just a short update from my workbench:
My amplifiers are working now and they do not oscillate anymore.
What I've done, was:
- complete dismount the board from the heat sink
- removed output circulator and both transistors from the board
- cleaned all of them (circulator, transistors, board AND heat sink) very carefully with alcohol
- mounting the board with all screws (changed to M2.5x4mm) on the heat sink
- mounting and soldering output circulator
- mounting and soldering transistors (bend the gate and drain pins to get good contact!) as mentioned by ra3s in this article
- Implement the common BIAS voltage modification as explained by DH1DA in this article
- connecting the GND from the power supply only to the heat sink
I used a 10kOhm multi-turn trim pot to be able to set the BIAS voltage very exactly.
I believe, in my case there was some residue from adhesive tape (used to fix the output circulator to the board for transport) on the circulator and the board. This and some small gaps between the transistors and the heat sink are responsible for having no perfect ground connection at those elements.
So many thanks for all those helpful postings here, guys!