> A TO-3 / TO-204 has a feature over most plastic transisters, higher "Tj".
Yes, but don't go there in audio.
> Moisture absorbtion! but when is this a problem really?
You can take a TO3 to 200°C a few times. But in most audio amps, especially Class AB, the transistors get hot-cool-hot-cool-hot-cool-hot-cool every few seconds, as the music swells and fades or bump-y-booms. Early transistor amps failed because this constant temperature cycling eventually cracked the seals and let Oxygen and moisture in, which ate the silicon. Yes, even in Hot Dry Sweden (and worse here in muggy New Jersey).
Things have changed. The 150° rating on plastic packs "can" be exceeded. I proved this on a PC motherboard, which had a regulator heatsink not really big enough for the hot-rod CPU I was trying to use. I checked the CPU voltage at the regulator pin and noticed that the regulator wobbled. But it was soldered in? The solder was melted but the regulator was still working! (Molten solder conducts electricity; obvious, but I'd never noticed before.) What is the melting point of generic electronic solder? Far over 150!
The modern plastic is pretty tough. And the silicon inside is no longer naked: they have pretty good surface treatment ("passivation") to seal the moisture out and the magic inside.
Even so, my old-timer rule of thumb is that audio transistors should not have more than 50°C rise from Off to Hot (about 75°C base temperature in a typical room). Or for US-heads: about 170°F, just on the edge of burning you. 200°C is 400°F: hot enough to bake a cake!!!! There is no excuse for that kind of abuse. Heatsinks are not that expensive.
Use TO3 for military and heavy industrial use. It may take more abuse. Use TO3 if you think it looks groovy, or if the device you want only comes in metal. For any ordinary audio use, plastic works fine. Use what you can get, or is cheap, or easy.
Yes, but don't go there in audio.
> Moisture absorbtion! but when is this a problem really?
You can take a TO3 to 200°C a few times. But in most audio amps, especially Class AB, the transistors get hot-cool-hot-cool-hot-cool-hot-cool every few seconds, as the music swells and fades or bump-y-booms. Early transistor amps failed because this constant temperature cycling eventually cracked the seals and let Oxygen and moisture in, which ate the silicon. Yes, even in Hot Dry Sweden (and worse here in muggy New Jersey).
Things have changed. The 150° rating on plastic packs "can" be exceeded. I proved this on a PC motherboard, which had a regulator heatsink not really big enough for the hot-rod CPU I was trying to use. I checked the CPU voltage at the regulator pin and noticed that the regulator wobbled. But it was soldered in? The solder was melted but the regulator was still working! (Molten solder conducts electricity; obvious, but I'd never noticed before.) What is the melting point of generic electronic solder? Far over 150!
The modern plastic is pretty tough. And the silicon inside is no longer naked: they have pretty good surface treatment ("passivation") to seal the moisture out and the magic inside.
Even so, my old-timer rule of thumb is that audio transistors should not have more than 50°C rise from Off to Hot (about 75°C base temperature in a typical room). Or for US-heads: about 170°F, just on the edge of burning you. 200°C is 400°F: hot enough to bake a cake!!!! There is no excuse for that kind of abuse. Heatsinks are not that expensive.
Use TO3 for military and heavy industrial use. It may take more abuse. Use TO3 if you think it looks groovy, or if the device you want only comes in metal. For any ordinary audio use, plastic works fine. Use what you can get, or is cheap, or easy.
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