Vacuum tube noise source

I don't get the physics of a DC field causing fluctuations, unless it som ehow interferes with striking and extinguishing the arc cleanly.

Anyway, now that you've got me interested...

The 1957 Sylvania datasheet has this to say about it (using a different t hyratron designed for your intended use):

"In this application, the tube is placed in a steady magnetic field which is directed through the tube at right angles to the conducting arc. Proper magnetic field alignment will be obtained if the field is directed paralle l to a center-line through the tube base. This center-line passes half-way between pins numbered 3 and 4. Preferably, the north pole of the magnet sho uld be adjacent to pin 7. A magnetic field strength of approximately 375 +/

- 20% gauss should be used."

You were thinking of a solenoid co-axial to the tube so you can turn the effect on and off or modulate it? I don't know offhand how the tube element s are arranged, but I don't see a plate cap in pictures of the 2D21, so you 'll have to figure out the direction of the arc inside the tube. If it's tr ansverse to the tube axis (as implied by the magnet stuck to the side of th e tube in your link), it should work. Beware of where the external part of the field goes- you might need an iron can to contain it. There's interferi ng with air cooling to consider, too.

The Sylvania sheet says 375 gauss, which isn't trivial, though your tube

Mark L. Fergerson

GR did that in the 1960's.

Looks like the mag field was perpendictular to the tube axis.

Elgenco did as well;

Never found a service manual for it, so you'll have to scan the circuit in-situ. It's 3.5 sigma and a few MHz, IIRC.

As you can see, it uses two 6D4s, which solves GR's statistical problem.

Tim

Tim, Any idea where the noise comes from? Amplifying the shot noise of the tube?

George H.

I think GR did a later generator, with two anti-phase zeners to improve the histogram symmetry.

Perm mags domaim browmiam motiom, atoms move Fe

It's usually a gas tube biased into the proportional region, so avalanche multiplication does the job of a preamp. There's similar devices that live in a waveguide, give out microwave white noise.

Reminds me of Barkhausen noise. But the magnet has to move I think to create the noise, then can be picked up and amplified.

Related maybe ?

boB K7IQ

Plasmas almost always have some form of thermal motion.

Three effects, A. The plasma is hot, so you have thermal noise radiated. B. You have free electrons rotating in the magnetic field, so you get magnetron style effects. The fill pressure on noise tubes, both microwave band and in the Thyratrons, is very low, encouraging effects that would be suppressed in a dense plasma. C. You have impact effects from hot electrons hitting metal.

Steve

If E and B are perpendicular, as it appears from the quotes from the manuals, then the electrons want to move at a constant velocity perpendicular to both. It's called "E cross B drift", and is independent of both charge and mass, so the whole plasma drifts in the same direction. See e.g. and .

When E and B are inhomogeneous (as in the tube case) it gets a bit more complicated, of course.

The magnetron motion (spiralling round *B*) should damp out pretty fast in a gassy tube. but in any case, the magnetic field allows you to use a much higher bias voltage without the tube firing, and so increases the particle energy, which in turn increases the multiplication gain.

Fun--I's never looked into how noise tubes worked.

Cheers

Phil Hobbs

I think the magnet near the thyratron breaks up periodic oscillations, improves the spectrum, somehow.

Yeah I think I read that in one of the manuals you posted. I didn't know about thyratrons... avalanche's!

The 6d4 manual mentions the magnet, but doesn't say what it does.

The noise doesn't go much beyond 1 MHz.

George H.

Probably too much capacitance. Gas discharge tubes have been used in waveguides, way up in the microwaves, as reference noise sources.

I wonder if a neon bulb is a very optically-noisy source, as in above photon shot noise.

I don't know much about neon bulbs, why would you think they would be noisy? Do you also get avalanches.. with a bunch of photons coming at the same time?

I've made an RF discharge lamp that is mostly at the shot noise limit. (mostly 'cause when the bulbs are new you get some "aging" effects or something that gives big spikies... well almost every thing is big compared to shot noise.)

George H.

Probably a bit over shot noise. You do get some plasma fluctuations, and the ion current is sustained by avalanche, whose gain is stochastic.

The upper-state lifetime of the emission line provides some lowpass filtering, of course. The main effect is that your collection system will have an efficiency much less than unity, so I would guess that the photodetection statistics generally dominate with low-pressure bulbs.

High-pressure arc lamps are super noisy (+- 10% power fluctuations are common). The beam pointing is also unstable, because in some bulbs the arc travels aimlessly across the electrodes at speeds up to 50 m/s or so.

Cheers

Phil Hobbs

Neons are very electrically noisy, so some of that probably spills over into light.

One more thing to try on a dull rainy afternoon.

Fair Radio used to sell a surplus noise jammer for aircraft. It transmitted a broadband noise to keep the enemy planes from talking between themselves. They used a phototube and a florescent lamp to generate the noise. That was followed by a broadband power amp. I think the output was about 20 watts.

Some of the jammers used a 931A PMT, in the dark. They get real noisy as the voltage is cranked up.