The tube monotricton (magic eye spinner) problem

[neazoi]

I am trying to build this circuit, to make a magic eye (EAM86 in my case) oscillate.
https://www-elektronik--labor-de.translate.goog/.../pl21...

I had to place a 10uF instead of 0.1uF, to decrease the frequency to around 1Hz.
Thyratron generates a sawtooth from 200 to 250v.

However the EAM86 beam, swings only a little and this swing is in the overlap region of the two beams (i.e beams not separated).

What can I do about it?

 

[FvM]

1: Find out the required st electrode (pin 7) voltage range, should be about (20 - 150 V ) according to datasheet
2. Modify the generator to achieve a sawtooth of this range

 

[betwixt]

Is that PL21 part of the schematic correct?
It is a Thyratron but I've never seen one connected like that, normally the timing capacitor goes anode to ground, cathode is grounded and there is an anode load resistor.
Try it with an ordinary neon lamp, ground one side, 1M to 300V on the other and connect 100nF across the neon. A simple 3 component oscillator with equal output as the Thryratron.

Brian.

 

[neazoi]

Is that PL21 part of the schematic correct?
It is a Thyratron but I've never seen one connected like that, normally the timing capacitor goes anode to ground, cathode is grounded and there is an anode load resistor.
Try it with an ordinary neon lamp, ground one side, 1M to 300V on the other and connect 100nF across the neon. A simple 3 component oscillator with equal output as the Thryratron.

Brian.

PL21 is identical to 2D21 https://www.radiomuseum.org/tubes/tube_pl21.html
Indeed this is the connection I have seen too http://microwave/giannopk/synth.htm
Indeed it can be done with a neon relaxation oscillator and in fact it would be better to safe this 0.8A of current that the 2D21 draws!
I just found this schematic and I though it would be cool to use this thyratron (I have 60 of them)

The circuit shown in the page on post #1 works and it produces a reverse sawtooth of around 200-220v (correction). My anode voltage is 220v since I used a back to back transformer and a single diode and capacitor for rectification. I thought to do so, so as not to stress the tube since max anode voltage is 250v.

I do not know exactly how magic eyes work, but FWM pointed to the voltage range and offset, which I do believe the problem is. However, I have no idea what to do in order to get that range/offset.
If I cannot find a solution I will switch back to a neon.

I have also some 0B2 and 0A2 tubes, can I use these instead of a neon to create a relaxation oscillator?
The only reason I would avoid these, are for their radioactive cathodes in case they break (this is for a decoration unit)

 

[D.A.(Tony)Stewart]

Too much grid bias? Reduce 2.2M ?

--- Updated Dec 9, 2023 ---

Too much grid bias? Or increase 2.2M ?

 

[neazoi]

Too much grid bias? Reduce 2.2M ?

--- Updated Dec 9, 2023 ---

Hm... I changed the 2M2 to 180K. I did not notice any change

 

[D.A.(Tony)Stewart]

Hm... I changed the 2M2 to 180K. I did not notice any change

Try 10M

 

[neazoi]

Try 10M

No, exactly the same behaviour

 

[betwixt]

Thyratrons are gas filled tetrodes or triodes, they have a breakdown voltage as the gas inside them ionises and it is this that causes the sudden anode current, dropping the voltage across the tube. The grids control the avalanche point, much in the same way as xenon flash tubes are activated by ionising part of their length with a trigger electrode.

Magic eyes are very simple triodes, the anode is open, usually a half cylinder with the open side facing the viewer, it is painted with fluorescent coating. The cathode is hidden behind the center support and has a single wire control grid between cathode and anode. Electrons from the cathode hit the anode and make the paint glow, putting a voltage on the control grid blocks them, at first behind the support and as the voltage increases, it widens the shadow and makes the eye 'open'.

0.8A is the maximum short period discharge current rating, they will run quite happily at microamp currents as well.

If this is just for decoration, you can get the same oscillation effect at much lower voltages by using a diac instead of a thyratron, typically around 32V breakdown, they are commonly used to control warning beepers in hidden lighting applications such as UV water treatment plants.

Never heard of OA2/OB2 being radioactive, they may have thorium or strontium coated cathodes but so do most vacuum tubes, including thyratrons and magic eyes.

Brian.

 

[neazoi]

Thyratrons are gas filled tetrodes or triodes, they have a breakdown voltage as the gas inside them ionises and it is this that causes the sudden anode current, dropping the voltage across the tube. The grids control the avalanche point, much in the same way as xenon flash tubes are activated by ionising part of their length with a trigger electrode.

Magic eyes are very simple triodes, the anode is open, usually a half cylinder with the open side facing the viewer, it is painted with fluorescent coating. The cathode is hidden behind the center support and has a single wire control grid between cathode and anode. Electrons from the cathode hit the anode and make the paint glow, putting a voltage on the control grid blocks them, at first behind the support and as the voltage increases, it widens the shadow and makes the eye 'open'.

0.8A is the maximum short period discharge current rating, they will run quite happily at microamp currents as well.

If this is just for decoration, you can get the same oscillation effect at much lower voltages by using a diac instead of a thyratron, typically around 32V breakdown, they are commonly used to control warning beepers in hidden lighting applications such as UV water treatment plants.

Never heard of OA2/OB2 being radioactive, they may have thorium or strontium coated cathodes but so do most vacuum tubes, including thyratrons and magic eyes.

Brian.

Hm... Are you sure about most tubed cathodes to be covered with radioactive material?
I thought only the cold cathode ones are, or some high power directly heated triodes.
On my site http://qrp.gr/safety.htm I have a list of such tubes (at the end). I do not think all of the tubes like magic eyes, contain radioactive materials.
How about these little NE2 bulbs?

 

[betwixt]

Thorium and Strontium are common cathode coating materials, they improve electron emission over plain metal. They are not radioactive in the compounds used for coating although there are isotopes of both that are radioactive. I used to live close to a strontium ore quarry and none of the staff or nearby residents were deemed to be at danger.

I'm not sure about NE2 and similar as being in gas, there is no need to optimize electron emission.

Brian.

 

[D.A.(Tony)Stewart]

Try 10M

No, exactly the same behaviour

Hmm It's being over driven into saturation with overlap, so if you have a scope measure pin 7's then disconnect and find what changes, , get the relaxation oscillator working as @FvM said then connect R's between pin 7's to adjust and 10M or a bit less as a divider.