubes). Just throwing that one out there if anyone knows of DC filaments causing a problem (obviously not with yours).

[technoriff]

If I can manage to record a definitive example of the sound, I'll post it.

Some more thoughts/observations - mostly conjecture:

o I realize that the pre-amp tubes are simulated, so one thought I had was that this fizz might be very accurately duplicated as a real model might produce. Maybe the amp's digital nature produces a large contrast to the relatively silent noise floor when the signal has faded.

o I will definitely try a new phase inverter tube as perhaps that would help. Along that line of thought, perhaps there's some component variation in my particular amp that is creating a circuit asymmetry? Maybe taking a scope to the signal might reveal more.

o Perhaps my description of "swirling" is misleading. What I mean is that the fizz interacts with the decaying guitar signal to create an intermodulation effect - sort of like an oscillating buzz; not so loud as to be in one's face, but hard to miss once noticed.

o Turning on the Chorus does seem to make the fizz more pronounced.

[JC1]

If I can manage to record a definitive example of the sound, I'll post it.

Some more thoughts/observations - mostly conjecture:

o I realize that the pre-amp tubes are simulated, so one thought I had was that this fizz might be very accurately duplicated as a real model might produce. Maybe the amp's digital nature produces a large contrast to the relatively silent noise floor when the signal has faded.

o I will definitely try a new phase inverter tube as perhaps that would help. Along that line of thought, perhaps there's some component variation in my particular amp that is creating a circuit asymmetry? Maybe taking a scope to the signal might reveal more.

o Perhaps my description of "swirling" is misleading. What I mean is that the fizz interacts with the decaying guitar signal to create an intermodulation effect - sort of like an oscillating buzz; not so loud as to be in one's face, but hard to miss once noticed.

o Turning on the Chorus does seem to make the fizz more pronounced.

Does your amp have an effects loop? Where are you running your chous, front of the amp or FX Loop? If you have a FX Loop, do you here the "fizz" on either hook up?

BTW, does anyone know how to add an attachment or pics here? I'd like to attach my modded schematic once I finish drawing it and some pics. Thanks.

[surfnorthwest]

When you click on "post reply" or anytime you are entering a new thread just below the box you type in there is another box where you can browse for your file and upload it to the site. This site accepts many different file types.

[technoriff]

JC1 - I'm just using the 501's built-in Chorus; decided not to complicate things by introducing another factor into the mix. Otherwise, just straight guitar into the input.

[surfnorthwest]

Just a thought here as I don't have this issue with my JMD. The speaker will sound a bit fizzy until its broke in, pretty common with any new speaker. Maybe you can connect the amp to a external cab and see if it still does it.

[JC1]

JC1 - I'm just using the 501's built-in Chorus; decided not to complicate things by introducing another factor into the mix. Otherwise, just straight guitar into the input.

OK then, one more question...does your amp use Power Tubes for the output? If so, what type are you using? 50 or 100W? I recall I think you said something about a phase inverter tube so I'll assume your amp is solid state preamp & Tube output type hybrid.

[Beck-Ola]

OK then, one more question...does your amp use Power Tubes for the output? If so, what type are you using? 50 or 100W? I recall I think you said something about a phase inverter tube so I'll assume your amp is solid state preamp & Tube output type hybrid.

The JMD has a digitally modeled preamp and a standard Marshall tube output section and uses EL34s. Technoriff has the 50 watt combo IIRC.

-- Fri Feb 18, 2011 3:39 pm --

o I realize that the pre-amp tubes are simulated, so one thought I had was that this fizz might be very accurately duplicated as a real model might produce. Maybe the amp's digital nature produces a large contrast to the relatively silent noise floor when the signal has faded.

o I will definitely try a new phase inverter tube as perhaps that would help. Along that line of thought, perhaps there's some component variation in my particular amp that is creating a circuit asymmetry? Maybe taking a scope to the signal might reveal more.

I thought about that first idea too but it's kind of doubtful because the design team would have noticed any extraneous noise or artifacts and corrected it, and also every JMD would be exhibiting the same noise. And maybe they are but you're the only one that can hear it, lol.

Your second idea might have some merit. Every individual amp is bound to have some slight variation.

[JC1]

OK then, one more question...does your amp use Power Tubes for the output? If so, what type are you using? 50 or 100W? I recall I think you said something about a phase inverter tube so I'll assume your amp is solid state preamp & Tube output type hybrid.

The JMD has a digitally modeled preamp and a standard Marshall tube output section and uses EL34s. Technoriff has the 50 watt combo IIRC.

-- Fri Feb 18, 2011 3:39 pm --

o I realize that the pre-amp tubes are simulated, so one thought I had was that this fizz might be very accurately duplicated as a real model might produce. Maybe the amp's digital nature produces a large contrast to the relatively silent noise floor when the signal has faded.

o I will definitely try a new phase inverter tube as perhaps that would help. Along that line of thought, perhaps there's some component variation in my particular amp that is creating a circuit asymmetry? Maybe taking a scope to the signal might reveal more.

I thought about that first idea too but it's kind of doubtful because the design team would have noticed any extraneous noise or artifacts and corrected it, and also every JMD would be exhibiting the same noise. And maybe they are but you're the only one that can hear it, lol.

Your second idea might have some merit. Every individual amp is bound to have some slight variation.

Correct, his amp might have a slight variation that wasn't noticed but now he hears it.

If you have a FX Loop, you could bypass your output stage and send your preamp tone into another source (PA mixing board, another amp) just to isolate the preamp from the output. You would go "Send Out" to another amp's "Return In" to see if the fizz doesn't appear on another amps output stage.

I'd take a look at the PI section and power output tubes. Since you have a digital preamp & I have tube (s) preamp, and both of ours have the same problem, then let's rule out the preamp section as the source.

I don't think it's your PI tube because I already tried that. It may be a something else that's getting onto the PI or output tubes audio. The power tubes should have 5.6k grid stopper resistors, the PI won't likely have one. You can start by adding one to the grid of the PI. I'd try something very high first, like 470k just to see if it works. If so, the drop it in half until you can go as low as possible and the parasitic is gone. If a 470k doesn't work... :cuss

You could place a .1 or .22uf cap across the 100uf filter cap for the PI section and see if the fizz goes away.

You could try a snub cap across both the PI plate resistors, 82k & 100k with say 120p. If that doesn't work, add another in parallel like 500p, just to see if it will rid the parasitic. If those don't change anything & the fizz is still there... :cuss

It's possible that the 5.6k grid stoppers aren't big enough. Note that an output tube like a EL34, just as an example, has a maximum grid resistance it can operate. That resistance is a combination of the 220k & 5.6k. One way to do it would be to place a 470k across the 220k's (now there 150k) and add the left over resistance (220 - 150 = 70k) to the 5.6k or let's make it 82k just for chits and giggles. Don't forget to rebias the amp!

The whole point is to see if the problem can be cured without destroying the tone.

If you can do all of that, the congradulations, you've now become an amp tech!!

Listen, I know your amp is probably under warranty and if not, you might not have the expertise and tools to do all of this. My amp is on the bench & I'll be doing those things above soon. Just knowing that you're experiencing the same thing with an entirely different amp and I'm fairly sure now the problem is in or after the PI section, will help me troubleshoot and I'll be able to let you know if I can locate the problem.

Fair enough? :beatdeadhorse

-- Fri Feb 18, 2011 10:38 pm --

Quick and easy try to rule in/out parasitic oscillation, connect a larger capacitor across the phase inverter then is there (should be 47p right now, add say 120p to it). If 120p & 47p in parallel (167p) don't work..

Go higher, maybe a few hundred pf's like 330p. The amp won't sound great if you go too high...you'll lose high end...but if this fixes the problem you've probably got a parasitic oscillation.

This should be the first step since it's the easiest next to a .1 or .22uf 600v minimum across the 100uf PI filter cap to ground.

[Vavoom]

New to this forum, so this is my first post. Firstly the fizz. I just recently bought a VM 100 Watt head and am on the fence on keeping it or not, but that is for a different thread. However, about a month before I bought the VM, I got the JMD 100 watt head. Here is my take (and it has to do with the "Fizz".)

-Very versatile and has IMO a great Marshall crunch
-Programmability is limited but still stellar for what you pay
-The footswitch is very useable for selecting your tone
approximation with the flick of a button
Overall I rate it highly but I did NOT keep it because of the fizzy artifact I would get on mostly the high gain channels and mostly with sustain harmonic content. Rarely single note passages did not give this fizz. Heck I think I'm going to call it digital artifact, because that's what it sounds like to me.

No such thing as a free lunch and I think this is the compromise of having the progammable, built in effects, swiss army knife of tones, all in one box. You have the software working with the hardware and it is NOT totally seamless and organic. Hence fizz. But will the patrons notice at a pub or hot and heavy gig? No way.....

So I would even entertain re-purchasing the JMD, but I am a pedal freak guy and did not care for the limitations on the effects provided by the "guv". Not bad, but to bucket brigade for me on the chorus, the delay was very limiting and flanger was ..........was that a flanger???

You don't have to use those effects though...right? Anyways I returned mine but miss it....cuz you could get rock tones....Marshall rock tones in my opinion quicker than my JVM or my new VM.

So there you go. My take on the fizz factor.

Now to go fiddle with my VM..............a personal review on that soon.

See ya!

Bill
Newbury Park, CA

[surfnorthwest]

Welcome Vavoom, what speakers were you using with the JMD head? I have a few different cabinets and find that the G12H-30s sound great with the 100 watt head, I don't get this fizzyness some of you are getting. The eq is very responsive on the JMD and would just tweak there to remove it.