sstc changing frequency issue

[MarkE]

The MOSFET gate to source capacitance appears in parallel with the gate winding.  Adding more capacitance from the gate to source or using a MOSFET with more capacitance slows the circuit down.  If you do not have a lot of inductance in the wiring between the gate winding and the MOSFET and/or any additional capacitance you add, then the additional capacitance reflects through the transformer.

[gyulasun]

Hi Jeg,

Please do the tests as MarkE suggested of course, I did not mean to dissuade you from anything what others tell you.
My opinion is that if you increase the gate-source capacitance you may reach an oscillation limit whereby the oscillation frequency may start decreasing to a certain degree indeed due to the reactance transformation action (I mentioned this in my first post) what both coupling coils do towards the 1000 turn tank coil  but then the increasing shunting effect of the high capacitance value you connect between the gate-source would eventually stop the oscillator:  always test the start-up by switching off and then on the circuit when increasing the capacitor to check the normal start-up.  The shunting effect means that the AC voltage feedback to the gate from the 1000 turn tank goes lower than a certain treshold which would be just needed and just enough to start and maintain the oscillations for the tank.

Addition:  the biggest frequency change from your 5 MHz to the lower frequencies like 3 MHz could also be made by adding a 'capacitive load' to the upper 'hot' end of the 1000 turn coil which is now floating.  Even adding a HV capacitor of a few picoFarad like 8-10 pF in parallel with the 1000 turn would reduce the 5 MHz significantly.  But Tesla did not do it: he used capacitive load (a meatal ring hat )  to fine tune his secondary but this loading capacitance was not connected to the grounded end of his secondary. The question is what you wish to use the HV from the 1000 turn coil, maybe you can use some HV capacitors in  parallel.

Gyula

[MarkE]

The increase in capacitance needed is roughly 2.7:1.  A high voltage capacitor can be made by stringing lower voltage capacitors in series.  This is probably not a very good idea with a Tesla coil where the voltages can get very high.  Fashioning a parasitic capacitor as just a disk or a ring connected to the high voltage is a good solution if you have some metal stock around and are handy with tin snips and a file to smooth the edges.  Increasing the diameter of your ball will also increase the capacitance.

As long as the primary and secondary are tightly coupled you should be able to increase capacitance on the low voltage primary without much trouble.  You will have problems if there is a lot of leakage inductance in the transformer and or the wiring from the transformer to the capacitor as added capacitance will then increase the phase shift of that low pass network and enough of that will kill the oscillator.  So keep whatever you use for a capacitor close to the transformer primary.

[TinselKoala]

All of the above, plus you could also use more wire on the secondary, like going to 1200 turns. This is very similar to my "Sassy ClassE" sstc.
You will get different quality in the corona as you lower frequency. If you lower frequency by adding terminal capacity, you may not be able to get corona breakout, but you may get lots of electric field power transmission to CFLs and neons, etc, near the coil.

[Jeg]

Hey guys
Every time i learn something new from you. Thanks a lot!

I fix it by providing more capacitance between gate and source as MarkE said, but also i will make a new secondary with more turns as Guyla and Tins proposed. It is really fan working on these Frequencies @ MHz range!!! What surprised me is the big burn on my hand just by touching Drain at 360V peaks of 3MHz! It was really a nasty burn!!!

My next try is to inject audio frequencies in to this little but powerful oscillator, for AM modulation. I will start by connecting the audio gen after the 15K resistor at where it meets the 1K resistor. At this point there is a DC voltage of about 2-5V (pot regulation in parallel with the 1K resistor), and an AC signal of the feedback coil at 3MHz. I guess i have to put some filter arrangement before injecting it, so to prevent the High freq. oscillation to come in to my audio gen. Have anyone try it? Are there any other critical points that i have to consider?

Tnks
Jeg