MA100C issues

[Nebulozny]

Hi everyone, my first post on the forum.
I have MA 100 watt combo and I love it, sounds incredible on gain channel pushed with Boss SD-1 overdrive + SG humbuckers and I am in heaven.
But, I do have some issues that are annoying, it has serious tube rattle. I have tried 3 amps in shop every had rattle,some more some less, even smaller 50C combo. Depends on what you play and volume, mostly on lower notes it just rattles as cabinet full of glasses,very annoying when playing at home on clean channel. Guy at shop took of tubes,rattle stopped, popped new ones, same thing, rattle comes in.
And another thing, as soon as I engage standby button ON, there is a weird hum from the amp, sounds almost like something is cookin inside, hard to describe,if you ever heard a gas flame oven thats the sound,just cooking there. Its there all the time, regardless of volume settings and it is annoying at low volumes really, tried again few amps all have same thing, and I am not sure is this normal for all MA amps or am I really a guy with bad luck? Guys at the shop are confused too, and cooking sound is getting louder with time. I thought amp should be dead silent with slight hum that increases with volume, but this is different.
I really like MA series ,sound,the looks and feel,but this small things are really annoying.

[SteveD]

Welcome to the forum Nebulozny.

Regarding the 'cooking sound', are you talking about just the boosted OD channel or the clean channel as well?

It is quite normal to have gain noise on a high gain channel although the MA is not particularly bad in this respect compared to some amps.

The valve rattle should be minimal unless you are cranking the Resonance up high.

[Nebulozny]

Hi Steve, as I see you work at Marshall?
"Cooking" sound I described in my post is there all the time, no mather what channel is on, while on standby amp is dead silent as it should be, turn on standby and cooking comes, it's quite loud now. I know how normal amp high gain noise should sound, this is not that sound, cooking is same on clean, gain or boosted mode, and not related to volume, even on zero it's still "cooking".Could this be bad tubes, maybe changing tubes would help?
And as I said 3 amps in the shop had same sound and tube rattle which is there regardless of resonance control position. If you knock on top of amp, you can hear tubes rattling, holding a finger on the tube stops rattle, so I think it's something due to vibrations from chasis being transfered to tubes. And I noticed some amps have more rattle some less.
Also switching channels produce loud pop, but I heard other amps do this too.
Since I can't find much reviews of this amp, I just wanna know if this is maybe some malfunctions related to few amps in one series or is it whole production line.

[SteveD]

It sounds like the jfet that mutes the input of the phase inverter when channel switching has gone. The switching noise should be quite low and there should be virtually no noise on the clean channel. If that fet goes then the amp will be noisy.

As far as I'm aware this is not a common issue.

[Nebulozny]

Thanks for reply Steve.
This malfunction is quite obvious and noticeable so I think it couldn't pass quality control in factory, or I am mistaken. Maybe I could record the sound and post it here so you can hear it, last night i was playing and it is cooking loud now, switching is loud too so I guess it's back to shop for this amp too. I will tell them about jfet thing, but they will contact Marshall factory anyways, so I hope this problem gets solved, cause I really like the amp.

[kingbee]

Just so everyone knows, I've been using the MA50 Head and MA412 cabinet since January. Bought it because my son likes using my Dsl50 and 1960 Vintage cabinet in one of two bands I play in (classic rock band and a southern rock band). I've been using it in both bands since January on every weekend. I use every ounce of it, it works flawlessly and I love it!!!!!

[Nebulozny]

Maybe this problem I have is related to combos, local Marshall dealer doesn't have MA heads so I tried only 50w and 100w combos, they are checking 10 more they have in stock, I am expecting their call today to solve this issues. This is only for channel noise ofcourse, heads don't have tube rattle anyway.
I can only imagine how 412 sounds, 212 combo is just blowing out crunch sounds, love it, and when i switch on overdrive pedal in front, now that is a killer.

[okgb]

I got excited thinking i could afford a Marshall head, but
when i talked to someone at the local dealer , they started
shaking their head right away mentioning noise issues, and
as soon as they mentioned china , i knew all bets were off
it would seem you have to pay extra if you want Quality control
from them [ I've been burnt enough with Chinese crap that
doesn't turn out to be worth the money no matter how cheap ]

where do the transformers come from ? [ i don't think they've
copied enough of them to get good yet , still trying to save pennies
on wire ]

Up close i thought they looked cheap as well , but no matter
for those who got good sounding ones , betterthan an MG eh ?

[kingbee]

For one thing, not made in china

[okgb]

Well that's positive , still looks cheap
but it's one more Marshall than i got
sure it's not " assembled " somewhere else ?

[kingbee]

I think Vietnam. Maybe it looks that way to you because it is particle board instead of plywood. Build quality is still great. As compared to my JCM 2000. Sounds awesome too! It's lighter also. Makes a difference when you have to move it around all the time. :party

[Nebulozny]

I like how they look and sound, only problems are those I mentioned above, and if they find one without those issues I'll be glad to use it. If not, I have a backup plan, DSL 401 is slightly expencive and I'll go for that.
For me, the amp that makes you play and feel great while doing it is the right one, no mather where it's made or what name is stamped on the front. :rocker

-- Wed May 12, 2010 8:06 am --

Update about my MA100C adventures, got the new one last night, this one is noiseless, dead silent on clean channel, tube rattle is there and channel switch pop which is present on all MA amps I tried (combos), but with this things I can live with, noise on clean was frustrating.

[pacAir]

I recently bought an MA100C that was damaged in shipment. After fixing 2 or 3 minor problems (which included a blown B+ fuse and bad pair of output tubes) I got the amp to basically function. However, I did find two problems with this particular amp that may or may not be attributable to some or all of its brethren:

1. On the clean channel, it gets a visible crossover hump (not a sharp notch) relatively early with a skewed sine-wave (1kHz) output as you approach clipping (not a very hi-fi clean).

2. I can only bias the output tubes to approximately 31mA of plate current each (the controls won't allow both sets to be adjusted any higher and they are extremely interactive).

3. The amp will only develop about 55-60 watts RMS into 8-ohms before clipping despite the fact that the B+ supply is putting out well in excess of 450Vdc (480Vdc in Standby, unloaded).

It is as if a mistake was made in a resistor value in a Bill of Materials or a typo on the schematic diagram (which I do not have).

I am going to compare the configuration of this output stage to some other Marshalls I have worked on recently (JCM800, Haze 40, JVM410C) to see if I can discover what the problem is. I don't expect Marshall to play fast and loose with specifications so I am assuming there are some real problems at work here.

I have seen a number of other people complain that an MA amp couldn't compete with a Peavey Classic 30 on its clean channel output and I am wondering if I am looking at the reasons for this. If the performance of this particular amp is any indication then there is indeed something amiss. It is entirely possible I haven't discovered a problem that only this amp has but the symptoms match other people's reports of weak output and a muddy-sounding Clean channel.

All other functions work including the spring-reverb, which became detached from its rubber shock mounts and was stuck to a speaker magnet (which probably save the reverb tank and the tubes from further abuse).

All front panel controls do their job, all switching and indicators work. The clean-channel signal coming out of the Effects Loop SEND jack look pristine so the problem doesn't appear to be in the preamp stages.

I have verified all the current output tubes on a Hickock tube tester and tried swapping the P.I. tube but no noticeable problems were found and no change in performance was seen.

Barring anyone having an idea (or a copy of the schematic) I will muddle through my evaluation and see if I can discover the problem by careful inspection & testing (or by osmosis) and will report my findings here.


Steve

[Beck-Ola]

Very interesting post. Especially the inability to bias them any hotter than that and the lack of clean output. Very interested to see what comes out of this once you've investigated further.

[pacAir]

Well I had some time to dive back into the MA100C and didn't find any amplifier component problems. What I found instead were output tube problems!

I found that one of the Svetlana EL34 tubes had insufficiently soldered pins! A pair of tubes would intermittently lose their heater connection. I fixed that problem but the amp's original signal and power problems remained.

I removed the Svets and put back one of the original Marshall-branded tubes and added a used Marshall-branded tube from a JVM410C I worked on a few weeks ago. Now that all 4 tubes were more matched (same manufacturer, anyway) they biased properly, the skewed waveform disappeared and the amp put out 85Wrms. The crossover "hump" was still there and it wasn't putting out 100Wrms but the amp sure looked a lot happier. A low-level hum that was independent of gain or level settings could be a ground loop or insufficient filtering but if playing at live levels or more than 3 feet from the amp it isn't noticeable.

I hooked up my Strat and found that the clean channel actually sounded quite good! It was very dynamic and multi-dimensional sounding. The gain channel sounded a bit buzzy (probably the cheap preamp tubes) but there was definitely some Marshall crunch in there.

With some better preamp tubes, a matched quad of output tubes and a Les Paul the amp might sound great! I think that the amp can be really bright with a Strat so the extra "darkness" and power of a good Les Paul might be a better match. It will be some time before I can get around to messing with it anymore but I think it definitely has potential! At least it is now fully functional and usable.

Steve