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Old 1st August 2004, 01:10 PM   #11
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Quote:
Originally posted by SY
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 that will send "bad" distortion-causing electrons right back to the cathode.
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Old 1st August 2004, 05:21 PM   #12
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Oh boy,

Who or what is magnetising my tubes? Some magnetic bugs? Or is it the earth’s magnetic field? In the latter case turning the amp by 180 degree on a daily basis will suffice to neutralise it me think.

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Old 1st August 2004, 06:55 PM   #13
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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
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Old 2nd August 2004, 04:22 AM   #14
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A fine analysis, Tim.

Quote:
(hence the circlotron).
Cyclotron. A circlotron is the perfect amplifier nobody really seems interested in building.

John
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Old 2nd August 2004, 04:26 AM   #15
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Hi,

Quote:
A fine analysis, Tim.
Unfortunately it's also totally beside the point...

Quote:
A circlotron is the perfect amplifier nobody really seems interested in building.
Hmmmm.....Cost is all that's keeping it from being popular IMHO.

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Old 2nd August 2004, 04:39 AM   #16
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Quote:
Originally posted by jlsem
Cyclotron. A circlotron is the perfect amplifier nobody really seems interested in building.
Oops

Quote:
Originally posted by fdegrove
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
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Old 2nd August 2004, 09:57 AM   #17
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Great info.

Now why don't you guys do this simple demagnetising experiment and tell us what you find.
If you have already tried it - what did you find ?

Cheers,
Ashok.
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Old 2nd August 2004, 02:26 PM   #18
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Hi,

Quote:
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.

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Old 2nd August 2004, 04:14 PM   #19
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I usually use ............. with good results.
Thanks Frank, that's the kind of reply I was hoping for. How about some response from the others. Surely some may not have found much difference (?). If so it might be interesting to find out
( or guess) why.
Cheers,
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Old 2nd August 2004, 08:19 PM   #20
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Quote:
Originally posted by fdegrove
I usually use a .. demagnitizer CD ...
Um, .... oooookay.
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