Marshall Mosfet Lead 100 reverb noise

[ChewyNasalPrize]

Just got one of these and I love it. A little dirty but otherwise seems to work beautifully.

The only thing so far that gives me some concern is a LOUD clanking from the reverb tank when the head is moved even just a bit while it is on.

None of my other amps seem to do this- I mean, you can hear the springs jiggling and all but this is LOUD- like firecracker loud.

The simple solution is to just not move it - or turn it down or off if I need to shift it around but I wonder if this is normal for this model or might there actually be something amiss?

Thanks.

[DrGonz78]

It can be normal on an older amp where the reverb tank springs are worn out and vibrate easily. The noise should not hurt the amp unless for some reason it is at super loud levels. Even then it is more just a nasty sound that you don't like to hear. One solution is to replace the reverb tank or just keep doing what you are doing. But first...

There could be an issue if one of the springs came off it's clip. There should be rubber o-rings that provide isolation from the cabinet to help reduce the vibrations. How does the reverb sound as you play loudly and then turn up the reverb pot? Is there any hum or vibration crashes while playing the amp? At least a close inspection of the tank is first on the list before any replacement.

[J M Fahey]

Visually inspect the tank.
For that to happen springs must be **almost** touching something and actually doing so on the slightest movement.

Maybe stretched reverb springs or suspension ones or what DrGonz suggests, any of them product of a long life traveling bumpy roads.
Maybe you can put the tank upside down or on its side or inside a bag.

[Roly]

All springlines can be made to crash if you are rough enough, and the longer they are the more likely they are to allow the springs to strike the side of the tank and produce a crash.

They are normally mounted on a sprung sub-assembly, but if something fouls the free movement of this sprung assembly they can become very sensitve to outside movement.  I remember a Fender Twin being brought to me with this problem, previously fine, and when I returned it I asked if he had been playing a Greek wedding.

"Er yeah, how did you know?"

"By the broken half dinner plate jammed under your reverb tank".   :lmao:

If the tank is okay then a thin layer of foam glued to the case in the middle of the spring span where they strike the case will at least reduce the impact noise; but a high sensitivity to spring crash is unusual.

This is a particularly long tank;


Inside you can see the floating sub-chassis on four springs;

...and this must be free to float.

But frankly, horizontal mounting of such a long tank in an amp head is simply asking for trouble (and sadly typical of how Marshall have allowed their standards to slip).

It appears that this tank is mounted in the top of the head cab so you don't have the option of spacing it off the case; the spring strike will be against the internal floating sub-chassis, and apart from some thin padding there isn't much you can do about it (apart from replacing it with a shorter tank, or perhaps as JM suggests re-mounting it inverted so the springs hang down into free space).

Please post all the numbers etc stamped and printed on the tank.

[ChewyNasalPrize]

Thanks for the quick replies-

There are no isolation rubber washers between the tank and wood of the cabinet. I pulled it apart today to give it a really good cleaning and inspection for strangeness but it all looks ok other than the reverb tank- which looks like it was applied as an afterthought. The screws were driven in at a 45 degree angle at the front and looks kind of crummy but it holds I guess. 

The tank is an Accutronics- It looks to be the same tank in my Lab L5 which does have this issue. It is in a bag though and not rigidly secured to the cabinet.

I took a pic of the springs with the tank propped on it's side and one of them appears to be a bit saggy- but I don't know if this is normal saggy or not. Maybe the tension could be adjusted on the springs without damaging them?

Nothing appears to be out of order otherwise and does not make any odd noise when I play- just not when I move it. It isn't a big deal but just seemed unusually noisy- I was more concerned it might be an indication of something more serious but it doesn't sound like it.

Thanks!

[ChewyNasalPrize]

LOL- Why are my pics so freaking big?

[Roly]

That spring sag doesn't look excessive to me, considering the length of the line, and no, I wouldn't go fooling with the springs trying to make them tighter.  I don't think you will be able to, it will change the sound of the line, and the taut wire suspensions are rather fragile anyway and won't like increased tension.

Now, from the mouth of the horse;

Accutronics
http://www.accutronicsreverb.com/

Your type number, from your pic;

4DB3C1D

Decodes as;

Type 4
D - 250 ohm input
B - 2250 ohm output
3 - long decay time (2.75 - 4 sec)
C - Input Insulated / Output Grounded
1 - no locking device
D - Vertical mounting plane, connectors down

So there you have it - this line is intended for vertical mounting (on edge), and if it's Marshall original fit they are very naughty indeed (and drop yet another notch in my estimation, bad Marshall <smack>).

So possible solutions are a) to mount it on the inside of the front face with the connectors down, if it will fit, or b) on the inside of the back, ditto.  Alternatively, if there's room, c) on a couple of L-brackets on the chassis itself, but it needs to be both soft mounted on rubber grommets o.n.o., and must be insulated from contact with the chassis or you will get an earth loop; also it should have the top facing the power transformer to provide screening for the coils from stray magnetic hum field - just try propping it in that position before you start drilling holes to confirm there is no hum pickup.  Personally I'd try (c) first.

In either case this should give the springs greater slosh freedom before they strike anything, and that is what makes a line crash.  Notice how the springs are somewhat offset away from the socket side allowing sag space on that side, hence "sockets down" mounting.

HTH

[ChewyNasalPrize]

Thanks for the detailed replies!

Yes- It appears to be the original tank- I see no evidence to suspect otherwise. The "clanking" noise, I'm pretty sure, is from when the saggier spring strikes the sub-chassis when moved. If it hanging horizontally on it's edge, I don't think this would be an issue.

It appears there is enough room behind the faceplate to mount it there. It won't mound directly to the face place because of a bracing block at the center top. So it would have to be set back off about an inch or so and I don't know if that will be too close to the transformer yet or not.

Mounting it to the back seems less desirable because the back panel is just a thin metal swiss-cheese type of thing.

I think the best option is the one you suggested to try first Roly- assuming the tank will not conflict with the transformer- which is to mount a couple of L brackets to the top of the cab and secure the tank to that. Before I do this, I think I'd like to get a bag for it since it would be so much more open to dust and bugs, etc.

For now, I've remounted it in the same location but with the addition of rubber washers for a bit more added insulation but I know this won't have any meaningful affect on the clanking.

I will try living with it for a while as is and if it still bothers me, I know what to try first.

As an aside regarding build quality, as I was inspecting the innards, I noticed one of the jacks was missing a plastic washer on the inside part which made the jack's shaft stick out a little further than the others. I thought that was a little strange. But as I was reassembling this morning, I flipped the amp over to put it back in the cab and a little black washer fell on the floor!  :lmao:

I guess whoever last worked on it (maybe not even Marshall's original assembly) the washer got "lost" and they were like "screw it, I'm just putting it back on without it."

Thanks again everybody! 

[Roly]

 :dbtu:




{naughty, naughty, Marshall <smack>}

[phatt]

Not sure if this is a head or a combo?
But if you invert the tank and it faces the electronics then it will hum to billyo.
These things pickup mains hum like crazy. :duh

If it's a combo then mount it in the bottom of speaker box springs facing down away from electronics.
If it must be near electronics Then keep the pickup end of the tank as far away as you can from transformers and like. Even with the cover between a transformer and pickup end they will still pickup hum.

Move it around while it's on and test the best place for least hum.
Phil.

[ChewyNasalPrize]

It's the head version.

Jammed on it yesterday and it sounded fine- I just didn't move it around.

Even when it's off and I'm just lugging it- you can hear that spring clanking around. I'll probably try to remount it eventually.