Fundamentally there is no problem of course. However, most tube preamplifiers are designed alongside tube power amplifiers which are always AC coupled. As a result, many tube preamplifiers lack precautions against turn-on/turnoff transients. Some are liable to produce the full anode voltage at their outputs during turn-on and turn-off. We find that a large proportion of repairs are due to tube preamplifiers discharging their output caps into the UcD input circuit. Note that most solid-state amps dislike such treatment, not just UcD or NCORE amplifiers.