copper foil a workable sub for tube shields?

[Jim R.]

Has anybody used copper foil instead of aluminum tube shields on your Eros, and if so has it been effective?

Here's the problem:  I have two very nice NOS GEC cCV 4086 tubes to use in the ef86 positions on the Eros, but these are military, flying-lead tubes that have been pre-mounted to 9 pin bases.  Trouble is that these bases are almost as wide as an octal base and thus standard tube shields won't fit.

Of course I don't know if I'll need the shields or not as my system is below grade and close to a concrete wall with earth on the other side, and I've gone to some lengths to get RFI noise sources out of my listening room, including my wireless router, switching power supplies, and have relocated the music computer to a closet in the diagonal opposite corner of the space from where the rack is.

I do plan to build a 2-compartment copper box for the inside of the wood frame, but not sure this will be enough or not -- and no, I don't expect anybody to tell me whether this will help or not as I will just have to try and see.  BTW, the box will separate the power supply from the gain circuit.

I've used the copper foil with reasonable success on some octal tubes in the past, but never on a noval, but not sure what implications that has, if any.

Thanks,

Jim

[porcupunctis]

Copper foil is used a lot to line the cavities in guitars where you fit the pickups and other electronics.  It seems to work very well in that application. 

You might even try a piece of copper pipe.  Several diameters are available at most hardware stores and they even have copper caps you could solder onto one end.  This would make a custom copper "silo" for just about any sized tube.  You might want to drill a hole in the top for ventilation.

[Paul Joppa]

We've used it experimentally with success. You have to ground the shield of course - pins 2 and 7 are the internal shield of the EF86, so connecting the copper tape to either or both would do the job nicely.

The main issue is heat; the best tube shields ( no made for decades I think) acted as heat sinks as well. The EF86 does not dissipate that much heat, but especially in a smaller envelope the surface temperature will be high - make sure the adhesive can perform acceptably at the temperature, and that the glass temperature does not exceed the spec. I say this with tongue in cheek of course, since it's going to be damn difficult to find the specs for any of these things!

[grufti]

I have used all kinds of shielding for the regular EF86 tubes on my Eros to no measurable or audible effect. I no longer even take the usual RFI/EMI precautions inside the house. Outside the house is suburban California with airports, etc.

This outcome was a complete surprise, but I'll take it. Cans or no cans the result is the same for me. Based on my experience you might not need to worry about shielding until the very last. ymmv.

[Jim R.]

Thanks all for the responses.  Hans, that's what I'm hoping for too -- that I don't need the shields.

Paul, no adhesive needed here -- I have some 6 mil (I think) copper foil and I'd just take a few turns around a form, solder a ground wire along the length and then solder the visible seam with paste solder -- works well.

Thanks,

Jim

[Noskipallwd]

Hey Jim, are those tubes CV4085s that are military spec,(with flying leads), or are they a different tube altogether?

Cheers,
Shawn

[Jim R.]

Shawn,

They are actually CV 4086 -- but the CV 4085 is listed as a sub, though I don't know what the differences may be. They are premium military grade, true mesh plate EF86s w/flying leads.  Made in Marconi Osram factory.

-- Jim

[Doc B.]

Guys, Buddha was copper taping tubes like this 13 years ago. Check out the Buddhafied Dyna FM-3 article in Valve issue 1, 1999 for a detailed procedure:

http://www.bottlehead.com/VALVEarchive/1999/va011999.pdf