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Brushless motor with vacuum tube as feedback sensor

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Avi

Sun Jul 13 2008, 11:06AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

I had an interesting idea to test if i could deflect the 'beam' in a pentode with a magnet. i used a 6BM8 tube and connected a series LED, and connected the grids such that the led was on. I then placed a neodymium magnet on the side of the glass and noticed the led got dimmer. This worked best when the magnet is placed over an opening of the metal plate inside. The effect is the same for both north and sough poles of the magnet. If the magnet is left on the side of the tube, and a secondary magnet is placed somewhere else near the tube, the effect is polar specific. I then fed this to a comparator which fed an H bridge, which powers the electromagnet.
Here is a video of the effect and the motor.

I have tested this effect with an electromagnet at 1Khz and see a more or less square wave. The amplitude decreases with distance from the tube, and also with frequencies beyond this, though this could be for many reasons, eg coil impedance and what not.
What i am unsure of is how to prove with certainty what causes this effect, is it 'beam' deflection, or is it movement of internal structures inside the tube. (note: I am not saying it operates like a reed switch and goes closed circuit with a low impedance as if some things inside are coming together and touching, this is NOT happening at all). The series LED actually fades brighter and dimmer as I rotate the wheel of magnets around, even at a slow speed (very similar to what you'd expect the output of a hall sensor to do).

Nik

Sun Jul 13 2008, 03:25PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

That is the coolest home made motor I have seen in a while, you deserve a MacGyver award for that!

Proud Mary

Sun Jul 13 2008, 05:41PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I wouldn't doubt that a concentrated magnetic field is affecting the performance of the valve, as it does on purpose in klystrons, magnetrons, and CRTs.

Whilst the magnetic field is actually in motion as you hold the magnet in your hand, it will not only affect electron migration directly, but will, of course, induce currents into the three grids of the pentode, and so affect the electron current in that way.

Avi

Sun Jul 13 2008, 06:16PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

in this case the induction is not relevant, you can turn it as slow as you like and watch the tube current go up and down. (i am not implying there is none, its, just not the effect i was trying to study)

Steve Conner

Mon Jul 14 2008, 11:49AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I also nominate Avi for a McGyver award

I'm fairly sure it's deflection of the electron beam, BTW. My other hobby is tube guitar amps, and I've heard it said that tubes sometimes malfunction if they're mounted too close to the speaker magnet.

Proud Mary

Mon Jul 14 2008, 01:28PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Sounds like you've got the makings of a magnetometer, Avi!

Avi

Mon Jul 14 2008, 01:45PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

hm, so has anyone heard of/seen a pentode/triode being used like this or am i the first? LOL

Dr. Slack

Mon Jul 14 2008, 02:21PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I think you'll find that magnetic deflection of a beam of electrons in flight has been done since the invention of the CRT. The first A/D converters were based on deflecting a beam into one of a selection of collectors, though whether magnetic or electric deflection was used for that applciation I don't know.

It was over 40 years ago that I wound a solenoid over an EY51 TV rectifier diode (my early start in HV), and noticed that the forward current dropped as a current was passed through the winding. The electrons are forced to spiral as in a magnetron, rather than travelling in straight lines from the central cathode.

You can regard the magnetic deflection of electrons as either magnetron behaviour, or as a Hall effect. The Hall effect in metals is very small as the large concentration of carriers means very low speeds. In semiconductors it is larger, lower carrier density than metals means higher carrier speed. In an electron beam in a vacuum, the very few carriers go very fast indeed.

Steve Conner

Mon Jul 14 2008, 03:03PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Avi could well be the first to do it with an ECL82, though

Avi

Mon Jul 14 2008, 03:09PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

wow, that is quite interesting about the A/D convertor.
Yes i actually did try also a winding around the tube, not much of a noticeable effect compared to the side of it.

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