Shield inside of head cabinet?

[rp]

I've never found the need for it, I put it in because everyone else does. Figure it makes my amps look more professional less kitchen table. Never built a combo though, just heads, and simple circuits, not high gain. I like going home on the subway with a big floppy piece of sheet metal.

[Richie]

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?

I think you are.. Its good to have the shield plate in the cab. seen some amps even use screen.
And yes you can have those problems sitting the amp on a speaker cab,and not having the shield plate,can cause those noises.

I've seen some people have amps in a road case, and leave the amp in the bottom part of the case,and the metal of the road case acts as a shield. Sometimes its even better if the amp has a cover plate. It can help odd noises.
Some amps may sound like they have a ground problem, and a cover plate takes that noise away, Ampeg V2 and V4s have those..

[M Fowler]

Well Crap Richie it was going so good not having to cut all that sheet metal.

I have not used aluminum tape yet, I wasn't sure if the last discussion we all had on certain shielding metals got solved.

[solderstain]

... seen some amps even use screen...

Vintage Fenders for example...

[Structo]

The problem with the metal tape is the adhesive is not conductive, so if you have several rows of tape, they are not connected electrically unless the chassis bridges all of the rows.

You can find light gauge aluminum at a home improvement store that you can cut with scissors.

[solderstain]

The problem with the metal tape is the adhesive is not conductive...

Keep looking. There are MANY metal tape sizes and brands that have fully-conductive adhesive. For example, the shielding tape that StewMac sells for shielding guitar control cavities is VERY sticky, AND conducts VERY well. Love it. I've had large rolls of aluminum tape with conductive adhesive as well. It's out there!

However, that's a lot of work to put into an entire cabinet floor, etc. I'm a fan of the 'scrap of aluminum flashing from a construction site for zero dollars' school.

[Phil_S]

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?

If you are building a combo, where the open side faces away from the speaker, generally speaking, it shouldn't be a problem. If you have a head that sits with the open side up, on top of the speaker, and there is no florescent or RF bombarding it, that should be OK, too. If you build a head with the open side on the bottom and want to put it on top of the speaker, you probably need shielding because the working speaker puts out some kind of signal that affects the amp.

[paolojm]

I think people have missed an important factor here.
The most likely reason that this amp squeals in one positions and not another is that cables are getting moved slightly within the amp.
Any amplifier should not require shielding to prevent oscillations.
I would recommend using a chopstick to move cables, identify the offending one and take steps to ensure that it cannot move back to the oscillating position.
Just my 2c

[Colossal]

I think people have missed an important factor here.

I just read this thread and agree that it is probably not shielding related. I have had this very issue myself and have observed presence whistling in real Marshalls. Check the physical location of the feedback wire coming from your speaker output back to the NFB resistor and presence pot. I also found the region near the two Bright and Dark channel (470k) mixing resistors to be quite microphonic. On a clone amp where I had the whistling oscillation at high volume and presence up past 8, I also had a VVR and the cap to keep DC off V2 was very microphonic. This cap was connected directly to the 470k/470k mixing resistors. The region itself was very sensitive to field effects like (carefully!) waving the shaft of a screwdriver around it. A short length of shielded cable cured this completely. Another builder also encountered the same thing and located the pair of mixing resistors in his clone off the board and directly onto the V2 grid pins with good results. I am doing that in a 50W build I am currently working on (see attached drawing). I would chopstick around, tap components, and also check that your lead dress is ok and sensitive wires cross each other at 90degree angles, etc. BTW, shielding did not work in my case, it was definitely a combination of components probably being too cramped and suffering unwanted interactions as well as lead dress. Shielded wire and lead dress cured it completely. When I got it sorted out, it was like lifting a blanket off the amp; real added clarity and it was stable with every knob dimed.