.1 Cap for presence .

[Tone Junkie]

What voltages are needed at this spot.
 standard I see 400 in some amps but what is needed.
Thanks Bill

[HotBluePlates]

Depends on the circuit, and what is the worst-case voltage that could ever be present.

Except for short-circuits, the worst-case should be the maximum voltage which could ever be present in the feedback loop. At an extreme, that's the voltage present at the speaker terminals (though it should be less due to the voltage divider used in the feedback loop).

The highest speaker voltage in an amp with a given power will occur at the highest impedance tap, usually 16Ω. Let's say the amp is 100w.

Power = Voltage²/Resistance  -> 100w*16Ω = Voltage²
Voltage = 40v RMS

So a 100v cap will be plenty in this case. Smaller output powers will have correspondingly lower voltages present.

However, folks often already have the needed cap value on hand at 400v (or 630v) and use that. Manufacturers may use a 400v part because they're already buying hundreds of thousands of that part for other positions in the amp (or other products) and have an economic incentive to use a part not absolutely needed.

Last possibility is some folks may desire to have a part that will never fail. If the amp's B+ is under 400v (even at turn-on), then now matter how a short circuit is formed inside the chassis, or from any point within the amp, then the 400v cap will prevent a big fault current. However, it could make it harder to find the odd short circuit because of fewer destructive clues.

Be careful thinking massive over-rating will produce a more reliable amp. There's generally a cost, size and weight penalty for pursuing this approach, and a well-built non-overrated amp will likely live just as long for cheaper. Besides, as soon as you build something that's idiot-proof, someone will come along and build a bigger idiot.

[Willabe]

Besides, as soon as you build something that's idiot-proof, someone will come along and build a bigger idiot.

 


                       Brad     

[Tone Junkie]

agreed about the bigger idiot.(LOL)
The reason i was asking is I was building a plexi clone with mustard caps i had aquired over the years and had one in 160v and when I looked on fleabay for .1, 400 volt I was a little shocked by the price and wanted to use what i had if possible.
Thanks Bill

Now if I can just score those mullerd xf1 el34 cheap enough I will have it made (LOL)

[Tone Junkie]

Oh and Hotblueplates thanks for the math that goes with it.
 I will stick that on my wall of usefull knowledge, there is so much math that goes along with these amps. Its hard to remember what equation goes with what section. so I put it where I can see it often and then hopefully remember what goes were.
 But either way its always there for quick referance.
Thanks Bill