Question about decoupling capacitor

[Tom Vyborny]

Hi guys!
I've just buildt a tube preamp for my bass guitar and at the last tube stage, the signal is decoupled by decoupling film 100nF capacitor from the plate. I'm bit worried about the fact that if that capacitor faills, the high plate voltage will appears at the input of my amp. My question is - should I be worried about the cap faillure and add some sort of protection or is it ok and I shouldn't be worried about that? Thank you very much.

 

[JonSnell Electronic]

May be you are using the wrong terminology as there is confusion between Coupling and Decoupling.

As long as the coupling capacitor is of a suitable working voltage, everything should be fine.

 

[TMM]

What I've done in the past is put a bidirectional 15Volt TVS diode from after the coupling cap to ground. At normal signal levels, it might as well not even be there, but gives you protection from a DC fault.

 

[Mooly]

I'm bit worried about the fact that if that capacitor faills, the high plate voltage will appears at the input of my amp. My question is - should I be worried about the cap faillure

Yes, you need to be certain the cap is suitably rated.

and add some sort of protection or is it ok and I shouldn't be worried about that? Thank you very much.

The output side of the cap should always be tied to ground via a high value resistor to ensure no charge is present on the cap which could/would cause damage to any solid state equipment it was 'hot plugged' into.

A lot depends on the value of the cap but something like 470k would be a good start.