Yet another tube q.

[JimRayden]

There's about half an inch space between the tube and the power tranny in my Tube Driver layout. Is the space enough?

Too little space between = bad hum?

------------
Jimbo

[R.G.]

Too little space between = bad hum?

Yep.

Try shunting some of the magnetic field around the tube with as thick a bit of steel as you can stick in the space between the two.

[puretube]

toroidal xfmr?

[JimRayden]

A usual one.

------------
Jimbo

[puretube]

toooo close...

[JimRayden]

Damn...


------------
Jimbo

[puretube]

MU-Metal (µ-metal) ...

[JimRayden]

No thanks, I'm more of a rock n' roll guy... I'm not into that heavy stuff.

------------
Jimbo

PS: okay okay... I can't get mumetal around here.

[Peter Snowberg]

Definition: MuMetal is a nickel-iron alloy composed of 77% nickel, 15% iron, plus copper and molybdenum that is highly effective for shielding magnetic fields. It is used to shield vacuum chambers for experiments and other magnetic instruments. MuMetal is one trade name for a high-permeability, magnetically "soft" alloy. Other trade names include Hipernom, HyMu-80 and Permalloy.


more:

http://digilander.libero.it/paeng/what_mu_metal.htm

[puretube]

it`s actually lighter than steel, IIRC, and sometimes used for shielding monitor loudspeakers...

I would never recommend "Heavy Metal", because:
"Most Of All We Need Da Fonk"
(I do hope, nobody is offended... )


http://diystompboxes.com/sboxforum/viewtopic.php?t=29866 HURRY!

[Eb7+9]

There's about half an inch space between the tube and the power tranny in my Tube Driver layout. Is the space enough?

Too little space between = bad hum?  Jimbo

That distance is standard in many tube amps, just look how close the power tubes are to the power tranny in classisc tube amps ...

Steel is usually garbage for isolating H fields, you want silicon-iron, aluminum, or mumetal - note: many tube shields were made of Aluminum or Si-Fe alloys ... Aluminum being most easily found in scrap yards ... just stick a vertical plate between the two if in doubt ...

The difference in Magnetic Remanance plays a big role in explaining the variation in sound between Steel-chassis Plexi's and Aluminum-chassis Plexi's - the plane messes with the OPT fields ... Ken Fischer wrote a great article about this in Vintage Guitar way back around '94 ...

[puretube]

such small distance may work with power-tubes that handle large grid voltages, but NOT with small signal input tubes...

[JimRayden]

Thanks, puretube, I'll work out another layout.

-------------
Jimbo

[R.G.]

Steel is usually garbage for isolating H fields, you want silicon-iron, aluminum, or mumetal

What you want is the world with a white picket fence around it. What you live with is what you can get. Or, as they say, a bird in the hand is worth two (thousand) birds in the bush.

Steel is of course not the optimum shielding material. Neither is aluminum, by the way; it shields M-fields only eddy current losses, and has steadily decreasing effects as the frequency gets lower. Aluminum has *no* M-field shielding whatsoever to a permanent magnet (DC) field. Mumetal is good - as long as the field strength does not saturate it. It has quite high permeability, but saturates very quickly.

The best shielding is distance, but as we have all noted, distance and stompbox innards are antithetical.

Input transformers - great pickup coils in their own right - are usually shielded by multiple layers of copper and transformer iron - the SiFe you mention. Transformer iron is better than ordinary carbon steel, but only if you anneal it after cutting and bending it. Otherwise, it rapidly approaches the permeability and hysteresis of carbon steel. It is possible to measure the change in magnetic properties of SiFe from the fully annealed state after it has been hit by a hammer --even once -- with out being noticeably deformed.

So, as a practical matter, if you're trying to save a layout you already have, sticking in as much steel that you have and can get your hands on and get into the shielding space is likely to be a better solution than sending off to Denmark or Denver or Tierra del Fuego for fully annealed mumetal shields that have been CNC laser machined to fit your space, especially if you have less than $10M to play with.

note: many tube shields were made of Aluminum or Si-Fe alloys

Yep - and quite a few, if not most were made of ordinary low carbon steel sheet. Aluminum is $#&^*@# effective at shielding RF, which is what most shields were used for. The **plating** on SiFe is $%%^*& effective at shielding RF in most cases at RF.

The difference in Magnetic Remanance plays a big role in explaining the variation in sound between Steel-chassis Plexi's and Aluminum-chassis Plexi's - the plane messes with the OPT fields

Remanence is the property of a ferromagnetic material that it does not relax to a fully zero M-field when the excitation is removed. Remanence vanishes, becoming hysteresis under AC excitation, so it does not and can not have an effect on AC (sound) operation.  A steel chassis *does* "conduct" M-field hum from the power transformer and to a lesser extent the choke into the OPT if you are not careful about placement and orientation of the OPT with respect to the PT and choke.

Ken Fischer wrote a great article about this in Vintage Guitar way back around '94 ...

I'll have to go read that - but just off hand, Ken Fischer is not the kind of authority I'd go to for finding out how magnetic materials work. I'm sure that he's pretty good at hearing differences, but I suspect that he may not be the best reference for the science behind what he hears. Just a guess. He may have multiple PhDs in EE, metallurgy, field theory, and molecular physics, but if he has, he's been remarkably quiet about it.

As I said, Jim; before you rip it up, at least try sticking in a hunk of steel you can get and see what happens. It may be OK. Maybe not. If not, all you've lost is the time to find a hunk of steel and try it.

[zachary vex]

even a small amount of steel can do a great job shielding against 60Hz magnetic noise.  i know... i hardly have any room in this thing to keep the two toroidal power transformers inside from interfering with everything else.  minimal hardware store steel solves my magnetic interference problems.



iMP amp. class ab tube stereo hi-fi amplifier, 1W/channel. June production.