Big capacitor's voltage rating in DIY amps

[Mooly]

If in any doubt measure the voltage across the caps with the amp on to see what you are dealing with. Circuit diagrams are not always correct particularly when regional/country variations are taken into account.

If the caps really are 75 volt and original then there is absolutely no problem fitting 80 volt devices.

Bypassing with a film does nothing with regard to voltage ratings. It's only of use in reducing the impedance of the supply at high frequencies... which probably isn't needed as there will be more local decoupling in the amp.

[AmCan]

Thank you Mooly, I really appreciate it. I will measure the voltage.

[sreten]

Hi, 80V capacitors will be fine on +/- 55V rails, rgds, sreten.

Bypassing is pointless, look at the main board decoupling arrangements.

[Conrad Hoffman]

If you use the maximum line voltage you ever expect to see for moderate durations (ignore spikes), and under unloaded conditions for the PS, that should give a reasonable minimum for the cap rating. Most of the time it will be operating with a generous factor of safety and will even be OK if the circuit is disconnected. Most caps also have a higher surge rating, so the FOS is actually even better. The manufacturer has already put in some margin because building caps isn't as precise or consistent a business as one might hope.

[AmCan]

Thanks guys, I appreciate that not only have you answered my question directly but I have learned something about making determinations.

I also like my Canadian flag up there I just noticed that.

[AndrewT]

Conrad is correct.
One must look for worst case operating conditions and design for those eventualities. The design decision could be to accept that some conditions are acceptably rare that the designer accepts responsibility for future failure when the foreseen risk is achieved. That's part of what a Manufacturer's warranty is about.

Guessing at a Factor of Safety (FoS) does no good. Determining a FoS based on testing and experience and actual variations in product specification is quite different.
There can never be an assurance that a guessed at FoS of 1.3 (that's 30% over the rating) will prevent failure. Particularly so when the worst case conditions may already exceed the "normal" operating conditions by more than that 30%.