Discussion in 'Amateur Radio Amplifiers' started by KQ0J, Feb 25, 2021.
Hint: all 4 were but the last one exploded to let me know it was time to replace them....
What was the cause of failure?
Why were there transistors from different manufacturers in there?
Thats not right.
You want 4 "matched" transistors (usually they are matched for dc beta) and they should be from the same manufacturer and lot.
Has this amp been repaired before?
You need to lap the heatsink as in this W8JI video
Heatsinks were improperly machined causing poor heat conduction and transistor failure.
The only reason I don't favor SS power amps is they're generally air-cooled and the fans make noise.
Now, if they were liquid cooled, that would be quieter (and also run the transistors cooler).
I've had a project brewing (in CAD only, not built) for a couple years for a liquid-cooled SS HF amp. Not as difficult as a water-cooled tube amp because the voltages are much lower and if the coolant becomes slightly contaminated...kinda "so what?" But I never actually did it (yet). Might make a great retirement project, if I can still have access to the CNC VMCs I have access to now, to machine the ductwork in the heatsinks; I'd use copper, not aluminum, and not only "machine" the mounting surfaces, but probably EDM and polish them.
Regale us with pictures if you do this. Sounds like fun!
The heat sink is fine.
Somebody botched the previous repair job.
Unless Tommy is putting floor sweepings in new amps...
My tube amp is in the garage and I can hear it in the living room. That way I don't forget to turn it off.
Yes Steve. Life is tough without a VMC and CNC lathe w/ live toolong.