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First 833c VTTC, 99% complete. Requesting tips for tuning and safety. Pictures

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LAN_403

Down with Umbrella

Tue Nov 01 2016, 01:08AM

Registered Member #54480 Joined: Sun Feb 15 2015, 01:02AM
Location: Kennebunk ME
Posts: 7

Hey folks,
I know there is an excellent knowledgeable community here. I started out building a decent SGTC, then I moved onto solid state. That was fun, Next on my agenda was a VTTC.
I've researched and read many articles and decided to loosely base mine off Steve wards vacuum tube five schematic.

SAFETY CONCERNS:
This is my first MOT project, I know unlike an nsts they can produce >2000V especially after the level shifter diode @ possibly .5-1A. Then the mo capacitors store a lethal charge too.
I'm using one large copper grounding buss that is connected to mains ground. Excuse my novice experience but is the buss dangerous? Which capacitors in the build will store the power if there is no oscillation.
In what scenario will the tube be at risk?
Parts list:
-The tube is a Taylor 833C from amazing1 where I also sourced the 3 doorknob caps. 3X 1000pf 30kV for a measured capacitance of 3.5nf.
-1200W mot from an old microwave, hope it works ok..
-2 mo hv capacitors for a total capacitance of 2.34.
-1amp hv diode(lever shift) Sourced off eBay.
-2 .001 ceramic disk caps across that diode.
-The primary has 30T of stranded 10g wire with taps from 20-30.
-The secondary is 1624T of of 30g magnet wire, resonance at approximately 300kHz.
-choke to grid 50ohm 10W resistor 24T of 22g hook up wire.
- grid leak circuit- 1 2000pf 30kv doorknob.(eBay) plus 1 150W 5kohm variable resistor.

Feedback coil, 30T of 22g solid core hook up wire.

-the filament heater, core from surplus mot. 10T of 10awg primary wire. Rewound. It hums a little and after I scoped the secondaries the waveform was meh but not super clean. I suspect it will be ok. 10.52v then I used a variac to dial in exactly 10v(In retrospect I should have used this mot. It was salvaged from a 1600W microwave and the geometry of the core was more difficult to wind.) most spec sheets for this tube state 10vac/10A supply thought it draws only 4.29A

At the moment it's built but I have not flipped the switch and am reviewing my schematic.
Any suggestions on start up procedure? I know I might need to flip the primary.

YouTube video of the walk down.

Photos
Two separate plugs, one controlled by the variac with the grounding buss and the filament supply mot has its own 120mains and ground.
https://4hv.org/e107_files/public/1477961911_54480_FT0_img_6163.jpg][/link]

Top view of lay out

Polonium210

Tue Nov 01 2016, 02:52AM

Registered Member #3450 Joined: Sun Nov 28 2010, 05:01AM
Location:
Posts: 61

Looking good!

Can't wait to see it running!

Down with Umbrella

Tue Nov 01 2016, 11:52AM

Registered Member #54480 Joined: Sun Feb 15 2015, 01:02AM
Location: Kennebunk ME
Posts: 7

Tonight is the night! It's gonna happen. Something will...

Down with Umbrella

Wed Nov 02 2016, 11:53PM

Registered Member #54480 Joined: Sun Feb 15 2015, 01:02AM
Location: Kennebunk ME
Posts: 7

Gahhh, first light then spudder-spudder-quit :(
Here's the video. Shouldn't have powered up to 70% during this phase of testing.

Edit, we're back in business. Just blew the level shift diode. I ordered a 6amp diode off HVstuff.com we'll see how that goes..

Bud

Tue Nov 08 2016, 12:00AM

Registered Member #54930 Joined: Tue Apr 28 2015, 01:38PM
Location: pittsfield ohio
Posts: 25

I just got thru with building an 833c tube coil. Also a dual 833A coil. Blowing the diodes was always a given. To avoid this make darn sure the bypass cap is big enough to withstand the kickback from the collapsing field. Both size and voltage. I did blow a level shift diode the other day and I paralleled 2 together and it works fine. I think they are only good for 350ma @12,000v? You might want to add taps to the primary to adjust the tuning easily. So far you are on track....at least it fired. Keep going and experimenting. I have done this for 10 years on my first tube coil. Finally got the hang of it. Here is a link to my youtube video on the dual 833 coil.

Bud

Tue Nov 08 2016, 01:31PM

Registered Member #54930 Joined: Tue Apr 28 2015, 01:38PM
Location: pittsfield ohio
Posts: 25

Down with Umbrella

Wed Nov 09 2016, 07:32PM

Registered Member #54480 Joined: Sun Feb 15 2015, 01:02AM
Location: Kennebunk ME
Posts: 7

Hey thanks bud, excellent video, I plan to watch more of it after my shift. I do have primary taps from about 25-30T.
The new diode is on the way but just to tune I used an extra diode from the microwave. Much to my amazement that little diode is still going strong! I'm definitely pulling >350ma.
My meter was bouncing between 20-22A current draw from the wall. (Hm not sure why mains breaker didn't pop) my poor variac probably hates me.
Here's a full power shot.
Also I have a much more robust MOT. It older with thicker secondary windings. 108ohms of resistance vs 86ohms on the first mot. Shunts are still in place.
The poor plate is turning slight orange. I'm hoping to mitigate the stress on the tube with further tuning and adding a fixed 5kohm and using the 5k rheostat to trim.
I have a mo fan blasting on the back side of the tube.
Is there an optimal angle of air flow?

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