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filament supply for tubes

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LAN_403

M.A.D.

Fri Sept 30 2011, 12:12AM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

Hello, I have some tubes (rated for 4Kv) a MOT to power them and another small transformer that can supply the 12v 2.5 A needed for the filaments.

so here is the question, the cathode can only be 150v off of the filament so how do I keep the high voltage cathode supply from destroying the low voltage filament transformer? I do not know the exact ratings for the 12v transformer, however I do not think it would survive the secondary floating at 4KV for even 10 seconds.

dude_500

Fri Sept 30 2011, 12:49AM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

In a VTTC, the cathode is grounded, so it does not matter. Connect one filament wire directly to the cathode and then earth it (in many schematics the cathode and filament are even shown as the same element)

M.A.D.

Fri Sept 30 2011, 11:06AM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

I am not building A normal vttc, however, I think that grounding the cathode will still work. so thanks for the advice.

Also, I am going to need a drive circuit, which will undoubtably be far easier with the cathode grounded, it says that the grid needs 125v for complete shut off, so can I just make a GDT with a lot of secondary turns, or is there a better way of providing this voltage?

Dr. Dark Current

Fri Sept 30 2011, 10:02PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

It's an indirectly heated tube? Just connect one heater connection to the cathode. The grid drive is usually done with a separate winding and a self-biasing RC circuit.

M.A.D.

Sat Oct 01 2011, 07:02AM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

Yes, I think that I am just going to ground the cathode and the heater center tap.

However, I am not building a vttc because the tubes I have are pulse tubes and are not rated for high frequency. The tubes are indirectly heated double beam tetrodes. the grid needs -125v for complete shut off and the screen needs up to 800v of positive bias.

I do not know a lot about the supply and driver circuits of tubes. So what would be the best way to run the tubes at 40-400Hz with a pulse duration of no more than 5 microseconds?

Dr. Dark Current

Sat Oct 01 2011, 09:07AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Make a bias supply for the control grid and bias it to some -200 volts in series with a resistor; to turn the tube on just ground the control grid with a PNP transistor or a P-MOSFET. The drive circuit for the transistor is not hard to make.

Note: if you are pulsing a VTTC primary this way, you won't get any sparks.

M.A.D.

Sat Oct 01 2011, 08:33PM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

All right thank you.

I know that pulsing a vttc primary would not work, however I am going to use a tuned primary secondary like in a sgtc, and I am thinking about using as high of inductance as I can on the primary.

Also, does the grid bias need to be highly smoothed (I think I might just rectify mains), and is there a good way to provide the positive bias (the limit is 800v) for the screen without another transformer.

Dr. Dark Current

Sun Oct 02 2011, 03:54PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Use voltage multipliers for the grid supples, if no galvanic isolation of voltages isn't of concern.
If you want to replace the spark gap with a tube, you will need 100's of us pulse to transfer some energy to the secondary. A few uS is about the length of one rf cycle and you won't transfer almost any energy in one cycle.

M.A.D.

Sun Oct 02 2011, 09:15PM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

I am not directly replacing the spark gap. As the primary coil is going to have a tuned capacitor directly in parallel with it. The tube is just going to dump some power into the primary to start the resonance with the secondary.

I ran this in a circuit simulator and the primary cap actualy ran at about 7KV. However it seems that for this circuit to work it needs a relatively clean square wave from the tube.

astralhighway

Tue Oct 04 2011, 02:10PM

Registered Member #4107 Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53

Hello, you are really complicating things by using a beam tetrode. I have never seen a VTTC circuit that successfully uses one of these. Connect your beam tetrode as a triode and away you go.

Connect control grid 1 to cathode. Now grid two is your control grid - you have a triode! The simplest VTTC use zero bias tubes such as the Svetlana 572B. Not sure what biasing arrangements you will need for your triode, but Dr Kilovolt has outlined a fool-proof method.

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