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Arcstarter

Sun Jul 20 2008, 03:44AM

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

I am going to be getting an 833c(Thank you Chris!!!) soon and i figured i would start working on it and i also figured i would begin my project thread. Today i made the filament power supply transformer. It is just a simple rewound MOT, but it had a slight problem. After about one minute of running with no load the primary was too hot to touch. I rewound it with 10 turns of wire that seemed to be the same thickness. It helped alot and the heating has decreased significantly. Now i can run it for 2 minute no load and it is only lukewarm. I have about 12-15 turns of 14 gauge(i is a very small mot!) and it puts out 10.3 volts. I am hoping for 10 volts under 5 amp load(the 833C).

Here are the pictures:

I am planning on having it all built on something like either a 1x12 or just plywood. I am going to have 3 different tiers. Bottom will have the power supply transformers and switches and stuff, the second will contain the 833C and all of the circuitry and the top shelf will have the primary and secondary. I will likely put some cheap casters on it so i can easily wheel it around, away from the house so i don't set the burglar alarm off like i did with my 811A vttc :P. The reason the filament supply mots have those extra wires coming from the plug is to hook to the plate supply mot. I am going to plug it in and the filament supply turns on and then i flip a switch and the plate supply turns on so i can allow the tube 11 seconds or w/e to warm up. I am going to make some money and buy 120 feet of 12 gauge wire for the primary and i have some wire i am going to use for the feedback. It will be wound on a 6 inch form and the secondary will be my 4 inch by 16 inch. I know this may be a little too small, but i believe it will be fine. Chris used a 4 inch by 19 inch so i should be fine :D.

I will start trying to find wood tomorrow cause i have no money :( and i should have the supply transformers mounted after i find some wood.

Dr. SSTC

Sun Jul 20 2008, 09:57AM

Registered Member #1407 Joined: Fri Mar 21 2008, 07:09AM
Location:
Posts: 222

wait so you only have to warm the plate up for several seconds and then turn the supply of and the coil can start

Arcstarter

Sun Jul 20 2008, 05:08PM

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

jovica93 wrote ...

wait so you only have to warm the plate up for several seconds and then turn the supply of and the coil can start

No, you have to turn the filament supply on and let it warm up for several seconds and then you can start the plate supply, but you must keep the filament supply on.

Dr. Drone

Sun Jul 20 2008, 05:57PM

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

Arcstarter

Sun Jul 20 2008, 08:13PM

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

THANKS!! I already know about not moving the filament, but the rest was very educational

. A few questions. Were do you get a variable resistor strong enough for the feedback capacitor bleeder? Also, how should i start the filament up? Should i get something like a soft start thing? Or just start it slowly with a variac? I am about to go put the filament transformer under a 10a load and see how the voltage decreases. That is kinda weird, 100 watts for just the filament

.

Also, where should i get the tickler wire? And, i have no way to check the running freq so i will have to guess by overall performance.

Arcstarter

Thu Jul 24 2008, 07:13PM

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

Well, i added a few turns to the secondary on the filament supply mot, and i tested it under an 8 amp load and i got 10.3 volts, hopefully i will not have to add another turn to the secondary to get 10 volts under 10 amps. I also made the base for housing the tube, circuitry and power supplies as well as the primary and secondary, so everything :P. All that is left to do with the base is stain, polyurethane and add the casters or whatever they are called

. I also took a picture of the secondary and the base.

The secondary. I need to get something to varnish it with. Would regular polyurethane work? I covered it in tape because i didn't have it varnished and the windings started to overlap. Stupid on my part...

The 'base' or whatever you want to call it. I settled on two tiers. The bottom will house the supply transformers and the 833C as well as all of the circuitry. The top will just have the primary/secondary. Just stain and polyurethane for protection, perhaps sand it and add the casters. It can hold up a matthew so it is quite strong

. I used two pieces of 5 ply and 4 1 1/2 by 5 1/2's.

Dr. Dark Current

Thu Jul 24 2008, 07:29PM

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

Knock the shunts off that MOT; you'll get better load regulation (less voltage drop under load).
The no-load input current may get a little higher, this can be fixed by adding a few turns on the primary.

Tiberius

Thu Jul 24 2008, 09:58PM

Registered Member #1484 Joined: Wed May 14 2008, 03:24PM
Location: Cary, NC, USA
Posts: 27

wrote ...
You can run [the filament supply] a little over 10 volts, but never below!

Admittedly I'm not familiar with the construction of this particular tube, but I recently read a study regarding the filament supply of large transmitting tubes and it seems to be that the relationship between filament supply voltage and filament lifespan is inversely geometric. The study found that decreasing the filament voltage slightly below the specified voltage increased the lifespan significantly, while increasing the voltage resulted in similarly significant decreases in the lifespan. The determination was that decreasing the filament voltage to the lowest value where measured output remained within specifications, then increasing slightly to allow for aging, was the best operating practice. Is there any reason this finding wouldn't be relevant with the tube in question? I'll try to locate that study if there's any interest.

...

Fri Jul 25 2008, 01:32AM

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

that sounds about right, you will get more output with a hotter filament, but your tube won't last nearly its specified lifespan.

However, for this application where you are only going to run the coil for like 10-100hrs, it might be worth it to overdrive the filament just a tad. BTW, I tried running the filament in a damaged 811a that i had at 2x its rated voltage and even after a day of letting it run it didn't burn out.

Kristian

Sun Jul 27 2008, 02:35PM

Registered Member #611 Joined: Wed Mar 28 2007, 11:40PM
Location: Hudsonville, Michigan
Posts: 79

Hey Arcstarter,

I'm building an 833C VTTC right now also. I was wondering about this comment you made in your post.

Arcstarter wrote ...

Well, i added a few turns to the secondary on the filament supply mot, and i tested it under an 8 amp load and i got 10.3 volts, hopefully i will not have to add another turn to the secondary to get 10 volts under 10 amps...

I have wound a filament supply on a 1200W mot with the secondary removed and the original shunts in place. I tested it under no load. It measures 10.2 volts. I did not consider the voltage drop under load would be significant. Did you experience a significant voltage drop under load? How much? Did you remove the shunts or leave them in place?

Thanks,
Kris

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