input cap V rating

[dsavitsk]

In a basic grounded cathode circuit with an input cap, what sort of voltage rating should the input cap have? I guess the question is, in the event of a catastrophic failure (of the tube) how much voltage might that cap need to block from getting to the input, or is this even a concern?

[Giaime]

What kind of input? If the input signal has no DC offset in it, you can remove the input capacitor.

If the input is a previous tube stage, the input cap should be able to sustain the full B+ voltage.

[dsavitsk]

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.

[Miles Prower]

Originally posted by dsavitsk
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.