I´m currently working on a Nemesis-esque amp using a single 2SK133 lateral Fet per channel, mounted on a pair of questionable junkbox heatsinks. At 29,8V and 0,715A (~21W) I can put a finger on the TO3 "hat" but after ten seconds or so it´s not very comfortable. The thermal coupling seems ok, the heatsinks are almost as hot as the transistor casings.
Is this too hot for a good long-term reliability? The heatsinks in question would work well for the chassis design I have in mind.
I tried to measure the temp by applying some thermal grease on the sensor of a regular house thermometer and leaning it against one of the transistors, giving a 47,5 degrees C reading. Propably not the most trustworthy method...
Is this too hot for a good long-term reliability? The heatsinks in question would work well for the chassis design I have in mind.
I tried to measure the temp by applying some thermal grease on the sensor of a regular house thermometer and leaning it against one of the transistors, giving a 47,5 degrees C reading. Propably not the most trustworthy method...
If you didnt use sil pads then you need thermal grease between transistor and mica pads and heat sink.
50 degree's isnt too bad.
If you went for a class AB amp it would ne more efficient.
50 degree's isnt too bad.
If you went for a class AB amp it would ne more efficient.
I use mica and silicon grease between the transistors and the heatsinks, AFAIK sil pads are not very good in power applications.
I just did a second attempt to measure the the temp and this time I managed to get the sensor tightly wedged between the transistor casing and a heatsink fin, resulting in a 50,0 C reading.
I just did a second attempt to measure the the temp and this time I managed to get the sensor tightly wedged between the transistor casing and a heatsink fin, resulting in a 50,0 C reading.
Well... The temperature is suspiciously stable at 50.0 degrees...even after increasing the quiescent current to 825mA, which leads me to the conclusion that 50.0 degrees is the upper limit of the thermometer😀
Now I´m left with the marginally useful information that the transistor operates at some temperature higher than 50 degrees...
Now I´m left with the marginally useful information that the transistor operates at some temperature higher than 50 degrees...
By all means it´s probably not much higher than 50 degrees, but still annoying. I´ll see if I can find some larger heatsinks that leave at least some margins if I want to try the amp with higher voltages and more current.
In my Mosfet SE amplifier with a H7 bulbs IRFP260 has about 60 degrees and it is stabile at 2,5 ohms. Power supply is from 24V/16,6A transformator and two groups of capacitors. Between them is 1ohm/400 watt resistor, also very hot.
