why this capacitor

[pula58]

Old ampeg G12 (Gemini) I am working-on.

There is a 50nF cap across the AC mains voltage (the cap is connected between chassis ground and PT side of the mains fuse.

What is this for? Seems to serve no purpose!? Is it a vestige of the old ground switch that a lot of old amps had? I am gonna remove it.


See this schematic:

http://www.el34world.com/charts/Schemat ... eg_G12.pdf

Also: See the center tap oif the heater winding? It "connects" to ground through a 0.1uF cap. Why not wire it directly to gnd instead of AC coupling it?

[Cliff Schecht]

This is the venerable "death cap" that you'll see us refer to every once in a while. It provides an AC path to ground for the input voltage which in some situations can help reduce hum. This was used before three prong cords became commonplace and the GFI was introduced. Before the three prong cord if you plugged the power cord in backwards there was a good chance you would hum bad (sometimes a polarity switch was added to correct this).

Now that we have a fixed hot and cold side on the power plug, the polarity problem isn't an issue and these caps serve no purpose. Nowadays it is commonplace to snip out the death cap and make sure that the hot side of the wall voltage (black wire IIRC) is fused with a properly rated fuse.