On Fri, Apr 28, 2023 at 10:03 AM, John Z wrote:
Have you considered also using thermal paste under the BS170s ?
you could also sprinkle over the BS170s some Water out of the Holy Fountain of Lourdes from southwestern France. It would help as much as thermal paste, but it will have the big advantage of not doing no harm and cost less.
Thermal paste is good between a radiator and a transistor designed to be mounted on a radiator. The BS170s are plastic transistors and your thermal paste will neither help nor harm, because the main thermical mismatch, the main barrier will be the plastic itself.? ?So using such a paste is just a mess, nothing more than that.
Sorry to have put it so sharp, but this is an issue often debated and discussed in this forum.?
What Sverre LA3ZA instructs us to do is the only way to stay out of harm's way when it comes to the lifespan and wellbeing of the transistors. And strange enough, what?Sverre writes above is much easyer to do and much closer to any common sense as smearing some paste without knowing why. It is prudence, modesty and wisdom at the same time. Following this path and increasing the efficiency of the PA instead of it's big-bang-power figure will result in transistors runing cool, smooth and remaining on the safe side both in terms of breakdown-voltage as also in what their thermical load concerns.
Aiming for power levels beyond any common sense, driving currents trough those small transistors against any sanity is not the way to go. It could be, for a test, for trying beat all records. This kind of motivation might work for a short period of time and for a very well behaved load that stays put at 50Ohm resistive and Zero Ohm reactive, but it would imply that you know what you are doing and have a shelf full of spare transistors. They will blow away, sooner or later.??
Have fun but stay cool. Staying cool does not need refreshments.
Yours friendly, Razvan dl2arl
DL2ARL