4x12 cabinet interference w/Express build?

[Tone Daddy]

I just purchased a very nice complete Express build and I thought it initially had noise/squeal problems.

To make a long troubleshooting story very short, I moved the head off of my 4x12 cab and all the noise/squeal issues disappeared. <whew>

To get my impression of the build out of the way, let me say this…. OMG, what a monster this amplifier is!!!

Okay, But what I first suspected was a vibration/coupling issue, is not. I’m running this head thru a well broken in ‘90s 4x12 Greenback RI cab. It’s a great cab and has worked well with my NMV 50 watt Marshalls for years.

But w/this TW, whenever I get the head within 2” of the top of the Marshall cab, the squeal starts up again. It starts squealing w/o me placing the head on the cab, even just hovering w/2”of air between the head and the cab, it starts ringing like a microphonic tube.

To solve the problem, I simply relocate the head away from the cabinet. On the floor or a foot away on another cab solves the problem.

For the record, here’s a description of what I WAS experiencing:
I play my amps pretty loud and un-attenuated most of the time. This one seems to have a squealing issue anytime the volume is beyond 12:00 and any presence is dialed in. This is with or w/o a guitar plugged into the input but doesn’t matter if the volume on the guitar is open or fully closed. Squeal. This is with 3 of my humbucker equipped guitars. It even squeals when nothing is plugged into the input. It’s not a loud, feedback type squeal but sounds more like a microphonic tube ring to it. I swapped tubes and it’s not a bad pre tube. Also, anytime the bright switch is engaged in the “up” position, the amp has a squeal and tone is distorted and not in a pleasurable, amp overdrive way. It sounds more like static, especially when I’m playing the guitar cleanly with the guitar volume backed down. The bright switch seems to be pretty much unusable.

So I’m no longer worried this is an amp issue but now an environment issue which I can easily solve.

Has anyone else experienced anything else like this? Is this an isolated issue with this particular Marshall AX cab?

[Rimy]

hi
welcome to the ampgarage.

mine did exactly the same.
this happens when the bottom plate / shielding of the amp chassis is missing.
i just put double layer of aluminum foil with tolex glue on the bottom side of my head cab and the noise/squeel was gone.

ralph

[Ron Worley]

A lot of folks have similar problems and don't have the head on the cab.... but it's typically due to mechanical issues.

As rimy noted, if you don't have a cover on the chassis, install one... it eliminates all kinds of noise. The foil thing works, but it tears easily when installing the chassis, so I'd op for the cover route.... see the BOM for the part number from Mouser.

[Structo]

Is it from the magnets of the speakers?

He didn't have it sitting on the cab so that pretty much rules out mechanical coupling.
Strange.

[Rimy]

when i didn't had the shielding installed i turned the head 90 deg. to the speakers and the noise was gone.
could be the magnet of the speakers or in which direction the ot is turned?
just a guess...

[Tone Daddy]

Thanks for the suggestions. I've got the shielding plate from Mouser in my cart. I've got a new flamed maple head box coming later this week and I'll try shielding the bottom of the head box when I switch boxes.

I'll post some pics once I get it in the new head box because I'm hoping someone can do the rear dyno lables for me...

[M Fowler]

Tone Daddy,

Walmart sells the old style labelers.

If you don't mind my asking who is building that flamed maple head cab?

Mark

[Tone Daddy]

Thanks for the info on the label maker. I'd rather not purchase one for this single use I have. If anyone can make me some labels, I'd appreciate it...

The box was built for another clone a while ago. The owner was gracious enough to ask his friend to craft for me another non master volume panel from the same tree, that's what I am waiting for. Here's a pic of the cab:
[img681]http://i199.photobucket.com/albums/aa14 ... G_2591.jpg[/img]