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4hv.org :: Forums :: General Science and Electronics
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Desperately need help with base fed vttc.

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Arcstarter
Wed Dec 22 2010, 01:01AM Print
Arcstarter Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I am trying to get this 'plasma tweeter' to work with my GU-81M. I thought the tube was dead, but i put 2kv on the plate and variable 0-100v (using a pot and DC supply) on grid1 and grid2 and monitored the current on the plate and it seems fine... Thank God.

Anyway, i will attach the schematic, it is a basic plasma tweeter schematic. I used this exact schematic with the same resonator but a different tube and it worked fine.

The screen supply is 535vac halfwave retified, and the plate supply is a MOT halfwave rectified. I scoped the plate current with a shunt resistor, and saw 500ma peak, which is 176ma RMS. The control grid in this case was at 0v.

What can i possibly do to get this thing to oscillate? I have no other information to give, seeing that this circuit is so simple. The only thing i can figure is that the control grid is getting no drive, which is obviously true considering the 0v potential... This is exactly what happened the last time i attempted this circuit with this tube.

EDIT: I forgot to say a few things. For one, the control grid pulldown resistor was 20k, not 39k, and i have even tried without it. I have also tried a fairly large aluminum plate for capacitive pickup instead of a small loop and still nothing happens. My resonator is about 5MHz res freq, not that it matters. Also, idk what the heck happened to the schematic, it was not fuzzy like that before, but here is another schematic from someone else which is the same other than some extra filtering.

Link2

And, another update, fixed oversized image, and i tried a magnetic feedback with a coil around the resonator and it *still* did nothing, so i think i will just give up, unfortunately. I am completely out of ideas now!
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ubuntupokemoninc
Wed Dec 22 2010, 02:58AM
ubuntupokemoninc Banned on 1/22/2011 for repeated rule violations after multiple warnings.
Registered Member #3299 Joined: Sat Oct 09 2010, 08:11PM
Location: Bantown, USA
Posts: 220
dude i could never get my 811a's oscillate and i found out there is nothing wrong with them.

I am praying that my GU-5b works

What if you wired the tube as a triode? if you did does it matter witch grid you use?

is it possible for a tube to be electronically good, but it still decides to play dead?
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Weston
Wed Dec 22 2010, 04:36PM
Weston Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
Good to hear you gu-81m is finaly working.

Could you provide pictures of you setup? I remember my 4-400a plasma tweeter being finicky to the layout and the position of the feedback loop. Have you tried taping the output terminal with a peice of metal that is well insulated from you while the setup was powered? A lot of these plasma tweeters need a little extra capacitance to get oscillating it seems.
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Arcstarter
Thu Dec 23 2010, 12:00AM
Arcstarter Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Ubuntu, if it is good electrically, there is nothing else that can be wrong... Running a pentode as i triode uses all of the grids. The suppressor AKA grid 3 is always wired to the cathode, but the screen (grid 2) and the control grid (grid 1) are hooked together and used as a control grid.

What would pics help? I have done so many different things it would take an impractical amount of pics to show the assorted feedback schemes, dimensions, etc. The only thing you'd see with a pic is the tube, resonator, and three transformers.

I have tried everything, different resistors, loading the resonator with more topload... I suppose when i turn my plate supply on, the tube simply conducts and has no negative voltage on the grid to turn it off. I will try to touch the grid with some negative voltage and see if anything happens, or maybe even make an oscillator to attempt to get some feedback until it self-sustains.

Edit: Sorry about all the edits, but this is so very confusing and it is important to me that i find an answer, and i want to make sure i can include every bit of info that i have. Anyhow, this is a youtube video of the same circuit running very well. Link2 Please no comments about the flowers and soundtrack, but apparently it was no too bad, it is a featured video despite only 800 views.

The difference is that it is an 811A, IE a triode, IE less gain, so i used a single feedback turn straight to the grid for magnetically coupled feedback instead of capacitive. I tried the same thing with the GU-81M as mentioned before and nothing happened.

If a negative voltage on the grid is not enough to get it to oscillate, i will just run it as a tride with the screen and control grids tied together with magnetic coupling. If that still doesn't work, i guess that means the tube is not working right. I can clearly see all three grids, however, and they look like they are in mint condition.

Edit: I put 100v negative and 100v positive in relation to the cathode on the control grid, and the current it pulled stayed exactly the same... Dead control grid i guess? It may be worth noting that when the control grid is left floating the current is less stable than when it is at ground potential or 100v negative or positive.
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