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Cosmic Induction Generator Build Thread


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#1 johnpolakowski

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Posted 12 August 2013 - 09:27 AM

Hey guys,

 

This is a build thread of a 1KW version of the Cosmic Induction Generator, as envisioned by Eric. Almost all the parts have been acquired already, and some of the components of the build have already been completed. The rough design has been done, but there are some details that need to be worked out yet. I see this as sort of a group build, as I am building the project, but hopefully the group here can help contribute to some of the design elements that are still incomplete. Any constructive advice or criticism is welcomed and appreciated! I am not an expert. I have received help from Eric so far, but I am not always in contact with him, so I am hoping to make a group effort to complete this project.

 

Overview:

The CIG basically consists of an AM amplifier/transmitter which powers some Tesla coils. The transmitter consists of vacuum tubes providing the power amplification. Solid state components are used very minimally. The Tesla coils are in a unique configuration, two Tesla Transformers aimed at each other creating an extremely intense dielectric field in the middle of the two units. Also where a normal Tesla coil would be grounded to the earth in order to transmit, these coils will be referenced only to each other.

 

Between the two coils is where the magic happens. For the CIG, the objective is to put a bulb containing a gas conducive to plasma creation -  in the middle of the intense dielectric field and see the "galaxy in a bulb". It is reported by Eric that inside the bulb the plasma takes on a life of its own, and the electricity has a memory, and displays intelligence. The primary objective of this project is to recreate that display. The nice thing about the CIG though is that it is multipurpose. It can also be used to create "singing" arc discharges when modulated with an audio source. Also, all the coils(primaries, secondaries, extras) are being built on independent frames and stands, so they can be used in a different configuration than just pointing at each other. If spread apart and grounded they can be used to transmit and receive to each other.

 

Another goal of the project will be to recreate the "radiant matter" experiment that was at the recent Bedini-Lindemann conference. I personally witnessed this experiment of charging a capacitor to over 5000 volts through space. It was quite shocking! Also I felt the "radiant matter" being emitted by the incandescent bulb. It was a strange warm, fuzzy, tingling feeling. I definitely hope to feel that again!

 

CIG Components

The Induction Generator can be simplified to 3 distinct components: power supplies, the transmitter section, and the tesla coils.

 

Power Supplies

The power supplies are pretty basic linear supplies. The transformers used are center tapped, and vacuum tube rectifiers are used to rectify the AC. Choke input filters consisting of a swinging choke and smoothing choke are used to regulate the DC. Oil capacitors and bleeder resistors go on the output of the power supplies. A variac goes on the primary of each HV transformer for adjust-ability and to dial in the proper voltages for the vacuum tubes. The power supplies are pretty basic. There is an integral safety interlock though that is required to cut all screen voltages if plate voltage is suddenly lost- to protect the tubes. If plate voltage is suddenly lost the screen suddenly becomes the plate and can be quickly destroyed by a current overrun. So there is a necessary feedback mechanism to the screen voltage power supplies. Part of the "power supplies" includes the filament supplies for the tubes. The objective is to make the filament voltage adjustable as well for two reasons:

1. To slowly warm up the tubes and prevent a large surge of inrush current.

2. As the tubes age the filament voltage can be turned up a little bit to boost the emission of the cathodes.

To provide this adjust-ability rheostats will be placed on the primaries of the filament transformers.

 

AM transmitter

The transmitter will be Class C operation. This particular design consists of 829B driver tubes and 4-250 power tubes. The tubes will be in push-pull operation, driven at their maximum permissible values. The operating frequency is chosen to be 1.8 Mc, or 160 meters. The 829B tubes will be driven by a WW2 era VFO/Exciter/Transmitter called a TCS. The TCS puts out approximately 25 watts at an output impedance of around 10,000 Ohms. It will be coupled to the 829B's through an input transformer. A tentative schematic for the transmitter I've come up with is below:

 

TransmitterSchematic_zps6d13a07b.jpg

 

The coupling and blocking capacitor values have already been chosen, as shown, as well as the RF choke component values. Eric specified 1/4 wavelength chokes to be used as RF chokes in the plate circuits. I'm not exactly sure why that design pattern was chosen.  The .05 uF caps are to pass the signal current, the .005 uF caps are to filter out higher frequency parasitic oscillations. The capacitors are all of the mica-type. The 5 mH chokes are to keep RF out of the power supplies, and the micro-Henry valued chokes are to filter out any high frequency oscillations that are able to pass through the 5mH chokes due to the distributed capacitance in the milli-Henry valued chokes. The mA circles are milli-amp meters that go on every electrode of the vacuum tubes. Split stator variable capacitors are specified to be used in the tuned tank circuits, in order to keep the networks balanced. The grid and plate tank circuit inductance and capacitance component values have yet to be designed. 

 

Overall I'd like this to be a learning experience for me as well as all who are involved. If anyone has any questions I would be happy to answer them if I have the knowledge to do so. I also have lots of pictures of stuff I've built so far, so if there are any pictures people would like to see, let me know. I could fill up page after page with pictures so I don't want to clog the thread up too much. Also if anybody has any questions on how I built something, or why I did something that way I would be happy to explain. Peer review is very beneficial!

 

Let the fun begin!

 

Note: As just a quick aside I don't want to address any of the recent drama regarding the lab in this thread. If someone wants to talk about that please do it in a different thread or send me a private message.


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#2 johnpolakowski

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Posted 12 August 2013 - 09:29 AM

So far I have a few of the power supplies finished. Here's a couple examples of some of the equipment I've built already:
PowerSupplyfinal009_zps6a6198da.jpg
PowerSupplyfinal006_zpsa3359d0c.jpg
PowerSupplyfinal005_zps67168857.jpg
PowerSupplyfinal004_zpsc126a084.jpg
 
 
PowerSupplyComplete001_zpsae0ee4e1.jpg
PowerSupplyComplete002_zpsf365e98a.jpg
PowerSupplyComplete003_zpsd4063112.jpg
PowerSupplyComplete004_zps03b65c5e.jpg
 
 
The primary and secondary tesla coils have been designed already, and I am nearing completion of building their coil forms. Copper tubing is being used as a conductor for the primaries, and coax is being used as a conductor for the secondaries. Some of my recent progress:
CosmicInductionGenerator187_zps16a44a19.
CosmicInductionGenerator192_zps42c8110d.
 

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#3 johnpolakowski

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Posted 12 August 2013 - 09:31 AM

CosmicInductionGenerator201_zps5ee05ed4.
CosmicInductionGenerator198_zps6e516357.
Primary bases finished:
CoilConstruction007_zps02585060.jpg
 
Secondary bases finished:
CoilConstruction006_zpsfae12489.jpg
A finished coil form:
CoilFormComplete002_zps1f30ec40.jpg
CoilFormComplete004_zpsc8603623.jpg
CoilFormComplete005_zps533550e2.jpg
 

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#4 johnpolakowski

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Posted 12 August 2013 - 09:31 AM

The transmitter itself for the CIG is being built on the chassis of an old 600 volt military power supply:
CosmicInductionGenerator003_zps13d15502.
CosmicInductionGenerator004_zps9b9ece1b.
You can see my supervisor in the background here:
CosmicInductionGenerator009_zpsa95583e6.
CosmicInductionGenerator028_zps6ac07c84.
The power supply was in amazing shape, looking like it had never been used. All the tubes were brand new. I probably would have actually used the power supply because it was in such beautiful condition, but its current output wasn't enough for my purposes. I started dissasembling to repurpose it as a chassis for the transformer. 
CosmicInductionGenerator021_zps50183c87.
CosmicInductionGenerator030_zps433852b6.
CosmicInductionGenerator033_zps4fdc380d.
When the transmitter is complete you should be able to see the 4-250's glowing through the metal screens here:
CosmicInductionGenerator034_zpsbe3af740.
 
All the original parts are being saved for use in another project. I should be able to use the rectifier tubes in another power supply I'm building:
InductionGenerator019_zps424e4b22.jpg
 

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#5 johnpolakowski

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Posted 12 August 2013 - 09:32 AM

These are the tubes being used for the transmitter. 
The 829B's:
InductionGenerator020_zps4b3b5c5f.jpg
 
The 4-250's:
InductionGenerator018_zpsd1f2d065.jpg
Notice the radiation symbol on the right of the tube, I think this is due to the thoriated tungsten cathodes:
InductionGenerator016_zps42a6d4e4.jpg
InductionGenerator013_zpse05e834d.jpg
 
Also I can't help but post a picture of this 5C22 hydrogen thyratron I recently acquired for a radar pulse modulator I will be building in a later project:
InductionGenerator026_zpsb3ad98be.jpg
 

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#6 johnpolakowski

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Posted 12 August 2013 - 09:33 AM

The first design task I'm trying to figure out at the moment is the 829B input transformer/tank circuit. Already specified is an output impedance of the driver of 10,000 Ohms, and the driver can put out approximately 25 watts. So the input impedance of the 829B tank network should be 10,000 Ohms as well. The transformer core will  probably be ferrite or powdered iron unless anybody has any other suggestions. I think Eric mentioned sourcing it from Amidon.
The grid circuit:
829Bgridcircuit_zps58f29b97.jpg
I figure the input transformer should be designed first, and then the tank capacitance can be calculated to resonate with the tank inductance at 1.8 Mc. I'm not sure how to proceed as far as a turns ratio calculation of the transformer vs input impedance of the 829B's. Also I'm not sure what kind of coupling the transformer should have: mutual inductance vs leakage inductance. Any opinions are welcome. 
Jim from the forum is experienced with vacuum tube equipment and was kind enough to relay to me that 829B's use a grid resistor value of 7,500 to 12,500  Ohms and apparently this approximately represents the input impedance of the tube. I'd like to first become clear on what exactly the grid resistor is, what its purpose is, and why it represents the input impedance of the tube. For me, learning is as much a part of the design process as actually completing the design. I think the grid resistor is this component:
gridresistor_zpsf4716607.jpg
From some reading I've done it seems like the grid resistor can be used for two things: to generate bias for the grid, and to reference the grid to cathode potential.
 
Anyways whoever wants to hop in and offer some suggestions or knowledge feel free!

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#7 Raui

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Posted 18 August 2013 - 01:14 AM

Awesome work John. I've been meaning to post up a reply here for like a week but have had a lot on my plate. If I were you the transformer would be the last thing I would design. The point of the transformer is to match the primary and secondary impedances so that they match so really you'd want to design your circuit and then design your turns ratio to match the two impedances which will likely be different. I'm not sure what is usually done in this regard. Maybe Jimm will? Not being very well versed with tube circuits I couldn't tell you exactly but another thing that I can see that it could do is limit the current going into the grid.

 

Check these out for more on impedance/matching etc:

http://electronicdes...matching-part-1


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#8 G4ΓΓ3ττ

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Posted 18 August 2013 - 02:29 AM

 

Jim from the forum is experienced with vacuum tube equipment and was kind enough to relay to me that 829B's use a grid resistor value of 7,500 to 12,500  Ohms and apparently this approximately represents the input impedance of the tube. I'd like to first become clear on what exactly the grid resistor is, what its purpose is, and why it represents the input impedance of the tube. For me, learning is as much a part of the design process as actually completing the design. I think the grid resistor is this component:
 
From some reading I've done it seems like the grid resistor can be used for two things: to generate bias for the grid, and to reference the grid to cathode potential.

 

John,

 

Things are looking great! I can't wait to see the finished project--btw I love the ginormous thyratron and Eimac tubes!

 

I believe the grid resistor is a "pull down" resistor. It keeps the grid tied low, as opposed to floating--floating is bad. This is common practice on BJTs and MOSFETs. Basically, when the applied input signal goes away the resistor drops the grid potential back to the off position. Though it may need to be tied lower than 0V--probably around -10V or so. Depends on the tube and the aplication. For amplification, you look for a spot that has good linearity and gain and use that as the bias point. For pulse application you want to have it cut off conduction completely. For the amplifier circuit, you would probably want to use capacitive coupling to the grid, so as to keep any DC offset currents from altering the grid's bias point. For good linearity, anode potential and grid bias need to be chosen carefully, otherwise you will add unnessisary distorion.

 

If the tube uses a shield around the filament you should draw that in as the cathode terminal. Direct filament tubes do exist though. Also, your drawing of the MOSFET is confusing: you should point out the drain, source, and gate. As well as use the standard symbol for it. Is the keying circuit just an on/off switch or does it need to be linear? If its just a switch you don't need the bias resistors, an opto coupled fet driver would be simpler. Finally, if you are trying to key the tubes on and off, wouldn't putting the fet directly in series with the cathode's connection to the annode supply's ground be better? You would want to leave the grid's circuit ground connected directly to the cathode and also not interrupt the heater supply.


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#9 Gestalt

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Posted 18 August 2013 - 05:32 AM

Check these out for more on impedance/matching etc:

https://www.youtube.com/

http://electronicdes...matching-part-1

 

That was an awesome video!!

If you find any more gems like that please post. I will watch them all.  :D

Might be good to be these in the archives. 


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#10 Robert

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Posted 18 August 2013 - 03:59 PM

Thanks John, for your fine example. I wish I could emulate what you have done.

 

Along the lines of the McKay re-enactment of Tesla's "radiant matter" pressure phenomenon:

 

I would like to know, and these are suggested tests that might be performed:

 

1. Whether it is a measurable mechanical force, or the mere sensation of pressure induced in the human body. It should be easy to "weigh" the pressure with a mechanical balance or spring.

 

2. Whether the force extends equally in all directions from the source. For example, might it be greater or smaller, from gently curving/convex surfaces as compared to sharper-curving parts of the apparatus?

 

It's not hard to see where I'm going with this: The pressure might be that means of propulsion described by William Lyne in his books.

 

The result of test 1 will indicate whether further enquiry is justified (although, it still might be interesting from a biological perspective).

 

If, after determining whether the pressure is a mechanically measurable force, then the result of test 2 will indicate whether it can be directed and thus used as a means of propulsion.

 

Then the men-in-black come to shut down the site and take us away. ;)


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#11 jimm

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Posted 18 August 2013 - 05:47 PM

Thanks John, for your fine example. I wish I could emulate what you have done.

 

Along the lines of the McKay re-enactment of Tesla's "radiant matter" pressure phenomenon:

 

I would like to know, and these are suggested tests that might be performed:

 

1. Whether it is a measurable mechanical force, or the mere sensation of pressure induced in the human body. It should be easy to "weigh" the pressure with a mechanical balance or spring.

 

2. Whether the force extends equally in all directions from the source. For example, might it be greater or smaller, from gently curving/convex surfaces as compared to sharper-curving parts of the apparatus?

 

It's not hard to see where I'm going with this: The pressure might be that means of propulsion described by William Lyne in his books.

 

The result of test 1 will indicate whether further enquiry is justified (although, it still might be interesting from a biological perspective).

 

If, after determining whether the pressure is a mechanically measurable force, then the result of test 2 will indicate whether it can be directed and thus used as a means of propulsion.

 

Then the men-in-black come to shut down the site and take us away. ;)

 

Robert,

 

The "pressure" is a dielectric/electrostatic effect as seen in the old Borderlands video where Eric charges a cap with it.

I worked with some interesting electrostatic devices back in the day which caught the attention of the late Charles Yost  of "Electric Spacecraft labs" and was invited to work with him, so I have some knowledge of these effects. Unfortunately the foundation died a few years after he did, but there is still a webpage: http://www.electricspacecraft.com/

If propulsion was your thing, he was "the guy".

 

Just as Tesla used a saline solution in wine bottles to create high voltage capacitors, the electrode in water ( even tap water has some conductivity) will set up a opposite charge on the outside of the glass if it is not "terminated" , so it becomes an "isotropic" capacitor instead.

 

None of this stuff is particularly  mysterious and is still within the boundaries of "mainstream science" , but there are some ***VERY*** creative things that can be done with it. That is all I am going to say...


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#12 G4ΓΓ3ττ

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Posted 18 August 2013 - 06:16 PM

Hey Jimm,

 

Do you happen to have any of the E.S. Journals, or know where one might find them? I wouldn't mind giving them a glance if I could get access to them. Btw, that's a pretty cool project to have been apart of! How long were you working there before they shutdown?


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#13 jimm

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Posted 18 August 2013 - 08:10 PM

Hey Jimm,

 

Do you happen to have any of the E.S. Journals, or know where one might find them? I wouldn't mind giving them a glance if I could get access to them. Btw, that's a pretty cool project to have been apart of! How long were you working there before they shutdown?

 

There used to be an archive but I don't know if it's still up.  I'll look and see if if I can find any copies around the house. I know I must have at least a couple. A lot of stuff got thrown out when I moved to the new house.

 

I never worked there physically as an employee, but was invited to participate ( which he also did with a few others whose work interested him) so we corresponded and worked on ideas back and forth. He was a great guy and we had many interesting phone conversations about the projects, bouncing ideas around.


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#14 jimm

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Posted 18 August 2013 - 11:19 PM

Take a look at the CIG schematics that were supposedly drawn by him. He has what should be RFCs  noted as lamda/4 ... NO!!!! The driver stage before the 4-250s is way over powered.  None of the component values are worked out either. There are other details that are missing altogether.

Then he leaves John P, in a lurch to figure it all out himself and to build. I suppose EPD will claim credit for it when it;s done, too.  Sorry, but I'm not impressed with the man, OK?


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#15 Raui

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Posted 19 August 2013 - 12:03 AM

Robert,

Jimm's reply to you in my mind had substance, he explained that the pressure is reportedly a physical effect in a dielectric medium which also effects how you'd attempt to measure the effect and gave you a reference to some electrogravitic material. While we would all like to see some of Jimm's handiwork it's not your job to go around calling him out on every post he makes just because he supports scientific ideas, which really isn't a bad thing.

 

I have asked that you both take personal issues off the forums, this will be the last time I bring this up before I change my approach. All posts which I feel are non-constructive have been deleted and others modified. Any issue with this action can be taken up with me over PM as this is a thread on John's CIG build.


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#16 Raui

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Posted 19 August 2013 - 12:13 AM

Garrett,

So the grid resistor is like the base resistor of a transistor, makes sense. I really should have thought of that :D

 

Gestalt,

I really liked that video too, impedance mismatching and reflections never seemed so straight forward until I saw a demonstration of a wave machine in a physics lab. Garrett originally found that video and posted it up on one of the Eric Dollard threads at the EF I just thought it had relevance here.

 

Also I would like to point out that had a brain burp and was wrong, the grid resistor does not act as a current limiter since it does not effect the current going into the grid.


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#17 G4ΓΓ3ττ

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Posted 19 August 2013 - 05:04 PM

 

Part of the "power supplies" includes the filament supplies for the tubes. The objective is to make the filament voltage adjustable as well for two reasons:

1. To slowly warm up the tubes and prevent a large surge of inrush current.

2. As the tubes age the filament voltage can be turned up a little bit to boost the emission of the cathodes.

To provide this adjust-ability rheostats will be placed on the primaries of the filament transformers.

 

John,

 

Use NTCs (negative temperature coefficient resistors) for inrush current limiters. These are cheap and are much simpler than the rheostat. They start out at like 10ohms and drop to around 100miliohms or so, once they heat up. Also, a timed relay wouldn't be a bad idea if you wanted to let the tubes warm up first, they make these in octal pin bases. You would use it on the primary side of the anode and screen power supplies--that way the filaments can warm up first then the other supplies can turn on. Finally, a current source can limit inrush current and can precisely control filament current--this is much, much better than the rheostat or NTC. It acts in a similar manner to the rheostat, but its resistance is dynamic and needs no human adjustment. Though you could add an adjustment knob to increase current for the tube wear as you mentioned. The current source can be made DC or AC but DC is the best for simplicity.


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#18 johnpolakowski

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Posted 20 August 2013 - 07:39 AM

John,

 

Things are looking great! I can't wait to see the finished project--btw I love the ginormous thyratron and Eimac tubes!

 

I believe the grid resistor is a "pull down" resistor. It keeps the grid tied low, as opposed to floating--floating is bad. This is common practice on BJTs and MOSFETs. Basically, when the applied input signal goes away the resistor drops the grid potential back to the off position. Though it may need to be tied lower than 0V--probably around -10V or so. Depends on the tube and the aplication. For amplification, you look for a spot that has good linearity and gain and use that as the bias point. For pulse application you want to have it cut off conduction completely. For the amplifier circuit, you would probably want to use capacitive coupling to the grid, so as to keep any DC offset currents from altering the grid's bias point. For good linearity, anode potential and grid bias need to be chosen carefully, otherwise you will add unnessisary distorion.

 

If the tube uses a shield around the filament you should draw that in as the cathode terminal. Direct filament tubes do exist though. Also, your drawing of the MOSFET is confusing: you should point out the drain, source, and gate. As well as use the standard symbol for it. Is the keying circuit just an on/off switch or does it need to be linear? If its just a switch you don't need the bias resistors, an opto coupled fet driver would be simpler. Finally, if you are trying to key the tubes on and off, wouldn't putting the fet directly in series with the cathode's connection to the annode supply's ground be better? You would want to leave the grid's circuit ground connected directly to the cathode and also not interrupt the heater supply.

Hi Garrett,

Thanks for the response. I'm not sure, but it seems in my application since I will be using a negative bias supply in addition to the driving signal, I would not need the pull down resistor- is that right? Because the negative bias supply already keeps the grid from floating, it keeps it negatively referenced to the cathode. Is a pull down resistor used when there's no external bias? 

 

This will be a pulse application, the tubes will be operated class C which I think is a conduction angle of 150 degrees (of 180) or less per tube. Initially, I don't think distortion will be a factor since we are not trying to audio modulate the signal, just looking to drive the Tesla coils. I believe the terminology for it is "hard switched tube" - running the grid bias much more negative than is required to cutoff plate current.

 

The 4-250's are directly heated, the cathode is the filament. I'm not 100% sure but I think the 829B's may be the same. In my next version of schematics I will include the correct symbol for the mosfet. I can't say with certainty but I think the fet will just be used as an on/off switch. There won't be any intermediate function. I don't know much about fet drivers, but I will have to start investigating that when I get to that stage. I could probably use some help or direction when I get to designing the circuit for it! :) Also I agree completely, the mosfet should interrupt the hv supply connection to the cathode and nothing else. The grid bias should definitely not be interrupted! Obviously this area of the design needs further work. But that's why I posted it, to get the excellent feedback and opinions of you guys, thank you!


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#19 johnpolakowski

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Posted 20 August 2013 - 07:49 AM

Take a look at the CIG schematics that were supposedly drawn by him. He has what should be RFCs  noted as lamda/4 ... NO!!!! The driver stage before the 4-250s is way over powered.  None of the component values are worked out either. There are other details that are missing altogether.

Then he leaves John P, in a lurch to figure it all out himself and to build. I suppose EPD will claim credit for it when it;s done, too.  Sorry, but I'm not impressed with the man, OK?

Hi Jimm,

I appreciate your opinion and can see why you feel this way. It would seem I'm "in over my head" right? Well it was done purposefully. Everything I've done with Eric he has never just handed me the answer. He's always given me a direction to go and then let me go work on it by myself. That way I actually learn, otherwise I would just be a robot copying his design. His goal is to make me actually learn this stuff in the process. By having to try to figure this stuff out, and think hard about things, I learn a lot more than if I were to just go and build it according to blueprints. So Eric usually gives me a push in the right direction and then if I get stuck on something he gives me gives me some pointers and then I go on and work on the next thing. But yeah it can be frustrating when you get stuck!

 

I'm not 100% sure why he chose to use 1/4 wavelength chokes as RF chokes but I'm sure there is one. I'll ask him next time I speak with him. I asked Eric about the driver stage- it is there for future audio modulation. So the audio signal can be input at the 829B's and not the 4-250's. Otherwise yes the 4-250's could have been driven directly. 

 

What other things are missing? I mean I would prefer to look at things in a constructive manner rather than - "shoot it's a crappy design, stuff's missing, and Eric's not here, I should just give up"


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#20 johnpolakowski

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Posted 20 August 2013 - 08:39 AM

John,

 

Use NTCs (negative temperature coefficient resistors) for inrush current limiters. These are cheap and are much simpler than the rheostat. They start out at like 10ohms and drop to around 100miliohms or so, once they heat up. Also, a timed relay wouldn't be a bad idea if you wanted to let the tubes warm up first, they make these in octal pin bases. You would use it on the primary side of the anode and screen power supplies--that way the filaments can warm up first then the other supplies can turn on. Finally, a current source can limit inrush current and can precisely control filament current--this is much, much better than the rheostat or NTC. It acts in a similar manner to the rheostat, but its resistance is dynamic and needs no human adjustment. Though you could add an adjustment knob to increase current for the tube wear as you mentioned. The current source can be made DC or AC but DC is the best for simplicity.

 By NTC's are you referring to thermistors here? I think that's what you're referring to. For the moment I'm keeping things simple and have separate switches on each supply for the filaments and for the hv, so everything will be done manually, but that sounds like an excellent future modification.

 

By a current source what exactly are you referring to? I mean that's essentially what a rheostat is doing- regulating the current flow into the filament transformer primary. Or are you referring to a transistorized version with feedback? I guess if it's not too complex I would be game for it. Are you suggesting regulating the filament current on the primary side or on the secondary side? Reason being is that the filament currents are quite high - for the 4-250's they require 15 amps per tube. So that would be a lot of diodes if using DC and regulating it on the secondary side of the filament transformer. 

 

Also, I'm still not completely squared away on the grid resistor. It seems it can be used to generate the tube's own bias via grid current flowing- the voltage drop across the resistor creates the bias. Same thing could be done with a cathode resistor. Is the resistor necessary in an external bias situation though? 


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