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Please explain "Safety" and "Death" Caps

U

UnGrounded

Well-Known Member
What is a "Safety" Capacitor?

I see "Safety Capacitors" rated in AC current, but I have yet to knowingly encounter any caps rated in AC. I recently acquired an Ampeg G-15 guitar amp and there is a ceramic cap inside which reads "AC .02", however, it doesn't read "VAC" and its location in the schematic should certainly be DC only. Is "AC" just a production code, and not a VAC safety cap? Do VAC / Safety Caps have something to do with old-style hot chassis applications?

Secondly, what is a "Death" capacitor?

I hear talk and see photos of what people label "Death" capacitors, but for me, without consulting a schematic, they just look like plain old ceramic caps.
 
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You're supposed to use a properly rated safety cap so that it will never short and turn into a death cap.
 
Your Ampeg will probably have only one ground cap and it is connected, either directly or through a ground switch, to one wire of the power cord. You flip the plug over in the outlet or flip the ground switch until the cap is connected to the neutral wire of the AC outlet. That's the lower noise position and when connected that way, there isn't any AC across the cap.

Without using a meter, the easiest way to determine the plug orientation or switch position is by ear. Turn on the amp, set the volume low, like to 2, and with your guitar cord plugged into the amp but not the guitar, touch the end of the plug with your hand, without touching the shell of the plug. The amp will make that hum or buzz that we all hate to hear. Now flip the plug orientation or throw the ground switch and repeat touching the end of the plug. Whichever position make the quieter hum or buzz is the correct position. There will be lower noise in that position because the chassis of the amp will be connected through the ground cap to the neutral wire out the ground rod by the electric meter.

If the amp has a three conductor cord but still has a ground cap, like silverface Fenders, the procedure described above doesn't work very well because the ground wire in the cord keeps the chassis well grounded (assuming the outlet is properly grounded) and both switch positions should be quiet. But you still want the switch positioned correctly so you don't have 120VAC across the ground cap. In that case, you can temporarily uses a 2 to 3 prong adapter to disable the ground connection, find the correct switch position that connects the chassis to the neutral through the ground cap, and just leave the switch in that position from now on. (This process assumes a correctly wired AC outlet that is grounded and hot is the narrow prong and neutral is the wide prong.)

In guitar amps with a single ground cap, as long as the cap is correctly connected to the neutral wire and not the hot wire (plug oriented correctly or ground switch in the correct position), even if the cap shorts there should be no hazard. That's because the neutral shouldn't be carrying any appreciable AC voltage since it's tied to the ground in the panel.

Be aware however that some hi-fi gear and older test equipment (tube testers, tube oscilloscopes, VTVMs, etc) use two ground caps, one on each leg of the power cord. Those caps need to be good and rated for AC, as described in the linked website. But I've never seen a guitar amp with two ground caps.
 
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