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Tweeks on tubes - bring out the demagnetiserss

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There is a thread on the Tube Asylum about this tweek. It is to do with demagnetising tubes. Many guys are skeptical about this.
So the only way to find out is to try it out. So all you need is an old speaker . You can break it and take out the magnet , unless you have a demagnetiser.

I used a ferrite magnet from a 6 inch speaker. Be careful because my 6922 tube almost crashed into the magnet when I took it close to it. I passed the tube through the middle of the magnet a few times and then slowly drew it away till it was about two feet from it.

The tube is used in a unity gain buffer stage .
When introduced between my DVD-A player and the amp it robs the music of 'air and hf cleanliness'. Simply put, it sounded duller than a direct connection between DVD-A and amp.

After the tweek it sounded far better. No other changes were made. It is still not as good as a direct connection but it no longer sounds dull. I am floored. I never expected this. Try it out and let us know what you find. I don't know how well this works with tubes that are run in for a long time.
I would really like to know what you find.
Cheers.
 
I've got some weird effects from magnets, putting blue glow spots on the glass in places even the sun doesn't shine :D and cutting off the tube by bending the electron flow around a bit too much (good way to burn up the screen grid in a beam pentode, I'm sure).

Nickel, the common metal used for plates and such, though magnetic, has an insignificant residual field. That's why it's used in many transformers and not permanent magnets (Alnico alloys aside, but those contain another ferromagnetic element or two). Any magnetic field will only be deleterious to the operation of a tube anyway. And lastly, you know what makes a magnetron work? Magnets! :bigeyes:

Tim
 
I passed the tube through the middle of the magnet a few times and then slowly drew it away till it was about two feet from it.

I could be wrong, but would this not *magnetize* the tube instead? :eek: I always thought you required an AC field for demagnetization. How can the unchanging, polarised flux of a permanent magnet cause demagnetization?

Take care,
Doug
 
I could be wrong, but would this not *magnetize* the tube instead? I always thought you required an AC field for demagnetization. How can the unchanging, polarised flux of a permanent magnet cause demagnetization?


A magnet has two ends; the north pole has positive magnet force and the south pole has negative magnet force. You can demagnetize a tube by waving the south pole end over it with a counterclockwise motion. To remagnetize it, just wave the north pole end over it with a clockwise motion. Vice versa for Australians and New Zealanders.

John
 
Ex-Moderator
Joined 2003
jlsem said:
A magnet has two ends; the north pole has positive magnet force and the south pole has negative magnet force. You can demagnetize a tube by waving the south pole end over it with a counterclockwise motion. To remagnetize it, just wave the north pole end over it with a clockwise motion. Vice versa for Australians and New Zealanders.

Sorry, but that won't work. When you try to demagnetise something by reducing the forcing flux to zero, you are always left with the remanent magnetism. Because this is a fixed proportion of the forcing field, the only way that you can demagnetise something is to take it through successively smaller cycles of demagnetisation until the remanence is effectively zero. That's why demagnetisers run off AC and why you gradually pull them away from the thing to be demagnetised (perhaps a tape head).
 
Doug is right.

Actually one will need an ac generated magnetic field that slowly decays to zero.
In my case it was a dc field but I did oscillate it out of the magnet where it would see a varying field and drew it away reasonably far from it. Not as good as an ac field but better than a static dc field.
It however made a difference !
Cheers.
 
SY said:
Tim, your point is a good one- if we want real, demonstrable effects, we ought to be talking about the use of magnets to modify electron flow in tubes. Now, THAT is a tweak.

IIRC, if you put the poles of a magnet at the very top and bottom ends of the tube so that the flux passes along the tube's axis, the electrons instead of moving radially out from the cathode to the anode will instead once they leave the cathode, move in a larger and larger orbit around the cathode axis, finally reaching the anode.

For a beam tetrode, I wonder if this would increase the electron cloud density and therefore space charge, or perhaps even make it not work at all?

I'm sure someone out there will start selling Magic Magnets tuned to some cosmic vibration :rolleyes: that will send "bad" distortion-causing electrons right back to the cathode. ;)
 
If you use a strong magnetic field, indeed, many electrons will be so sharply deflected as to collide with the cathode again (assuming a rectangular cathode and a beam tetrode here). The ones emitted from the cathode's corners with sufficient velocity will go out and either strike the grid supports, screen supports, beam deflection plates/supressor grid or some may actually reach the plate. All current levels (grid leakage, Is, Ip) will be out of whack and Ip will be lower.

Microwave action is probably quite unlikely as it takes a specially hollowed-out plate to form the resonant cavities, but you never know. Snivets distortion (in sweep tubes) is a rather foggy situation as I recall...

With weaker fields, the beam will be deflected less and will congregate sideways of the natural position they strike the plate at. This will reduce plate dissipation or a red spot because the beam is no longer striking the plate seam, where much of the heat can be dissipated with ease.

With a non-axial field, say one in line with the beam but perpendicular to the vertical axis, will concentrate the beam lengthwise. Electrons moving at an angle to the field will be deflected parallel to it, i.e. focused. If the field were more concentrated right around the plate, that is, the magnetic lines of force all generally pointing in towards a point on the plate - they will be concentrated here. Mind you this will also change the behavior around the screen and control grids, causing various unwanted results.

If the field is rotated another 90°, around the tube this time, the electrons will generally spiral out from around the cathode, be deflected into the grids, they'll still probably reach the plate though at an oblique angle. Whether they go up or down depends on the charge of the particles (electrons - negative) and direction of the field.

Think about it yourself, imagine magnetic lines of force etc... Remember that F = V x B * q, where F is force, V is velocity, B is magnetic field and q is the charge. F, V and B are vectors and x is a cross product; this makes the right hand rule. Generally, a charged particle moving perpendicular to a B field will be deflected perpendicularly again, such that the force causes it to spin in circles (hence the circlotron). Magnetic fields also cannot change the momentum of a particle as E-fields can, so an electron trapped in a constant field with no electric influence could circle forever and basically never move.

Tim
 
jlsem said:
Cyclotron. A circlotron is the perfect amplifier nobody really seems interested in building.

Oops :cannotbe: ;)

fdegrove said:
Unfortunately it's also totally beside the point...

Ya. Was gonna add that the nickel plate, cathode, grid supports if not copper, etc. will redirect the field, shielding what's inside to some extent. How much depends on the orientation, strength, plate dimensions (especially thickness) and magnetic properties (mu, saturation).

Tim
 
diyAudio Senior Member
Joined 2002
Hi,

If you have already tried it - what did you find ?

Errrr....that you don't really need a magnet to achieve the same results?

Removing magnetic remnance from ferrous, or paramagnetism from non-ferrous materials is been known to have a positive impact on sonics for quite a while already....
Why would tubes be any different?

I usually use a Densen demagnitizer CD once every couple of months with good results.

Cheers,;)
 
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