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

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.

I have an older Krell KAV amp that has been recapped and refreshed.  A technician that I respect very much said do NOT use a vacuum tube preamp with my KAV 250, or any other Krell amp for that matter.  Can someone with more technical knowledge than myself tell me why I should not be using a vacuum tube preamp with my Krell?  Are there some technical specs that I should be aware of when pairing?

@judsauce There's no problem using almost any tube preamp with a Krell. They have been promoting the myth that you can't for a couple of decades at this point.

Here's the truth of the matter: Regardless of the preamp or amps you own, ALWAYS turn the preamp on first and allow it to stabilize. With a tube preamp this means a 30 second warmup time. Then you can power up the amps. Solid state preamps can have a substantial turn-on thump too, which can easily damage loudspeakers or an amplifier, so this is simply good practice. Many modern preamps have a warmup mute function so people have gotten complacent and I'm sure this is how this myth got started; literally one person damaged their Krell amp in this manner and Krell decided to spread this myth.

People have been using tube preamps with solid state amps for decades (our preamps have a direct-couple output and we have lots of customers with solid state amps; we even make a solid state amp...); clearly this isn't a problem!

@erik_squires mentioned something about film caps (used at the output of many tube preamps; ours have a patented direct-coupled output so not us...) getting 'leaky' over time. To be clear, 'over time' means about 50 years or more. I've seen some film caps from the 1950s and 1960s that have gone bad but since then not so much, and FWIW, they usually open up rather than get 'leaky'. That's something that electrolytic caps, which are not as coupling caps in tube preamps, do.

@dogearedaudio 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'.