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

"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. ;-)

I had a krell ksa-100 that I used with a don sachs pre for 3 years and no issues. I now bought a simaudio 760a and it goes into dc protection mode within 10 min of operation. The sachs does leak some dc at the outputs. Some amps just don’t jive with some tube preamps due to dc leakage. We just put the sachs internally and everything is fine. A transformer coupled tube pre is the safest bet. Krell usually respond very well to tube preamps. It might be that since the recap your amp is now too sensitive. It’s a journey man, it can be frustrating but it sure is fun. 

I've used tube preamps with many a Krell amp over the years with no problem.If there is a problem the amp should detect it and shut down until the fault is removed. Years ago when I sold BAT and McCormack. The VK-3 would always shut down the McCormack amps as soon as they were turned on. Removing the pre from the circuit rectified the problem.

Does your direct coupled preamp put out a DC surge when it turns on? If someone has their direct coupled solid state amp on and you turn on your preamp.....could it blow up the amp? Long ago, I had heard that your preamps could blow up things (is this true or not?) I made a pair of custom mono block amps that used the original Hypex class D modules (modified by me). That pair of amps was sold years later to someone else and they were working fine when sold. The new owner had one of your preamps and had the newly purchased amps on and then turned on the preamp. One of the amps immeditately blew up. He called me and we discussed it....that is when I learned that he had one of your preamps and I told him that I had heard your preamps put out quite a DC surge.

@ricevs Our preamps don't make 'quite a DC surge'... During warmup our balanced preamps do put out a low frequency signal but its low in amplitude (about 1/2V is typical); less than most tube preamps make. I'm not aware of any amplifiers ever damaged by it in the last 35 years.

I'm sure you are aware that a DC Offset at the input of a class D can result in the amp attempting to put out thousands of Volts, hence any class D should have an input servo to kill any offsets that might be present. This should be built into the input buffer circuit. Our class Ds seem to be fine if the preamp is turned on first rather than last and our tube amps of course don't care either.

there were many krell amps damaged from BAT tube preamps, they got a bad batch of Jensen capacitors. If it would have only been one amp damaged, krell wouldn't have went to the trouble of putting capacitors on the input of their amps with jumpers to bypass the caps if not needed.

@invalid Ah! Thank-you for that. We were sent samples of various oil-filled capacitors about 20-25 years ago but all of them we tried seemed prone to some electrical leakage (as @erik_squires alluded to; when subjected to vacuum tube operating Voltages at the input of the part there was a slight DC Voltage present at the output); enough that we felt it unwise to use them in an audio coupling application; such leakage could throw off the bias in an output section or cause a DC Offset to be present at the output of a preamp. Not all the caps did this but we found over time that some that seemed OK at first developed this problem later so we never used them in any of our products.