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The 13,56Mhz Vttc

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LAN_403

Reaching

Sun Nov 25 2007, 04:41PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

Cause im into amplitude modulation for a while, i thought: why not build a vttc with a high power tetrode to provide modulation through the g2..

So i searched the web for infos and found a nice german site form a university.

the vttc circuit discribed is completely differend to the regular found on the web. the circuit is fully selfresonant, so you have everything in tune at all time without changing caps etc.

2: you dont need a primary coil, the power is fed into he "secondary" coil directly at the bottom .

after looking for parts in my room i had everything thrown together and gave it a try.

with my eimac 4-250A high power tetrode 13,56mhz would not be a problem. so i wound my resonator

Hfcoil13klein

heres my improved selfmade tube socket

Rohrensockelklein

and heres the result of a few hours of work

Entladung45klein

in big

and for all interested in the shematics, heres the site, in german of course

and a video of the first light. . the arc at end of the video which occured at the bottom of the resonator was a short between g2 and anode of the tube, causing a hf flame to appear between the two windings. but i noticed it fast enough and fixed the fault. nothing bad happend .

(Divx 2,8mb)

the tube is powered by a usual microwave transformer with shunts removed. i removed the heater winding for the magnetron and added 5turns of heavy wire to get the needed 5volts 14,5A to power the tube filament. the rest was easy, cause there are not to many ciritical parts..

the only thing i have to do now is to look for a bigger smoothing cap, cause you can hear the 50hz humm coming from the flame, cause i have only 2ÂµF of smoothing , and i need around 20 or more.

next is to solve these problems and find a high quality audio transformer(those transformers used in old tube amps and radios) to modulate the g2 voltage with the audiosignal . then iÃŸll have a very loud plasmaspeaker. .

heres a pic of my testsetup

on the left you can see a small transformer with 285v ac to power the g2 (its called screengrid in english right?) the 285v are rectified a smoothed via a 68Âµ cap on the pcb in the middle.
the big transformer is the mot, rectified with a mot diode on the pcb and smoothed with the 2 microwave caps on the right and a few 4,7nF 4kV caps on the pcb. the 6 grey resistors on the board are for current limiting. the big choke on the right consists of 20 turns 1,5mmÂ² wire ca 30ÂµH.. upper left, the big tube, right of the tube, the reaonator. . the black wire ring on top of the resonator is the feedback winding going to g1

the flame is absolutely silent, with just a 50hz humm, but thats due to the low smoothing capacitance. you can burn paper etc quite easily. . the copper electrode melts after a few seconds and starts to glow red or yellow. . i really need some tungsten electrode or so

Linas

Sun Nov 25 2007, 04:57PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

WaveRider

Sun Nov 25 2007, 05:45PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Excellent work! One comment tho'.. I would shield that coil in some sort of Faraday cage.. It could improve the spark power as well as reduce harmonic emission...but then I am a bit over cautious.

ragnar

Sun Nov 25 2007, 06:08PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Nice VTTC, Reaching -- he looks a bit like some of the coils I did a while ago (characterized by the white discharge). How rare and unique they each are =)

The discharge was hot enough to blow holes in just about anything I point it at. ^_^

I do also admire your base-feed topology a lot, something I imagine I will never tackle, even with my 1500V MOSFETs. Keep us posted!

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...

Sun Nov 25 2007, 07:24PM

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

Quite nice!

Amazing how much easier things can be when you are using tubes

But judging by that glowing plate, efficiency isn't too great...

Good luck with the audiomod!

Reaching

Sun Nov 25 2007, 07:54PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

its just a test setup yet and some things can be made much better. i can drive that tube to glow red, thats specified in the datasheet, but hopefully i can find some more smoothing caps and eventually changing a few resistors to improve efficiency. the screengrid voltage is not in the optimal range, such as the resistor from the feedback going to ground . many things to improve, but remember how much power you need to get a decent cw flame at 13,5mhz which is around 5cm high..

i added an additional 1ÂµF smoothing cap and the 50hz humm is almost gone. . mh. the small transformer has several taps from 100 to 600volts. maybe i should increase g2 voltage from 200 to 300 volts or so

Finn Hammer

Sun Nov 25 2007, 08:28PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Reaching wrote ...

Then iÃŸll have a very loud plasmaspeaker. .

You bet your sweet )*( you will-
I built U. Haumann`s version back when I was into tube amps and fieldcoilspeakers, and it is the best sounding tweeter I ever heard.
With more amplitude, you have a winner!
Congtatulations!

Cheers, Finn hammer

Reaching

Sun Nov 25 2007, 08:49PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

hrhr. .
Thanks :)

Made a few changes and now the plate glows in a dim red, like as it should for long periods of time. the eimac tubes have grphite plates, so dim red glowing is not that bad

Leuchtstoffrohreklein

in big

really nice pic.. i wondered why the fluoreszent tube took so long to light up.

heres a new vid.

the cam changed the colour of the dim red plate, so it looks like its glowing red hot. thats not the case. the plate is glowing dim red and nothing more and behaves like that a few minutes, . maybe i can change some calues to get it better, but the new electrode made it so far. i have a nice flame 5cm in height which is totally silent (ok some 50hz humm)

Steve Conner

Sun Nov 25 2007, 11:03PM

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

Yay Reaching, that looks awesome! I have a 4-400A that was pulled from an old induction heater, so I'm going to have to try this too... BTW, Blackplasma, stop threadjacking!

Marko

Sun Nov 25 2007, 11:16PM

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

What I just asked about, it could be cool to try using a halfbridge of tubes to directly base feed a resonator.

I'm actually wondered that none builds base fed VTTC's even though voltage is plenty.

Easier design and no need for tuning... direct CT may be enough for drive but you, steve, could maybe go a step up with PLL ;)

I don't know where could I get cool tubes like that here - adn shipping those over big distances appears to often be troublesome...

I never played with tubes larger than ECC81 myself.. so I find this all mysterious

Marko

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