sstc changing frequency issue

[Jeg]

Hi guys, i wish you a happy new year!

I recently built this Armstrong oscillator using an IRFP460 Mosfet. With 6 turns primary and 2 turns feedback, seems to work great and most important...cold! I tried to lower its frequency from 5MHz to 3MHz, by changing the primary coil turns, but things didn't go very well... I am thinking to leave the tank coil as it is, and change the caps. What is your opinion on this? Any drawbacks? Which is the most efficient and simple way to do it?

Tnks
Jeg

[MarkE]

You can slow it down by adding capacitance from the MOSFET gate to the MOSFET source.

[Jeg]

That's the definition of simplicity! Thaks MarkE, !!!

[gyulasun]

Hi Jeg,

I think it is your 1000 turn coil which basically defines the 5 MHz oscillation frequency...  You have an LC tank: L is the 1000 turn coil and C is the self capacitance of this coil (including any capacitive reactance transformed into the tank from the whole circuit), all the other components serve to maintain the oscillation energy in the LC tank via the coupling coils of 2 and 6 turns. 

Obviously you can vary some of the other C components, it will influence the 5 MHz a little but the main thing would be to rewind the 1000 turn coil: perhaps use a 4 or 5 cm coil bobbin to wind the 1000 turns using longer wire of course.  (I know it is not a pleasant thing to wind so many turns again...)

Alternatively, you could use a small piece of RF quality ferrite core and insert it into the grounded end of the 1000 turn coil (obviously Tesla did not use any core in his secondaries but then he used higher diameter coils like 1.5 cm)  but at least you can gain some experience what components change the 5 MHz the most.

Gyula

[Jeg]

My gosh Gyula! This is the worst scenario! I'll give it a try today starting with MarkE proposal. If it wont work as i 'd like then i will use the ferrite idea before i start winding again!
Thanks