mosfet oscillation

[gustav]

Hi there,
hoping for some advice.
Have a big amp with +-100V rails and 8 pairs of Hitachi 2SK176/J56 powerfets. None of the usual cures against HF oscillation are totally effective.
With a 4 ohm load it works ok with up to +-70V supply.

Has anyone used these devices with success?
( have used the 135/50 types earlier without problem but with +-60V supply only)
Regards Jonas

 

[Bricolo]

can you post the schematic?

 

[peranders]

What type of oscillations do you have, above over a certian current, only positive half period?

 

[gustav]

Typical 50MHz oscillation and yes it happens at some current level, maybee 10A or so. The amp is not up and running just now but I think only one half is effected, could well be the positive side.

I have no "digital" schematic but I am quite shure that only the output stage is involved .
There are gateresistors and there is also some RC-load to ground/heatsink (50 ohms ac-coupled)

Thanks for your interest !

 

[PMA]

Did you use the resistors in series with gate of FETs? - sorry you have just replied. How is the feedback loop (poles etc.)?

Do you use output coil and Zobel circuitry?