Ceska verze

VTTC II. with RE025XA (4CX250B, SRL460)

     After successful construction of VTTC with GU-81M tubes I have now tried to to build a VTTC with a completely different kind of tubes. I choose Czech coaxial ceramic tetrodes RE025XA made in Tesla (equivalent of 4CX250B or SRL460). They are designed primarily for power transmitters. The advantage is small size, low cost, lower power consumption of heater compared to GU-81M and also less risk of breakage during transport than the giant "bottles" :). The disadvantage is the need to getter them, longer heat time and higher sensitivity to wrong electric treatment. The maximum frequency is 500MHz and the anode loss of 250W. Due to their small size, it seems more than enough :). Not to think small, I joined 6 of these tubes in parallel :). Heater voltage is 6V (not 6.3 V, as usual). Electron tubes taken together almost 17A heating current and total heating power of six vacuum tubes does 100W. I used small rewound MOT (class 220 °) for heating. I rewound it to approximately 10.8 V and the voltage on the primary I limited using ballast for 125W mercury lamp. This makes output voltage under load drop to 6V. This type of power supply has several advantages. There is no large indush at power on, when the filament cold and have much less resistance. Reduced voltage on the MOT primary allows for continuous operation. Heater source must be rated for continuous operation, because ceramic tubes required gettering before use. If not used for more than a month, it is necessary to connected to the heater voltage for a period of 72 hours. On one piece of tubes I already convinced that without gettering it will not work - the tube at the anode voltage 250V flashed and welded g1 with g2 together :).
     The RE025XA datasheet shows the maximum anode voltage 2kV. The maximum peak voltage is not noted. External surface (creepage) distance between the anode and g2 is only 8mm, which is somewhat disturbing (risk of arcover). The problem I have solved the looping tubes with duct tape :). Start of duct tape should be cut obliquely to avoid the tunnel. This increased creepage distance to more than 2 cm, which already looks much better. As a high voltage anode power supply I used 3 MOTs in parallel with a voltage doubler made of capacitors 9 from the microwave ovens (I think that would be enough to use and 2 MOTs and 6 capacitors). Secondaries are connected in series with the primaries (it works as autotransformer). Input voltage is regulated by a variac. At the output of MOTs is situated protective spark gap, which protects MOTs and other components from high voltage pulses to tens of kV in the case of rapid withdrawal of oscillations. Spark gap has a distance of 2-3 mm. The voltage supply is blocked by high frequency capacitor 40n / 7,5 kV. It is compiled as MMC ten (5x2) of capacitors 100n 1500V type TC343.
     I connected the vacuum tubes directly in parallel (corresponding electrodes to each other). Theese vacuum tubes require forced cooling b air flow (see details in Datasheet). Anodes I have placed in the holes in the box, in which the fan (from the microwave) blows. Without forced cooling you can't run even the heaters only (solder on pins would melt)! The fan is therefore necessary also for gettering. First I tried to connect the vacuum tubes into triode wiring Like the VTTC with GU-81M. Thus feedback connected to g1 and g2 connected together. It turned out that this way is not the way. Failed to create a bigger spark than 3 cm. Then I tried to ground g1 and put feedback into g2. Discharges into the air were now 35 cm. After tuning better they extended to 45cm. I used primary from the first VTTC, It has several taps on the primary. This VTTC works best with the anode resonant capacitor connected to the tap with 13 turns. The resonant capacitor is 470pF 15kV 40kVAr made in Soviet Union. The primary is wound on a 16 cm diameter and is wound with insulated wire. Secondary is on 11 cm diameter, the layer of winding has a height of 32 cm and is wound by enamelled wire 1 mm. It has about 270 turns. The feedback winding is 11 turns diameter of 11 cm, placed under the secondary. The feedback winding is connected through a blocking capacitor 22n/1000V, a diode and incadescent lamp 230V 20W-40W. With a smaller bulb (10W) ruined oscillations, with larger (60W) the discharges were slightly shorter.
     Operating frequency of this VTTC is about 1MHz. The largest discharge (45 cm in air) gives at the input voltage 160V~. The more increasing of the input voltage does not extend the discharges. Current at this voltage is 16A. That means input power 2560W :) with heater not included. Before MOTs it is recomended to add a circuit breaker.

     WARNING !!! Tesla coil is extremely dangerous device! Without knowledge of the principles of working with high voltage, you should not construct it. All parts are connected directly to mains. MOTs produce high voltage with current of several ampers. Tesla coil is causing a broadband radio frequency interference. Electromagnetic radiation can damage electronic devices or storage media. During the operation it produces ozone (O3) and other toxic gases, must be ventilated! Everything you do at your own risk! For any injury I do not take any responsibility.

Schéma elektronkového teslova transformátoru (VTTC) se šesti RE025XA
Schematic of the vacuum tube Tesla coil (VTTC) with six RE025XA

Výboje z VTTC se šesti RE025XA (4CX250B)
Arcs from vacuum tube Tesla coil (VTTC) with six RE025XA

Výboje z VTTC s RE-025XA
vacuum tube Tesla coil (VTTC) with six RE025XA operating

Výboje z VTTC s RE025-XA Výboje z VTTC s RE025XA

Výboje z VTTC s RE025XA Výboje z VTTC s RE025XA
Arcs from vacuum tube Tesla coil (VTTC) with six RE025XA - shot with a long shutter.

VTTC s 6x RE025XA v provozu VTTC s 6x RE025XA v provozu
vacuum tube Tesla coil (VTTC) operating

Výboje do vzduchu 45cm. Výboje do vzduchu 45cm.
45cm (18'') arcs.

VTTC se šesti elektronkami RE025XA

VTTC se šesti elektronkami RE025XA

VTTC se šesti elektronkami TESLA RE025XA

VTTC se šesti elektronkami TESLA RE025XA

VTTC se šesti elektronkami RE025XA - Foceno s krátkou závěrkou za denního světla.
VTTC - short shutter, at daylight.

Vysílací výkonové tetrody v koaxiálním keramickém provedení RE025XA české výroby (TESLA) - ekvivalent 4CX250B nebo SRL460
Trensmitrer coaxial tetrodes RE025XA of czech made (TESLA) - equivalent to 4CX250B or SRL460.

Zvýšení povrchové vzdálenosti mezi g2 a anodou
Increasing the creepage distance between g2 and anode :) using a duct tape with askew cut end.

Hranatý plastový kýbl od barvy
Box with holes for tubes.

Elektronky zasazené do otvorů
Tubes in the holes.

Větrák z mikrovlnky pro chlazení elektronek
Microwave oven fan for cooling.

Větrák z mikrovlnky pro chlazení elektronek - po připevnění
Microwave oven fan attached.

Příprava na 72hodinové getrování - měření žhavícího napětí a proudu.
Getting ready for 72h gettering. Measuring the current and voltage.

Žhavící elektronky RE025XA
Heating RE025XA tubes

Propojení anod.
Anodes connected.

Propojení nožiček elektronek
Pins connected.

Sestavený VTTC
Completed VTTC with 6 MOTs, 9 Microwave oven capacitors, primary, secondary and rewound heater MOT. The heater ballast is under the table :).

Sestavený VTTC
One more look at completed VTTC

Ochranné jiskřiště chránící MOTy, VN kondenzátory a VN diodu před špičkami v případě vysazení oscilací.
Protective spark gap for MOTs.

VTTC - 45cm arcs after tuning.

VTTC - First succesfull power on, 35cm arcs.