Simple VTTC questions / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

Simple VTTC questions

1 2 3 4 next

Move Thread

LAN_403

Myke

Wed Jul 09 2008, 03:15AM

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

Hi,
I tried making a simple VTTC like the one here but I couldn't get it to oscillate.

here is the information on stuff about the coil:
4-400A tube
660V plate supply (going through a bridge and filtered by 1uF)
310V screen supply (same as plate but going through a 15K resistor to screen)
5V 15A filiment supply
17uH inductor to plate
30ohm wirewound resistor for plate
output coil connected to plate is on a 6cm by 5 cm diameter form with 12turns
the feedback is about 1cm away from the output wire
cap connected from feedback to the grid is 10nF
resistor from grid to ground is 39K
the schematics are the same except that there is no audio modulation section.

I don't know what could be wrong... Any ideas?

PS: I don't have a camera available right now.

Arcstarter

Wed Jul 09 2008, 04:35AM

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

I had a similar problem with the same kind of circuit. I used a 6dq6 pentode and i made this circuit:

I could not get it breakout, or even oscillate i don't believe. I used 38kohm from grid to ground, 6.8nf capacitor for the feedback to grid capacitor, the choke looked just like what he used, which is a small ferrite 'rod' with a few turns of wire on it, i didn't use t1 or anything below it, 230 volts and 16kohm for the screen grid power supply, and 22uf capacitor and no 150pf capacitor, a 6.8nf capacitor from 700 volts supply to ground, and about 700 volts input. I could get a small spark from grounding the plate, but that is it.

Myke

Fri Jul 11 2008, 05:47AM

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

Ok, so I tested the tube to see if it was even working. It seems like it is because I can adjust the voltage that it drops by the grid. Any way to test the screen grid?

The 17uH inductor was air cored. Would there be any noticeable effect if there was some core material like iron powder?

Steve Conner

Fri Jul 11 2008, 01:12PM

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

Try a larger resistor from grid to ground and more plate voltage. I don't know if the 4-400 will do much of anything at only 600V, it's designed for 2000V+.

Arcstarter

Sat Jul 12 2008, 03:17AM

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

Dr. Conner wrote ...

Try a larger resistor from grid to ground and more plate voltage. I don't know if the 4-400 will do much of anything at only 600V, it's designed for 2000V+.

Is there any way you could explain what could be goin' on with mine? It is strange how no matter how hard i try, i can never get anything to work....}:(

Steve Conner

Sat Jul 12 2008, 09:39AM

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

Well, I'll try to explain the reasoning behind what I said. An oscillator is just an amplifier with its output fed back to its input. If the amplifier doesn't have enough gain, it won't oscillate. Increasing the plate voltage and increasing the grid resistor both give the circuit more "loop gain".

So would making that 15K screen resistor smaller, for that matter, and making your 17uH RF choke bigger. A magnetic core wouldn't be recommended, since you're probably going to be up in the 10s of MHz with only 12 turns on your resonator.

I have a 4-400 too, and from looking at it, I get the impression it would just laugh at anything less than 2000V. I've not done anything with it beyond hooking it up to a MOT (and a 600V transformer for the screen) to get the plate good and red, and check that it actually worked without arcing.

Arcstarter

Sat Jul 12 2008, 09:22PM

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

Dr. Conner wrote ...

Well, I'll try to explain the reasoning behind what I said. An oscillator is just an amplifier with its output fed back to its input. If the amplifier doesn't have enough gain, it won't oscillate. Increasing the plate voltage and increasing the grid resistor both give the circuit more "loop gain".

So would making that 15K screen resistor smaller, for that matter, and making your 17uH RF choke bigger. A magnetic core wouldn't be recommended, since you're probably going to be up in the 10s of MHz with only 12 turns on your resonator.

I have a 4-400 too, and from looking at it, I get the impression it would just laugh at anything less than 2000V. I've not done anything with it beyond hooking it up to a MOT (and a 600V transformer for the screen) to get the plate good and red, and check that it actually worked without arcing.

Well thanks alot! I will try everything that you suggested and just hope it wants to work :P.

Myke

Sat Jul 12 2008, 09:48PM

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

Same, I'll try the things you said.
Thanks

UPDATE: IT WORKS! YAY! Thanks soo much.
The discharge is about 1cm high.

another update: It stopped working. heh

Marko

Sat Jul 12 2008, 11:01PM

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

Hi guys,

hope it is related enough - I'm trying to build an oscillator like this with ECC81 triode for testing Tesla resonators... I would only require it to oscillate, a watt or so of power is enough for that I guess, so I thought ECC81 (both internal triodes paralleled) would be suitable.

I used the typical plasma tweeter circuit, without modulation of course, varied g-k resistor 20..120k, supply voltage up to 300V, 1mh RFC (may it be too large?)

I was unable to get the circuit oscillating as well. Those tweeters keep confusing me in many things, like how can they oscillate at all without 'loading capacitor' or input network, which they seem to do without excellently.

When one looks into the circuit he realizes that tube actually forms an inverter gate, that is, feedback will always be 180 degrees out of phase with the output and it won't oscillate.

Also, without A-K loading capacitor, won't the tube see destructive high voltage pulses generated by RFC inductor?

Parasitic capacitances inside tiny tubes like those used are just way too small to justify for anything, even at few tens of Mhz.

So what is the ''right'' way to build these circuits, and what can be preventing the oscillation?

Why do they work at all even if they theoretically shouldn't?

Arcstarter

Sun Jul 13 2008, 01:25AM

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

Marko wrote ...

Hi guys,

hope it is related enough - I'm trying to build an oscillator like this with ECC81 triode for testing Tesla resonators... I would only require it to oscillate, a watt or so of power is enough for that I guess, so I thought ECC81 (both internal triodes paralleled) would be suitable.

I used the typical plasma tweeter circuit, without modulation of course, varied g-k resistor 20..120k, supply voltage up to 300V, 1mh RFC (may it be too large?)

I was unable to get the circuit oscillating as well. Those tweeters keep confusing me in many things, like how can they oscillate at all without 'loading capacitor' or input network, which they seem to do without excellently.

When one looks into the circuit he realizes that tube actually forms an inverter gate, that is, feedback will always be 180 degrees out of phase with the output and it won't oscillate.

Also, without A-K loading capacitor, won't the tube see destructive high voltage pulses generated by RFC inductor?

Parasitic capacitances inside tiny tubes like those used are just way too small to justify for anything, even at few tens of Mhz.

So what is the ''right'' way to build these circuits, and what can be preventing the oscillation?

Why do they work at all even if they theoretically shouldn't?

Does this circuit not require 2 or more grids? (tetrode,heptode,pentode and such)

1 2 3 4 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob