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.

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'.

@atmasphere  Thank you so MUCH for taking the time to reply.  I do feel a bit better.  My reply this morning on this thread at 8:16 A.M. lays out my problem that prompted this thread in the first place.  Something did happen to my KAV250a.  But there was no smoke, no smell, just a soft malfunction-left channel first, right channel second.  I'm taking the amp back to where it was re-capped and refreshed in 2022, so next week I'll have more idea about what caused it.  Might not have been the pre-amp at all.  But, thanks again for your input.

regards,

judsauce

Ralph,

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. He seemed to not know this about your preamp.

There is no way to make sure you always remember the WAY to turn on your stereo. We are living in another world/time now, we should not need to "KNOW HOW" to turn on the stereo. There should NEVER, EVER be a product sold to the general public that puts out a serious DC surge when it turns on. Every preamp and every CD player and DAC that I have EVER worked on has DC surge protection. They cannot hurt anything. The orginal Audio Research preamp had no protection....Instead you were suppose to plug in your amp to the back of the preamp and there were two on buttons on the front. You would turn on the preamp first and whistle dixie for a minute and then turn on the amp. And when you wanted to turn off the system you would turn off the amp first and then the preamp....Of course, this was the 1970s.....I owned one of those Audio Research SP-3A1 preamps things. Very soon after that Audio Research put a DC protection circuit on their preamps....so it was NEVER an issue after that......that was the early 80s.......This is 2024. By the way, i have never seen a solid state preamp of recent vintage (within the last 30 years) with a DC surge on turn on or turn off. Most of them are direct coupled and/or DC servoed.....and protected.

@atmasphere  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.