I've always used the plastic package transistors for low current stuff. And I was under the assumption that the metal case version, such as the TO-18, allowed more collector current...because the metal case could radiate heat better or have a heat sink clamped to it.

But when I look up an example that I might use - 2N2907A - I see that the plastic TO-92 version has a higher collector current spec than the TO-18 metal case version. This seems odd to me.

Okay, so maybe the metal case provides shielding. But unless the metal case is grounded, does it really provide any shielding? Doesn't it need to be connected to ground in order to shunt the interference somewhere?

 

From the datasheets that I looked at the TO18 is rated at 800mA while the TO92 is 600mA.
The collector is generally connected to the case.

 

These were the ones I looked at. Plastic 800mA, metal 600mA.

 

And if the collector is connected to the case...doesn't that make the case an antenna?

 

Technology and manufacturing methods advanced...

 

Hello there

And if the collector is connected to the case...doesn't that make the case an antenna?

In this schematic the antenna can be connected directly to the TO-18 metal case transistor collector. I would swap out the 2N3904 with a 2N2222, replacing 100kΩ resistor with potentiometer & 86ing the 365 picofarad variable capacitor all together.