Hello KoumparosJ,
each electrolytic capacitor has a very huge derating over the applied voltage. If you really use an electrolytic capacitor close to its rating, its lifetime will drops in the region of 10 years. This is what I found in the data-sheet: Voltage derating is expressed as the percentage that the applied voltage is less than rated voltage, e.g., a 450 V capacitor operating at 400 V would have 11% voltage derating. In applications operating at less than 45 ºC no derating is needed, and with up to 75 ºC, 10% is sufficient. For higher temperatures and with high ripple current, 15% or 20% is appropriate. Since operating life continues to increase for further derating, military and space applications use 50% voltage derating. Means if the capacitor is rated for 450V 300V would be a good figure for the very high temperatures, we have in tube amplifier.
Because the capacitances are connected in series, the current passing through the capacitors must be equal, so each capacitor receives an identical charge. If their capacitances are equal, then the voltage across each one of them must be equal.
Unfortunately, even if the capacitances are equal, the leakage currents in each individual capacitor will not be equal. To equalise the voltage, and prevent one capacitor from exceeding its rated voltage, each capacitor should be bypassed by a resistor so that the resulting potendial divider chain forces the voltage to be equal. Means you choose a value for the resitors, where its quiescent current is 10 times higher than the quiescent current of the capacitors. 220k Ohm is a good choice. If you want to test your amplifier without tubes, your votage will easely increase to 550V. 2 X 300V would than be really critical. 350V you mentioned goes in the right direction. Even the company weber uses in their soldano kits 450V caps. This was my first SLO100 I build.
Best regards
Olaf E.
