Hybrid Circlotron question

[ebuckley002]

Hi there,

I recently built a hybrid circlotron based on a schematic by "circlotron audio". It actually works, but I'm wondering whether the MOSFET bias will be long term stable given that it is set by the constant current sources for the tubes. I'm hoping that the current feedback to the tube cathodes might stabilize the arrangement, but I'd be grateful for any feedback here as I'm sure there are some hidden gotchas.

 

[vilfort]

Nope. Tubes deteriorate over time. So direct coupling from plate to gate is a bad idea. But you can insert a cap between the plates of the triodes and the gates of the MOSFETs. But you also need to create a BIAS supply for the gates of the MOSFETs.

 

[ebuckley002]

I did as you suggested and put some caps in and generated an independent bias supply via a current source, but the bias isn't stable and varies all over the place. Any ideas where I'm going wrong?

 

[vilfort]

This is looking much more complicated than it needs to be.
All you need is dual triodes to drive 2 MOSFETs.
The MOSFETs must be capacitor coupled to the tube plates.