Vacuum Tube preamp with my KRELL KAV 250a, a no no......WHY?

There was a thread in another forum recently wherein a fellow was using a highly-regarded tube preamp with a very sensitive SS power amp.  The amp kept going into shutdown, and after much discussion with the preamp's designer, it was determined that there was just enough DC leaking from the preamp to trigger the amps input sensors.  So, yes, it's a thing, unfortunately.

A while back I was using some very pricey output capacitors in my homebrew tube preamp.  One day the sound disappeared on one channel.  After a lot of fussing around I finally figured out that one of those pricey caps had shorted, sending 150VDC to my homebrew tube amps.  Fortunately, as Lynn says, tubes are pretty tough and there was no meltdown.  But I learned a) to avoid pricey boutique caps and b) that tube preamps can definitely be tricky with sensitive amps.

"The cap failure you experienced might have been caused by not speccing the cap right. If its connected to a plate circuit, it must be rated at a value 15% or so above the no-load DC Voltage of the power supply in the preamp! If not, it can short on turn-on.  I have seen some oil-filled parts that have developed 'leakage' but its worth noting that they are not considered to be 'film caps'."

It's possible that I cut it a bit close, with a 300VDC rated cap in a circuit that, worse come to worse, might see 300VDC before settling to 150VDC--though I took plenty of precautions.  I'm certainly more prone to err on the side of caution since then. ;-)