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Kolas

Sat Jun 24 2006, 08:06AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

Hi guys.
Ok earlier I asked your guy's opinion on my big akward tube.
Seems you guys think the normal armstrong circuit will work!
Though the grid needs pretty exact voltage control! How do I assure the signal from the grid does not exceed the voltage I need?

Pretty vauge question?
Sorry.
From the load line, it looks like the grid voltage should only be 100Vpp

could clamping work?

I need help, thanks.

Gavin

Fri Jun 30 2006, 09:50PM

Registered Member #478 Joined: Tue Jun 20 2006, 11:57PM
Location: Shrivenham, United Kingdom
Posts: 8

Hi,
I am no expert at VTTCs, I am just starting out myself. As I understand it the grid resistor to ground regulates rather than sets the grid voltage. When the plate current increases, so does the grid current, and so the voltage across the grid resistance increases, thus reducing the plate current, and so the grid current. This constant self regulation keeps everything set just right.

The bias voltage its self is set for class C operation, i.e it is set so that the grid is quite negitive, the voltage actually swings slightly positive when the feedback voltage reaches its peak. This then allows for efficient operation, as the valve is not conducting linearly all the time, as found with class A.

Sorry if you knew all that, or if I didn't answer your question!

Gavin

Kolas

Sat Jul 01 2006, 12:52AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

Yeah kidna knew that, but thanks.
As it turns out, conner has helped me a tad and reminded me that the tank circuit can, and will competly controll curent draw. IE it dopenst mater how much the tube turns on, or off, the tank circuit is designed for an exact I and so, wont pull more then that

As soon as I get a piggy, this tube will take life!!

Kolas

Gavin

Sat Jul 01 2006, 07:42PM

Registered Member #478 Joined: Tue Jun 20 2006, 11:57PM
Location: Shrivenham, United Kingdom
Posts: 8

Hi Kolas,
perhaps you could help me, as I said I am new to VTTCs, I'm more orientated toward SSTCs.

My question is regarding the choke or LR circuit in the plate circuit, I don't quite understand its purpose. Surely this choke will hinder the generation of RF by the tube, as this LR circuit will hinder the flow of an RF current?

Thanks,

Gavin

Steve Ward

Sat Jul 01 2006, 08:26PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

My question is regarding the choke or LR circuit in the plate circuit, I don't quite understand its purpose. Surely this choke will hinder the generation of RF by the tube, as this LR circuit will hinder the flow of an RF current?

Exactly correct, it will hinder *very* high frequencies. Its pretty much transparent to the operating frequency. The reason for it is to snub out parasitic oscillations that may occur during a fault in the tube, and help extinguish the internal arc. Im told its an old trick used in most high power tube circuits.

As soon as I get a piggy, this tube will take life!!

Why dont you just use the big transformer you already have? I think you said it was 6kV or something? Sounds like plenty to get started.

IE it dopenst mater how much the tube turns on, or off, the tank circuit is designed for an exact I and so, wont pull more then that

I do not agree, but it does set a maximum current when the tube is in conduction for say 50% of the time (you dont want to exceed that, and i think the way the oscillator functions, you cant anyway). But right, the LC has a defined impedance, but you will still see a large difference in current draw at 10% duty cycle vs 30% duty cycle. The grid leak bias is what controls this duty cycle. The larger the resistance, the greater the negative bias, so the peaks from the feedback winding turn on the tube for a shorter period of time.

In any case, you do not need any sort of ballast, and in fact this would hinder performance as all it would do is drop the tank circuit voltage.

Kolas

Sun Jul 02 2006, 03:15AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

Well I follishly made this transformer, and I need 6A form it at 6200V. the transformer cannot supply this.

I'm trying to meet the load line specifications given by the tube manufacturer.

lalala I figured out how I can use an OP amp to get 30% of the positive wave of the feedback, so I might try to operate in class C with controlled Grid V

more on that later, Kolas

Steve Ward

Sun Jul 02 2006, 08:23AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Is the load line operation that crucial? Seems like it would certainly work fine at lower currents though at 6kV. You could certainly run a higher tank impedance and reduce the current draw to something more reasonable. If a pair of MOTs will do 2 foot sparks, im sure your big hacked up 3-phase transformer would do something pretty respectable. Since you already have it, there doesnt seem to be any reason not to begin testing with it. You could also do staccato or something and use some sort of storage cap. It shouldnt need to be more than a few hundred joules for even the juiciest VTTC sparks.

Kolas

Sun Jul 02 2006, 07:48PM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

Well the data sheet of the tube gave optimum conditions for Class C operation at 512 ohms. Any combo of that on the load line looks GREAT!

But much higher or lower kinda looks like shit...(poor grid voltage line interception)
I still have to weld the box together to make the plate transformer, I'm waiting on either a piggy, or feedthrough insulators, since I found a place that will give me some free. I need info from my pareants, heh.

Even though the tube looks hard to drive at this point, fear not, I will try.

Kolas

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