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

Hey @judsauce -

If a tube swap fixed your issues you have your answer. Any leakage at the outputs would not change based on the tubes, AFAIK. In either case, this problem is super easy to measure. Get a multimeter that measures at least down to 0.01 V and put it across the preamp outputs. You should measure very little, if any, DC.

I could imagine it is possible that swapping tubes caused oscillation though, which is a different issue than DC.  That's when the pre or an amp starts outputing very high frequencies (the very opposite of DC) which are inaudible, but still heat up the amp and voice coils.

There are some good reasons for mixing tube pre/solid state amp, such as wanting to drive very difficult speakers. Electro-statics for instance can really shine this way.

judsauce

You will get some great advice here, and many of us use tube preamps in front of our tube and solid state amplifiers......the combination can be a beautiful pairing.

I'd recommend reaching out to some of the best tube preamp manufacturers and have a conversation with them; most of them will give freely of their time.....as Lynn has done already. Ralph at Atma-Sphere and Aric Kimball of Aric Audio sometimes post here as Lynn has.

Some of those that I'd recommend are: Bob at Backert Labs (Rhumba Extreme & Rhythm preamps), Lynn Olson/Don Sachs (Raven preamp), Mick at Supratek, (several great preamps), Ralph at Atmosphere (MP-1 and MP-3 preamps), and Aric Kimball at Aric Audio (Motherlode II and XL and Super 6SN7 preamps).

I spoke with each of them before I purchased my Aric Audio Motherlode XL preamp, and they all were very giving of their time.....and you cannot go wrong with any of those preamps

Regarding the power going out or flickering during storms: I have both preamp and amp going straight into fresh/new sockets in the wall.  Whenever there is even the slightest prediction of a storm, I yank the power cables on both until the threat passes.  And if I'm away for a long time, I pull them just as a precaution.  

This is good practice, but about a third of the time I've seen gear go up it has been stormless.  About half of the power events at this location in SC were in clear blue skies, due to some transformer issue, and I'm often not even aware of a short intense storm about to roll through.  YMMV, but I've seen enough gear lost to something coming down the power or coaxial cable that I'm willing to "sacrifice" for surge protection.

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'.