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Simple VTTC questions

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Myke

Sun Jul 13 2008, 01:52AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Is there a way to know if your tube is arcing over internally without looking into the tube (filament is kinda bright)?

Marko

Sun Jul 13 2008, 02:31AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys,

Myke wrote ...

Is there a way to know if your tube is arcing over internally without looking into the tube (filament is kinda bright)?

If the tube is gassy and arcing you should see blue plasma in it, you could probably check it by bringing it near a Tesla coil.

Since I'm not really building a plasma tweeter (no audio modulation) I didn't need anything but triode, thus one grid is OK.

I just got completely carried away - I got the little ECC81 to oscillate and light fluorescent tubes - not close getting a breakout though, as tube has way too high output impedance for a weakly loaded TC and just drops most of the voltage.and results in very poor efficiency.

The circuit lived after I used a loading capacitor between anode and cathode, I used a 500pF variable cap. I found that only one specific capacitance works*best* and tuning up or down diminishes the output.

I think that would also solve you guys's problems.

Alignment of antenna was important, as well as inductance of RFC - smaller RFC tended to work better for high frequencies and vice versa. I tested the circuit from 500kHz to 10Mhz and could always get it to work more or less the same.

I also added a potentiometer to adjust best grid-cathode resistance.. some values again worked better than others.

I'd really like to play more but I just don't have any medium size tubes to try.

Marko

Myke

Sun Jul 13 2008, 02:55AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I don't have a Tesla coil to test it with.
Would there be any difference in current draw if it were arcing?

So I ran it with 3KV on the plate and found that it leaks 24mA...

Is this bad?
My supply is a MOT rectified by a MO diode so it can supply more than that.
I just hope my tube is ok.

Steve Conner

Sun Jul 13 2008, 10:45AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi guys

If that 24mA figure is with both grids grounded, I'd say that sounds just fine. A tube arc is a dead short circuit, so you'll probably hear bangs and popping breakers.

Marko, that's neat that you actually got a VTTC to work with an ECC81. Do you believe me about the loading capacitor now? :q

If you're looking for more information, the plasma tweeter circuit is formally known as a Vackar or Tesla oscillator. You can find design info on ham radio sites, since it's sometimes used in radio transmitters.

Arcstarter

Sun Jul 13 2008, 06:26PM

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

Well i tried it again with an 811a and nothing different happens with or without feedback. I figure maybe the feedback signal isn't strong enough?

Myke

Sun Jul 13 2008, 07:32PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Ok, so I tried putting a florescent tube near the output and it glows

I don't know why there is not breakout though.
I'll post a video later.

Myke

Mon Jul 14 2008, 05:53AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

UPDATE: Here is the video

Sorry that it's a bit staticy and stuff. I think I need a better camera...

(is this a bad double post?)

Marko

Mon Jul 14 2008, 03:09PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Myke, have you started to use a loading capacitor as we said? Do you have any doorknob caps around there?

Me and Conner actually wondered how the circuit works at all without it, probably only due to stray parasitics when in 10's of Mhz range.

Use a potentiometer between grid and cathode, a and maybe a small few tens..few hundred pF capacitor too. Changing the resistance will affect the circuit greatly.

Should I draw a schematic, or you guys know what you need to do?

Myke

Mon Jul 14 2008, 11:05PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I know what I need to do. I got breakout by touching the tube to the output. I'm guessing touching the tube to the output adds capacitance and then after the "flame" has ignited, it adds enough capacitance to remain lit.

I had changed the screen resistor to 10K and the grid to ground resistor to 82K. I also changed the plate inductor to 90uH.

I'm a bit afraid of using a doorknob cap because aren't those kinda lossy? Since it is resonating, what would the plate voltage be multiplied by to get the output voltage?

How would I go about measuring the frequency of the coil? An antenna a little ways away connected to an oscilloscope?

Marko

Tue Jul 15 2008, 12:06AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Doorknob capacitors are most efficient for the application, they are specially made for high RF power so I don't know why you think they are lossy.

A typical HAM tube amplifier would be exactly that same circuit, except output network is parallel instead of series to match into low antenna impedance.

They often use vacuum variable capacitors as loading caps in that case, but some doorknobs would probably be just fine.

According to Steve Conner the capacitor is calculated to be 1/10 tube output impedance (working voltage/max current) at working frequency.

You must also note that in any case the minimum voltage the tube will ever see is 2x supply voltage (required to keep RFC current from integrating); pi*supply voltage with perfect loading.

Without the capacitor I'm unsure what is happening, the voltage is probably going very high and causing losses in the tube.

Steve Conner implied that over voltage can be tolerated by tubes to some margin but I'd rather be careful.

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