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Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
Well, College bogged me a LOT and i wanted to get my VTTC slowly put together so its not laying all over my desk un-touched for months on end.
Awhile ago i made a thread on this and kinda ended it due to lack of parts. Now I have better parts, so Here's some information.
The capacitor seems to work best at turn number 22 The tube is connected at 35.
Not saying there's a hotter spot, but so far this seems to go well. The feedback coil consists of 24awg wrapped around the 4.5" tube 18 times. When i bring up the power, I get about 4 inches while drawing 3-4 amps. But out of no where, those 4" sparks flick now and then, and suddenly turn to squealing 1cm arcs. I turned it off, it wouldn't go back on. So I lifted the feedback coil by 1 inch. It turns on and the pops are gone, but now its too high and carbon tracks up the tube. My suspisions to fix this error, is to lower the feedback coil, and add more turns. I'm thinking about putting it below the primary, and giving it 40 turns, in hopes to fix my issues.
So. I'm looking for some feedback from you guys on what you think would be adequate for a GU81M.
Also, I need to know if the parts I'm using are fine. Here's the schematic with my exact values.
So yeah. It's getting there!!! Just .. very slowly, Not really sure if there's rules of thumb im not aware of but I'm happy to see the 90kVAr cap made a world of difference.
Things are very crude still, as I wanted to bench test all my parts before even considering building a base and making it better wiring...Id like to figure out the parts and everything before hand, build it better, then final tweaking.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You need a RF bypass cap in your "Not installed" spot. This will increase output and prevent the MOT from catching fire later.
Squealing arcs could be a form of "squegging". If tinkering with the feedback coil doesn't help, try experimenting with the grid resistor and capacitor.
Connecting all three grids together can sometimes overheat the control grid. In really bad cases the glow from the grid can be visible as it gets brighter than the filament. To avoid this you can connect it to the cathode and drive the remaining two grids.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Yes, what you absolutely need to do first is to add the cap Steve mentioned.
Then, I like to put the grid coil below the primary, but it will work just as well above it. For me it usually works best with high coupling to primary. Maybe your grid coil could use less turns and if you still have stability problems, try decreasing the value of the cap parallel to the grid resistor. Your grid resistor also seems to be a pretty small value...
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
I put a grid coil below the primary, and it has about 28 turns ( I didnt really count ). Now things are running fine, and its a matter of tuning. When I added a topload it didnt want to break out, I wasnt sure if this was because of my tuning ( i tried multiple taps ) or if its because a topload will go from OFF to ON from 0" sparks to 6" sparks, instead of grow.
So. I put 22awg steel on the breakout as a test and kinda hit the limits, it started to melt and i didnt want to risk fire. So far, 5 inch sparks at 50V 6Amps. Easily could have done higher. I know if i change the tap of the capacitor, it will draw more amps at a lower voltage, and grow longer sparks at a lower voltage.
A very jank setup but it gets the job done.
I'm thinking though, now that it can run without problems, its getting near time to tare it all apart and build it for real...
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Back to work on my GU81M - Advise wanted
