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I want to build my first VTTC...

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LAN_403

Killa-X

Sun Jul 01 2012, 05:54AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

It's true that I have never made a vttc in my life. I've done sgtc, sstc, and drsstc. Well, I recently bought the GU81M tube. As it slowly makes its way into the USA I want to know, how can I test the tube and make sure the thing actually works when it gets here?

I've been researching as far as schematics go...

Most the parts I have, except the high power resistors and the inductor that goes to the tube from the upper tuning coil. I also lack the capacitors, but I may have some around, got some 2000v 0.1uF code capacitors, if those work.

But, any suggestions or tips? Won't be for another 2-4 weeks until I see it but, I never made one...not really sure how to besides schematics and all that. I've done only solid state coils really. What are some dos and don't about tubes that I should generally know? I notice the guy uses a soft start on the tube because it makes the filament last longer...I can do that, I have a 1-60 second time delay made just for doing that, giving a 30 sec delay for the filament to heat up, them tick, full power.

Thanks! Can't wait :)

kilovolt

Sun Jul 01 2012, 08:56AM

Registered Member #2018 Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74

Hello Killa-X

I would recommend to use doorknob ceramic capacitors for the tank circuit. They perform very well for VTTCs. I made my first VTTC with WIMA MKP-10 capacitors, but the output wasn't very well.

It is also recommended to make sure the heater voltage of the tube is as precise as possible for a long tube life. For this, I took an old toroidal transformer with 15V and rewound it to 12.5V under load. To be sure, that you don't overload the filament of your tube while trying to wind your own filament transformer, you could take a variac to slowly increase the main volatge of the filament transfomer and measure the output voltage under load on the tube filament.

In the first moment of turning on the VTTC, the filament should ony see about half of the filament voltage to heat up slowly, because turning on and off puts stress on the filament. I made this with a 1Ohm resistor in series with the filament, which is bridged after a while by a relay.

For the grid leak resistor you need something with a good power rating (about 100W). If you don't find a resistor, you could also take a light bulb for this.

You should plan enough space between every component. VTTCs tend to arc over everywhere (that was actually what I experienced with my fiorst setup).

If you are interested in my Dual GU81M VTTC, you could take a look here:

I wish you good luck and nice streamers

Best regards
kilovolt

Paul_J.

Sun Jul 01 2012, 03:57PM

Registered Member #5436 Joined: Sun Jun 24 2012, 01:44AM
Location:
Posts: 39

Killa,

I'm the opposite from you. I have only built vt tesla coils. I'm not too familiar with this tube but this is what I would do to burn it in before a heavy tesla coil work load.

Heat up to rated filament voltage. Leave it running for about a day, cooling if needed. Now after many decades of storage, some gas must have entered. To combat this, most high power tubes have a enter on the plate itself, opposed to one on the glass, to prevent arcover. To get rid of the gas, we need to activate the getter, which means heating it up to very high temperatures.

Connect MOT to varaic with high power lightbulb. Look at data sheet to find optimal voltage for screen (2nd grid) supply, usually around 600 volts. Create variable negative to positive bias supply for control grid. Apply plate voltage, raising it slowly looking for tube trouble. Keep control grid negative. Slowly raise voltage on control. Now plate current will rise and the plate will get red-orange. We want that, as that will activate the getter. At this time you can take current ratings and compare them to the graphs on the datasheet. Continue until you feel omfortable, but I would do it for at least 3-5 hours. Make sure you have cooling, or you will melt the solder on the tube pins.

I'm only a hobbyist, so all specifics are from experience and may be total rubbish. I'm sure others will have more info.

Killa-X

Sun Jul 01 2012, 05:43PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Nope, you did fine sir. I will take any information on proper tube running/testing.

Looks like this would be the datasheet.

Now, as far as filaments go. If its 12.6, i take its not good/smart to run it at a solid 12.00 on a LM7812 w/large filter capacitors? Though after reading that would be bad..11A plus regulator rated 1A.. POW

Also, yes, grid 2 voltage is 600V, anode 2KV.

Paul_J.

Sun Jul 01 2012, 10:16PM

Registered Member #5436 Joined: Sun Jun 24 2012, 01:44AM
Location:
Posts: 39

Hi,

I looked at the datasheet and good news! (no not professor farnsworth good) it has a tungsten uncharted filament, which basically means its a lightbulb. This means that it is very rugged.

1. You do not need a DC supply. A plain old rewound MOT would be fine.
2. To clean the filament of impurities, just run it at higher voltages. For about 30 sec- 2 min, run it at 12.5-14 volts, about white hot. VERRY IMPORTANT TO MAKE SURE NOT TO RUN IT TOO HOT. This is the most dangerous part of the process.
3. It is better to run a filament too high than too low. About 10-20% is the maximum.

Thanks,

Paul

Ps use common sense! Most of these figures are from the top of my head.

Dr. Dark Current

Sun Jul 01 2012, 10:41PM

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

The GU-81 has a standard thoriated tungsten filament, like 99% of other directly heated tubes. No need to run the filament at a higher voltage, just leave it turned on overnight, this activates the getters which might clean up any residual gasses which could possibly appear in the tube after some time.

You can tie all 3 grids together and use the tube as a triode - with similar circuits like the 833A, etc. The tube actually runs pretty well in this mode (NOT true for many tetrodes/pentodes, where tying the grids together will result in a relatively weak output).

Paul_J.

Sun Jul 01 2012, 10:48PM

Registered Member #5436 Joined: Sun Jun 24 2012, 01:44AM
Location:
Posts: 39

Hi,

Sorry, I stand corrected. I must have misread the datasheet. I was thinking of a pure tungsten filament. However, I would still recommend using it as a pentode compared to a triode

Killa-X

Mon Jul 02 2012, 03:15AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Well, my only mot I had that I would wind, was shot primary. For 20 bucks I bought a vintage ham radio tube transformer rated for 12v 12A output. I'll check the voltage of it open and loaded before plugging it to the tube. If its too high, easy to fix :). Never hurts to get a filament transformer...6 pounds of joy haha :D

2Spoons

Wed Jul 04 2012, 12:26AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

I too have a GU81M on the way. Only difference is this will be my first TC ever.
Doing a lot of reading up on valve theory and found a great resource for old, out of copyright books:

Understanding what is going on in a valve helps a lot with understanding why the oscillator circuit is the way it is, and helps with debug and fault finding!

Killa-X

Thu Jul 05 2012, 10:14PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Got the transformer today :) Has a strong vintage smell!

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