Hello guys, this is my first post on the forum.

I am trying to reproduce a simplified version of the circuit "OSCILLATOR" attached here in pdf. This is a Colpitts oscillator based a on triode ML6696-A which datasheet is also attached. The resonating tank circuit is connected between the plate and grid.

Based on the datasheet and some info I found online, I modelled the triode on LTspice as follow:



As of today my LTspice circuit looks like this :




I'm mostly interested in the resonating current across the work coil, the plate current across C2 and the grid current across C7.

Here are some observations/questions I hope someone very familiar with triode-based colpitts oscillators can help me with :

- I do observe oscillations but the resonance frequency depends on the order of the capacitors C100 and C200, if I inverse them the resonance frequency drops. To my understading this is not normal, the equivalent capacitance is C100*C200/(C100+C200). Someone has an explanation for that ?

- The plate current (50A peak) is indeed larger than the grid current (7A peak), I expect more values as stated in the ML6696-A datasheet.
However the plate current is equal to the resonating current across the work coil. If I remove the grounding between C100 and C200 I do get indeed a bigger resonating current (50A peak) than the plate current (7A peak) but then the plate and grid currents become equal.

- The current across R1 is way too big (130A) thus dissipating unrealistic amounts of power, however keeping R1 low is the only way I found to not have damped oscillations.

Does my problem come from the .sub definition of the triode, from the triode biasing, or from something I missed while trying to reproduce a simplified version of the oscillator circuit ?

Thank you very much !

/Boris

 

hi Boris
Welcome to AAC.
Please post your LTS asc file.
I do not see a DC path for the Grid circuit???

E

Update:
What is the Vsupply volts?

 

Hello Eric,

Thanks for your reply, I have attached the .asc and .sub here
What do you mean by DC path to grid ? Clearly I'm not an expert with triodes

/Boris

 

hi Boris
Your triode has problems, very poor bandwidth and gain.
I have used two circuits to compare my triode with yours.
E

 

Dear Eric, thank you for your precious help.
I took inspiration of your library to define a new subckt for my ML6696-A triode.



The current drawn by the filament (R1 and C3) is now much lower which is good.
I do get a resonant frequency which is now independent of the order of C100 and C200.
I also do get a plate and grid current much lower than the tank current, the grid current is however higher than the plate current which should not be.

After approx. 1s the resonance starts however suddenly dies out short after, what could this be due to ?

 

The circuit in post #5 has no return for the grid current, thus no grid current flows, so the grid charges up and the tube is quickly cut off. So it seems that it goes into cutoff, or possibly saturation. AND, if the scale is correct, at 230 amps the plate circuit melts. Also, there is no capacitor to resonate with L1at the operating frequency.

To me, the circuit does not look quite like a correct COLPITS oscillator.

 

Hi Boris,
Why are you using a 13,000V supply voltage on the simulation, also there is still no DC path for the Grid to 0V, as pointed out in my earlier post.
What is your required oscillator frequency?
E

 

HI B,
This is a list of the circuit error messages, there are also errors in the triode model.
E

 

FIRST, if you do not specify reasonable values in a simulation, certainly the results will not be reasonable. And certainly 13000 volts AC for a plate supply is not a voltage that a casual experimenter will be using. The device in the data sheet is a high power tube suited only for high power braodcast transmitter applications. It would certainly be a monster power oscillator by any standard.
Why was it selected for this application??