If a coupling cap is properly sized for the whole audio range, there should be NO AC voltage across it in normal operation. The entire reason for a coupling caps existence is to pass the AC and block the DC. At very low audio frequencies there will be some AC voltage across the cap due to phase shift effects. A cap sized to roll off low frequencies (guitar amp or crossover) could see significant AC voltage across it at frequencies below the corner frequency.
This can be demonstrated with a dual trace scope. Put one probe on each side of the cap, set the traces to add, then invert one of them. The scope will show the voltage (AC and DC) across the cap.
The worst case for a coupling cap is usually at turn on and sometimes turn off. Consider a fixed bias amp. The output tubes bias should come up quickly at power up, so the output tube's grid will be at the bias potential (say -50 volts for example). The driver tube is not conducting yet, so its plate voltage could be equal to the full B+ voltage (say 400 volts) the cap will see 450 volts at power up. As the driver tube warms up its plate voltage will drop to the normal operating voltage (175 volts). In this case the cap will see 450 volts at power on and 225 volts in normal operation. These are real voltages measured on a TubelabSE with 300B tubes, and I use 600 volt coupling caps in these amps.
This can be demonstrated with a dual trace scope. Put one probe on each side of the cap, set the traces to add, then invert one of them. The scope will show the voltage (AC and DC) across the cap.
The worst case for a coupling cap is usually at turn on and sometimes turn off. Consider a fixed bias amp. The output tubes bias should come up quickly at power up, so the output tube's grid will be at the bias potential (say -50 volts for example). The driver tube is not conducting yet, so its plate voltage could be equal to the full B+ voltage (say 400 volts) the cap will see 450 volts at power up. As the driver tube warms up its plate voltage will drop to the normal operating voltage (175 volts). In this case the cap will see 450 volts at power on and 225 volts in normal operation. These are real voltages measured on a TubelabSE with 300B tubes, and I use 600 volt coupling caps in these amps.
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