DIY Valve Guitar Amp Head

[Simon Barden]

And on no.1, the working voltage should ideally be half that or less of the caps rated value. Above that, the capacitance value drops off (so you might need to use a bigger value capacitor than normal if that's the case). And when working with AC circuits, if the cap only has a DC voltage value on it, then remember that the RMS voltage of an AC circuit is less than the peak voltage achieved by the AC signal. For a sine wave this is 41% greater than the RMS value.

[DrNomis_44]

And on no.1, the working voltage should ideally be half that or less of the caps rated value. Above that, the capacitance value drops off (so you might need to use a bigger value capacitor than normal if that's the case). And when working with AC circuits, if the cap only has a DC voltage value on it, then remember that the RMS voltage of an AC circuit is less than the peak voltage achieved by the AC signal. For a sine wave this is 41% greater than the RMS value.

Good point there Simon, I've actually got first hand experience of what happens to an Electrolytic cap if it's working voltage is exceeded, I've got a variable DC power supply that I built from a kit of parts that I bought from Jaycar Electronics, one day I powered it up so I could use it to power a circuit I had breadboarded on one of my solderless breadboards, about a minute after I turned the power supply on I heard a loud bang from inside the power supply, I quickly powered it down and disconnected it from the mains power, after I had recovered my nerves I took the top plastic cover off the power supply so I could have a look inside to see what caused the loud bang, it turned out to be very obvious just what caused the loud bang, one of the main power supply filter caps just after the bridge rectifier had exploded and blew it's aluminium casing off so that the casing was now lying on the bottom of the power supply's plastic casing, as luck would have it, I still have the pics I took of the exploded cap so I'll post them here.


Here we go, this is to illustrate exactly what can, and will happen when a cap exceeds it's working voltage:








As you can see in the pics, the cap in question was a 2200uF/25V Electrolytic cap, my guess is that the voltage across it was a bit too high and the result is what you see in the pics, the other 2200uF/25V cap is in parallel with it so maybe the one that exploded was inherently faulty in some way when it was manufactured, I ended up replacing both of them with a couple of new 2200uF/35V caps if my memory serves me right, and the power supply has never given me any more trouble since.


A very classic example indeed.