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833C vttc problems.

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LAN_403

Arcstarter

Thu Feb 19 2009, 10:41PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

well, i put my vttc together again yesterday, with my new secondary. I got about 1 1/2 foot sparks, no staccato.

Well, after about a minute of running, my power MOT destroyed itself. so, i replaced that, and the diode, and then it didn't wanna work right. I had to put the diode in backward from the schematic to make it run, which made the filament transformer arc, and the sparks where small. Well, i now made a new filament transformer, with good insulation, and i connected diode as the schematic, and just reversed the polarity by switching the leads that go to the primary/tube filament.

Now, the plate heats up worse, sparks are smaller and they branch out less and are thinner. Also, there is much less of them, and it sounds different. The filament transformer puts out 10-10.1 volts when loaded with the tube. there is no arcing anywhere there shouldn't be, and the feedback is phased right. the grid leak resistors do glow red after about a minute though. They are rated for about 10 watts in all. I measured them, and they read 5.20 kohms, but if i reversed the meter leads they would read infinity. I am not sure why this is.

The tube grid looks just fine, but at the bottom of where the plate ends, and where the filament and grid are attached ( a little white thing), the filament looks brown and burnt. Not too brown, but you can easily see it. When connected to filament power, it lights up the same color and brightness, but i am thinking maybe somehow the tungsten got stripped.

By the way, thanks to Dr. Spark for the tube in the first place!

...

Thu Feb 19 2009, 11:35PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I would suspect your resistors, they aren't supposed to start glowing

If not it is possible you damaged the tube, but i do not see a plausable way for that to have happened because a tube run in reverse is just a diode...

Dr. Drone

Thu Feb 19 2009, 11:55PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

Arcstarter

Fri Feb 20 2009, 12:08AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Resistors are 5k, cap is 1950pf. I have no resistors, time to find some cheap ones.

BTW just went to run it, and every time i start up, the spark length is different. I suspect there is bad feedback. When there is nothing grounded near it, the sparks are usually about 4-5 inches, but when something grounded it above it the sparks can grow to about a foot. Also, sometimes there is hardly breakout. It must be those darn resistors. A minute ago, they got so hot the solder melted and they just kinda fell apart.

Dr. Drone

Fri Feb 20 2009, 03:32AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

Arcstarter

Sat Feb 21 2009, 12:32AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Found the problem, and boy am i stupid! I connected doubler wrong, i think. I cannot test because i just blew everything that could be used as a resistor :P. Haha. I grounded the tube to the anode of the diode instead of transformer ground. boy do i feel stupid.

Hard to imagine it still have 1 foot sparks when a grounded something was near it :P.

So i am out of resistors for that coil, and the little one i made today, but oh well. at least i found out what was going on.

Dr. Drone

Sat Feb 21 2009, 01:45AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

Arcstarter

Sat Feb 21 2009, 01:57AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Thanks alot Chris, i sure appreciate your patronage.

As for the feedback (grid leak anyway), is 650pf not too low? If it is too low it will hold the tube off to short and the duty cycle will increase, causing higher current to flow. Steve Ward used 2nf on his single 833a, but those have higher dissipation ability. I don't know, just a though.

Dr. Drone

Sat Feb 21 2009, 02:53AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

Arcstarter

Sat Feb 21 2009, 05:28AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Dr. Spark wrote ...

Here is the secret of getting max out of 833

Built about 30 or so 833 VTTCâ€™s and have tried many, many configurations @ just a few grid feedbacks coils..heehee ..yep anal big time!

833's like a Fo of 300 to 400 khz on the secondary. More to the 400Khz as this well known in the Ham radio ring from the 40s.

25 turns on the grid feedback.

27 to 35 turns on Primary taped at 29 turns to start.

4 inch Secondary
6 inch primary
6 or 8 inch topload.

Tank cap of 1000pf 80kv (2 doorknobs)
Grid cap 600 to 700 pf
Resistor 5K ohm taped at 3.5k

BTW beat Steveâ€™s spark length on VTTCâ€™s 6 years ago @ grid cap was 200pf

First VTTC was in 1972 for Physics class.

Cheers,
Ch

EDIT: RF Bypass cap 10x tank cap min!

Well, good on the filament bypass, as i am using .22uf. Do you think 2nf would be too much for the tank cap? I don't have anything but that one, so i am kinda stuck with it. I know that tank capacitance can be made to run right with change in inductance AKA tuning. But, if anyone knows, it would be you that certain tubes like certain things. Not so great on the freq, as the coil by itself is 294khz. With the topload i have it is around 230khz. When i add the topload, i can sure see the spark get smaller, even if tuned.

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