What are tube covers for?

[mkruger]

What are the metal (steel) covers on tubes for? Do they provide any RFI or EMI protection? Or do they just protect against damage?

I noticed on my old Dean Markley guitar amp, the small pre section tubes are covered but the bigger amplification section tubes are not.

 

[soundguy]

definitely for RF in some cases, my 436 wont work without the shield on the mu tube, I recently built a guitar amp that oscillated like crazy without the shield on the phase inverter but is quiet with the shield on.

dave

 

[mkruger]

thanks, I will use them for all my pre sections i build from now on

 

[CJ]

they also provide mechanical protection by keeping the tubes in the sockets on bumpy road trips.
some people like them off, maybe because of increased capacitance to ground and also deflection of the electron beam.

 

[mcs]

Not only RFI. Some input stages hum like crazy without tube shields...

Best regards,

Mikkel C. Simonsen

 

[Guest]

at last a question I can help on
On my Marshall head they may be used for other things - but on many occasions they have stopped me burning my hand whilst accidentally poking my hand in at the end of a gig to lift it up

Nowadays you will also a find a grille across the back of all valve amps - some EU directive i think to stop straying hands going into the back of valve amps