The 4 transistors have arrived.I'm a bit worried about the 4th kelvin pin,because I do not know what is it,but it's shorted to the emitter and the datasheet says that they are not exchangeable.
An oscilloscope is getting pretty necesarry,and I'm going to buy a cheaper,because it should be enough for me.I found the 3 cheapest scopes in the EU which are two hantek:DSO5072C and DSO4072C and UTD2102CEX.
I think that the DSO4072C would be the best choice,due to its frequency generator function,but its also a bit more expensive than the two others.Which one of the other two would you recommand?

Also,is it important to mount so high power transistors like these on big heatsink,while running SSTC?

The secondary got 3 layers of acril-pulyurethane water based eco  varnish.It dried very fast and isolated,fixed the windings very good.I added an interrupter to the circuit,increased the voltage to 30V and got pretty good results,2-3cm arcs.
Here are some photos about the setup,and the GDT,which I'm planning to use.What is your opinion about it?

I made a driver,with PLL halfbridge(irfz44),running everything on 12V,so I can tune the frequency and measure it.At a range of 150-160kHz,a fluorescent lamp is lighted up at almost  max. brightness,current drawn is at around 0.1-0.2A at 5+ primary turns and under that the current increases rapidly and does not increases the output power.I did not expected anyting better than this,because the setup was just made in order to check the secondary's resonant frequency.
The feedback for the PLL is also working,it adjusts frequencies as even high as 180kHz down to the resonant frequency (150-160kHz).

The previous slayer exciter test loaded the 55v PSU to 50V,so that was not the problem.Even if I shorted the collector to the + rail,no current was flowing,so I think that the 47k base resistor was too big.In this case without a signal from the base of the coil,the transistor never turns on,and with less turns,the induced current into the secondary coil is not big enough,to trigger the transistor I THINK...

My new question is how and what kind of varnish can I use to insulate the secondary coil?I visited some nearly electronics and hardware  stores and nowhere could I find varnish for magnet wire.I have nitro based laquer,but I do not know if I can use it for this or not.Actually I want to protect the coil,and prevent the pretty lose windings from overlapping,not to prevent it from too high voltage.

So I made some measurements.The resonant frequency is 140kHz i think,but i'm not sure,because every frequency was measured with multimeter.The power supply is around 55 volts,large smoothing capacitor.
•Without topload:3 turns primary,current is 1A,and it outputs around 0.5cm arcs and they are pretty weak,can't pull a constant arc.With more primary turns,current drops,but the arcs gets more powerful,around 2cm.
•With topload <5 turns does not give any output,and by increasing the primary turns the current is dropping,arcs are increasing-the same thing as without topload,but the arcs are not as big as without topload,seems like it needs more primary turns with topload to work better.
The frequencies are pretty high(around 400kHz) in both cases,measured with multimeter so i think it's not accurate or it's not measuring what it should.

Also,the height of the coil is 50cm,a bit taller then I described,because I wound it until I ran out of magnet wire.

Now i have the components for the secondary let's see what can I do
The coil,as calculated should be:125mm dia.,485mm long(1:3.88),1620 turns.

I hope,that if I can make it,it will work with a SSTC driver,then maybe upgrade to DRRSSTC...

Thank you guys for being so helpful!:)
I'm upgrading to a better coil recently.A coil of 0.3mm magnet wire is coming,the primary is made of 6mm copper wire- 15 turns.I will make the secondary 110mm dia,550mm long,and around 1800 turns,as calculated.

I made my uploaded schematic on a perf board,but I realized that it's not working with the transistors,as they should.As the load on the IRFP460 full bridge I put two LEDs in different polarity,decreased the frequency to 1Hz and(if I remember),one of the LEDs was blinking,and the other was turned on fully,and never turned off.
I could't find the problem,so I made the circuit on a breadboard,now it seems working there.
The other problem:all of the IRFP460s are dead,due to my stupidity(or just they are crappy fake mosfets and give up easily....)

An oscilloscope could solve a lot of my problems,but I want to buy good one.

I do not use gate drive transformer.In theory,I think that the 74HC14 shifts the NE555's output level,and the IR2109 chips will never turn both the high- or low side mosfets on,since the input signal is inverted.The setup is also weak to drive a TV flyback transformer.