• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

input cap V rating

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dsavitsk said:
There is offset, but the voltage of the previous stage is low. The previous stage is an I/V, and the tube stage is a gain stage for the dac. I am really just concerned with high voltage from the tube stage damaging the previous stage.

For a basic common cathode amp, the voltage rating of any input DC blocking capacitor isn't all that important. Any AuriCap in that position will be just fine. Cheaper disc or monolithic ceramics (which you shouldn't be using in the first place) can be rated at 50WVDC. It's not a problem since vacuum tubes, unlike transistors, will almost always fail open. It is also far more likely to poof a tube by a grid short to cathode, not the plate. That, too, will remove any DC from that input capacitor.

Of course, it's a whole 'nother story if the grid is at some substantial DC voltage, as would be the case for an input LTP to increase the length of the tail or provide working voltage for a CCS in the tail. In that case, best make that voltage rating as generous as is practical.

I would always include one in case you connect to some sound card or preamp that uses its audio feed as a DC power line. 12VDC positive on the grid of most small signal tubes will poof them.
 
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