Metal plate in bottom of head shell

clarkefan

I've mostly grown up playing Marshall heads and they always have metal sheets inside the head shell underneath where the chassis sits.

What are they for?  Something to do with earthing the chassis?

I'm asking because my JCA doesn't have this sheet which is new to me.

Winny_Pooh

Shielding. Perhaps the JCA has conductive paint?

RiftAmps

When the chassis is loaded into the cab, it forms a Faraday cage

https://en.wikipedia.org/wiki/Faraday_cage

clarkefan

Thanks both.  I'm puzzled why some makers don't include this, perhaps it's more a nice to have and not that important.

ICBM

It can be important. I have to say I haven’t noticed it being missing in the Jet Citys I’ve worked on, but I may just not have been paying much attention...

In my workshop it’s obvious when the chassis is on the bench with the open side facing upwards, since I’ve got a fluorescent strip light over it - there’s always a lot of buzz.

So a head with no shielding could be a problem if there’s a strip light in the room below.

p90fool

I'm pretty sure all my Jet City heads have had very thick foil stuck in the bottom of the shell. 

clarkefan

Thanks both, sounds like something got missed, will sort it out cheers. Never thought of that downstairs problem!

ecc83

clarkefan said:

Thanks both, sounds like something got missed, will sort it out cheers. Never thought of that downstairs problem!

Yes, if a head was stood on some sensitive electronics or alternatively something chucking out **** you need the screening. BTW don't cheap out on chuck foil! Get some 20g ally or better, tinplate and make sure the front and rear edges are well stuck to the base, otherwise they can catch in the chassis and make it a bugger to get out!

Dave.

clarkefan

Ahh, "foiled" again, appreciate the advice thanks

clarkefan

I've been in touch with JCA UK and they don't think their amps are built with these plates.  If anybody has evidence to the contrary I'd appreciate it. 

ICBM

I've always used standard kitchen foil very successfully. As long as you make sure it's glued down properly - I use spray-on contact adhesive - it won't snag or tear. It can help to put some tape over the edge at the front, which is where it tends to catch most. As long as it makes contact on at least one edge it's fine.

Fender and many other amp companies actually use foil not much thicker than that too. 

ecc83

ICBM said:

I've always used standard kitchen foil very successfully. As long as you make sure it's glued down properly - I use spray-on contact adhesive - it won't snag or tear. It can help to put some tape over the edge at the front, which is where it tends to catch most. As long as it makes contact on at least one edge it's fine.

Fender and many other amp companies actually use foil not much thicker than that too. 

Yes IC, I can see that from the point of a service job the punter is not going to spring for extra metal! Good that SOME form of screening is used.  My comment was tainted because of struggles I had with one particular chassis...Yes! One of theirs! (there is also a trick to removing the chassis from the Series One 45 combo)

Dave.

clarkefan

Thanks both as always

ICBM

ecc83 said:

My comment was tainted because of struggles I had with one particular chassis...

I’ve had a few real struggles with shielding plates too - if they get kinked it can be a nightmare to get something into the gap and press it down. At least with foil, if you pull hard it just rips! Then you can replace it if you have to...

ICBM

@ecc83 - No shielding plate on the Blackstar HT20 head either. What there is, is some rubber sticky tape along the front and back rails of the chassis, very firmly sticking it to the cabinet and making it impossible to remove the chassis without forcing a knife blade under it and levering it up!

Intentionally 'service proof'?

ecc83

ICBM said:

I've always used standard kitchen foil very successfully. As long as you make sure it's glued down properly - I use spray-on contact adhesive - it won't snag or tear. It can help to put some tape over the edge at the front, which is where it tends to catch most. As long as it makes contact on at least one edge it's fine.

Fender and many other amp companies actually use foil not much thicker than that too. 

Yes IC, I can see that from the point

ICBM said:

@ecc83 - No shielding plate on the Blackstar HT20 head either. What there is, is some rubber sticky tape along the front and back rails of the chassis, very firmly sticking it to the cabinet and making it impossible to remove the chassis without forcing a knife blade under it and levering it up!

Intentionally 'service proof'?

Well no, don't think so. I have an HT-20  head here, bought it for my son and I have had the chassis out a few times no problem.

B's use a foam strip on front plates to stop rattles in combos and to stop liquid getting in, it could be that it denatures over time and heating? Like lekkie's tape? Poor choice of materials?

Dave.

ICBM

ecc83 said:

Well no, don't think so. I have an HT-20  head here, bought it for my son and I have had the chassis out a few times no problem. 

B's use a foam strip on front plates to stop rattles in combos and to stop liquid getting in, it could be that it denatures over time and heating? Like lekkie's tape? Poor choice of materials?

I don’t know, but this one was well and truly stuck! No amount of trying to pull it out by hand was going to shift it, I literally had to work a knife blade all the way along both front and back.

I’ve only worked on the combos before so it was something of a surprise to find it in a head where it’s not necessary at all. I’ve stripped it all off since it didn’t come away cleanly anyway...