Shield inside of head cabinet?

[Analog Assassin]

I just built a Ceriatone plexi50 with a PPIMV. Thing sounds amazing. But when I turn up the presence and treble, I get this high whistling noise. I've got the amp sitting on top of my cab, but not in the head cab. It's sitting tubes and transformers up.

So I flip the thing upside down careful not to mess up the tubes so I can tap around inside there with a wooden dowel to discern the source of the noise. It is dead quiet now. Flip it back to tubes-up, and it's whistling.
I stick the chassis in the head cabinet. Put it on top the speaker cab. It whistles. I move the head to the floor. Whistling stops. The speakers or their wiring is interfering with the amp. I'm new to Marshalls, which have the chassis "upside down" compared to Fenders.

So I'm thinking, I should shield the bottom of the head cabinet. Is this a common practice? How should I do it? I'm thinking some copper foil would work, as long as it makes contact with the grounded chassis?

[azzaaa]

I can't help but think that this is a paristitic osilation. Maybe a grid wire is a fraction to long or in the wrong position or any number of paristitic remidies. Also are the transformers positioned in relation to each other corectly.

[dukeamps]

Hi, Just glue kitchen foil on the bottom of the head case and it will be ok.

Dukeamps

[Sonny ReVerb]

I use aluminum flashing from the hardware store. Stick it on with some 3M Super 77 spray adhesive.

Some people use screen material, aluminum foil, or a full blown chassis lid, as well.

[srubelva59]

I picked up some aluminum foil tape from home depot and covered the inside bottom of the head cabinet, works great as RF shielding.
http://www.amazon.com/JVCC-AF20-Aluminu ... y_hi_img_b

[JamesHealey]

Shielding the bottom of the head cab in plexis is vital.. i've never had a plexi not do this.. you can shield the second stage grid wire and this will help also.. i'm a big plexi fan love the sound of a good super lead!

[Phil_S]

Hi, Just clue kitchen foil on the bottom of the head case and it will be ok.

Dukeamps

Well, I'd glue it, not clue it . I do this for every amp I build. You are picking up a signal from the speaker and the foil will give you the needed shield. All the materials mentioned work. This is the cheapest and I think easiest stuff to work with. You can cut it with a scissor or tear it against a straight edge. Lay down a thin layer of glue, rub on the foil and you're done.

[dukeamps]

Hi, Just clue kitchen foil on the bottom of the head case and it will be ok.

Dukeamps

Well, I'd glue it, not clue it . I do this for every amp I build. You are picking up a signal from the speaker and the foil will give you the needed shield. All the materials mentioned work. This is the cheapest and I think easiest stuff to work with. You can cut it with a scissor or tear it against a straight edge. Lay down a thin layer of glue, rub on the foil and you're done.

Sorry I meant glue. But I think he would have got the general idea

Dukeamps

[PCollen]

Hi, Just glue kitchen foil on the bottom of the head case and it will be ok.

Dukeamps

As often as I take my chassis out to tinker with the amp, I'd be tearing and having to replace that foil 3-4 times per year.

[PCollen]

I use aluminum flashing from the hardware store. Stick it on with some 3M Super 77 spray adhesive.

Some people use screen material, aluminum foil, or a full blown chassis lid, as well.

10ft of aluminum (aluminium, for you Brit's) flashing is cheap ($10, and often cheaper if you find a house being roofed) and enough to do 5-6 amps.

[ChrisM]

That Al tape is also good for shielding reverb tanks. Cut some stiff carboard to fit over the underside of the tank. Cover the cardboard that faces inside the tank in Al tape. Then use a bit of masking tape to keep the cardboard "shield" attached to the tank. Through it all in a reverb bag and use some good quality RCA cables and your golden.

Sorry for the OT post.

[Structo]

I use the aluminum sheet metal.

What I did on my D'lite is cut it to match the foot print of the chassis.
Then cut notches where the mounting screws go into the chassis.
I also took my Dremel tool and ground the powder coating off around the mounting holes so the shielding will make good metal to metal contact there.

Then I position the shield over the chassis and hold it in place by using four pieces of blue painters tape, just to hold it in place until it is screwed tight.
When the chassis is tightened down the shield is pinched between the chassis and head cabinet making good contact to the chassis, shielding the works from interference.

[rp]

Home depot has various sheets of sheet metal, steel and AL, small and big squares different gauges. Usually in a far back corner somewhere very well hidden.

[Structo]

Yep, if I recall the aluminum flashing I bought was in the correct width, I just had to cut the length with tin snips.

And really, if you have some heavy duty scissors, they will cut the light flashing fine as well.

[M Fowler]

I quit using the bottom plate and shielding.

Once the chassis is inside the cabinet I don't hear any shielding problems.

EDIT: Am I alone in the non shielding?