Grounding tube shields

[Noskipallwd]

Well going to be building the Eros soon, I don't have any experience with tube shields, and after reading Jim's thread on using foil for shields I wanted to make sure I have things right. I plan on changing the sockets and shields, and from what Paul J. wrote I need to ground the shields to the internal shields of the tubes. So is it sufficient to solder a ground wire to the shield base and run that wire to the corresponding solder lug on the socket? Thanks in advance for any help.

Cheers,
Shawn

[cvandyke]

The sockets in the kit have a metal cylinder which is attached to the chassis plate with screws. The tube shield is held in place by this cylinder and so is grounded to the chassis through the cylinder. I think it unlikely that you would notice any difference in the noise level if you grounded the tube shields in another place.

I'm at work right now and don't have the Eros schematic in front of me so I don't remember where the tubes' internal shields are grounded but I'm certain that it's properly taken care of if you follow the assembly instructions.

In the case of using foil on the outside of the tube, the foil should have a connection to ground. I've only done this as temporary thing to see if tube shields would make a difference and I just used a clip lead to supply a ground connection.

[Grainger49]

I changed my sockets in the Eros but still used the aluminum top piece because it lined up with the tube sockets I bought.  The shields go on, come off, etc.  I can't decide.

[Paul Joppa]

Grounding and shielding is still complex enough that half of it is best treated as voodoo, that is to say reason and analysis often rely on too much information that you don't have at hand :^)

Shielding acts to intercept electric fields, including the electric component of electromagnetic radiation (radio, TV, cell phone, etc) by draining the induced current to ground. The ideal shield would fully enclose the electronics. So to be effective, a tube shield must be connected to the chassis so it acts like part of the chassis.

Having said that, I hasten to add that the EF86 has an internal shield that surrounds the actual tube elements and is connected to pins 2 and 7. Those pins are grounded (p.60 in my manual) to T21 and T25. The external shield is redundant, and serves only to slightly improve the shielding - neither shield completely encloses the tube.

Incidentally, and for clarity, the 6922 has a different sort of shield. It merely separates the two triodes from each other, it does not surround them or separate them from the external environment. As we found out when developing the Seduction, the 6922 types need an external shield when in the presence of electrical interference.

Hope that helps a little!

[Noskipallwd]

Thanks for your replies gentlemen, oh and you to Grainger. Paul, this does help. I don't have any ef86s on hand but I have a couple cv4085s, is the internal shield the mesh surrounding the tube elements? Does the fact that these tubes are shielded imply they were designed to operate in a high RFI, EMI environment, or is the design more sensitive to interference?

Cheers,
Shawn

[Paul Joppa]

...Does the fact that these tubes are shielded imply they were designed to operate in a high RFI, EMI environment, ...

Actually, it just means they were intended to operate at very small signal levels (like microphone preamps) where even the smallest interference is important because the signal itself is also tiny.

[Noskipallwd]

Actually, it just means they were intended to operate at very small signal levels (like microphone preamps) where even the smallest interference is important because the signal itself is also tiny.
[/quote]

Ahh, that makes sense, just trying to build the Eros with the hope of producing as little noise as possible. I built a Hagerman Cornet2 and I chased hum in it for a month before I figured out the problem, like to avoid doing that again. Although, I did learn quite a bit about what causes hum.

Cheers,
Shawn