Coupling capacitor voltage rating - Calculating the 'proper' value?

[Leo_Gnardo]

If Marshall ever made an amp with 250V rated coupling caps Loopytoots is going to have to prove it..

I think Loopy's having us on. Or hallucinating. Or both. I call B.S. Photos, or it didn't happen.

[J M Fahey]

And if you connected one side of a 200v cap to a 300v supply and left the other end open? That almost infinite resistance will still let the cap eventually charge, but I don't think that the cap would ever be damaged from the overvoltage. That's all I was getting at.

If you leave one side open, then the cap gets *zero* volts, as simple as that.
We speak voltage all day long, but in fact it's always voltage difference between two points.
You have just one? ... no voltage.

Now a high impedance point is not the same as open.

A coupling cap of, say, .1uF and a 1M pot or grid resistor defines a time constant of 0.1 seconds , meaning after that time the cap will be charged to 63% of applied voltage .
0.1 seconds !!!!!!
So in , say, 10 or 20 seconds it will charge to around 99% of applied voltage.
As you see, the apparently high 1M series resistor provides little or no DC attenuation after a few seconds and the capacitor has to stand full voltage.

That said, I always used 400V caps and never ever had a problem, but I always used Siemens/Epcos or SIC/Mallory ones.

[pdf64]

I don't imagine Fender or Marshall ever had anyone less qualified than me on their design staff

The bright sparks that designed/approved/put into production the 1987 'bias supply after the standby switch' brainwave should have any qualifications revoked!

[Enzo]

And yet the amps soldier on year after year, never knowing how poorly they had been designed...

[Chuck H]

And yet the amps soldier on year after year, never knowing how poorly they had been designed...

Like many of us