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

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.

FTR:  When I mentioned that film caps get leaky, I meant the DC voltage leaked.  This may have no visible effect.

Electrolytic caps OTOH can literally leak, or blow open from internal gasses.

"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.