The input power is the output power plus the booster loss. With an output of 13 volts at 20amps, or 260 watts, the input power would be close to 290 watts (assume a 90% eff boost). If the input voltage to the converter dropped to 10 volts under load, the input current would be 29 amps. Even with 10 volts and 30 amp fuses, 100 watts PEP SSB should be OK because of the low average power....PSK would be 100%, but at lower power, so it should work too. Check the voltage on the input side of the converter. Do the fuses get warm? If they do the contacts at the holder may be bad.