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Re: [TCML] Massive VTTC!!!

Very impressive tube coil, Carlos !
However try pulsed operation. You will get a huge difference in spark length. More than two years ago I build a VTTC that uses a Brown Boveri T380-1 triode. This tube is very similar to the industry standard 5867A (TB 3/750). Its power ratings are approx. 75% of a 833A tube. Using a complicated Hartley oscillator circuit it produced approx 45 cm (=1.5 feet) sparks.That's not very much for such a tube, but I will try to improve the impedance matching of the tank circuit to increase spark length. However, unlike most VTTCs it uses filtered DC (from a voltage doubler). The filter cap has 5 uF @ 12kV. My VTTC would operate at 5000-7000 VDC. The interesting difference to "staccato-coils": 1. Anode supply voltage is DC instead of AC. 2. The oscillator is grid pulsed !! In this mode of operation the grid bias remains at highly negative potential (e.g. -800 V) for the "off"-phase and is switched to zero potential for the "on"-phase. During the "on"-phase the oscillator works normally with the well known grid-leak resistor / bypass capacitor combination for automatic grid bias. When the oscillator is forced into "off"-mode the additional negative grid voltage forces the tube into cutoff-condition (virtually no anode current). This principle was widely used in powerful industrial RF generators to control the average RF power output of the oscillator. The grid-pulser allows very convenient adjustments of the duty cycle and the repetition frequency. My grid pulser circuit comes from a discarded 10kW-RF-generator. I achieved longest sparks with a "on"-time off approx 2-3 miliseconds. Repetition frequency was between 1 and 40 Hz, typically 10-15 Hz. At low repetition frequency (e.g. 5 Hz) the average anode current would be as low as 60 mA ! This allows exremely cool operation of the tube and all other components ! 

If  you want to achieve long sparks I would try the following:
1. Use a pulser circuit. Instead of CW operation, let your VTTC run in "burst"-mode.
2. This allows you to increase the peak power output by increasing the anode supply voltage
I will upload some of my VTTC movies on youtube very soon. This helps to get a better idea of grid-pulsed operation. 

Martin
----- Original Message ----- From: "Roger Smith" <rwsmith@xxxxxxxxxxxxxx> 
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Thursday, November 04, 2010 1:44 AM
Subject: Re: [TCML] Massive VTTC!!!
Hi Carlos, It's hard for me to make suggestions without knowing more about your coil but I do know you should be getting sparks longer than 2 to 3 feet. I have a tube coil that runs in half wave mode and I'm getting 2 foot sparks with 4 kw input. These aren't straight line sparks either they have some branching.
There is one mistake I have seen a some people make. If you put the tickler coil above the primary coil like most people do you got to make sure the secondary coil is wound the right way or the whole thing will perform very poorly. The reason for this is that the tickler coil is effected by the electric field from the secondary as well as the magnetic fields. If you wind the secondary the wrong way the electric field will be working against you and not for you. The way to get it right is to wind your primary coil, secondary coil and tickler coil all in the same direction from top to bottom. The top of your tickler coil goes to the grid and the lead from the bottom goes to the grid resistor and capacitor. The top of the primary coil goes to B+ and the bottom goes to the plate of the tube.
I wouldn't worry about staccato mode. Staccato mode is for people with wimpy tubes. 

Roger
----- Original Message ----- From: "Carlos Van Camp" <carlos@xxxxxxxxxxxxxxxxxxxxxx> 
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, November 03, 2010 3:30 AM
Subject: [TCML] Massive VTTC!!!


Hi guys (and gals),
I have been building a VERY large VTTC for the past 2 years, and I am in the final stages of testing and tuning. This machine has been built with brand new high power components and has an input power capacity of upto 50kw. 
The valve is a tetrode (4CX15000a) with a plate dissapation of upto 15kw.
I am an experienced builder of large SGTC, but fairly new to valve coils.
The trouble is that dispite an input of 30-35kw of power (CW mode), I am only achieving arcs 2-3 foot long (6 arcs at once coming off the top electrode). I understand that VTTC's make much shorter arcs for the power applied, but I have heard of people making arcs 8 ft long with less power than I have available... 
So... What am I doing wrong...?
Does using a stacatto system truely make arcs longer...? Or does it just reduce the input power?
Does any body have practical experience using thyratrons in place of spark gaps, and would one be suitable to produce a musical coil? (ie changing pulse rate to play tunes etc) 

Thanks,

Carlos.
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