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

The combination is often used and can be a magical. The tech is being overly cautious, but not entirely making things up.

Solid state preamps and tube preamps produce the same output voltages when working correctly. The issue with tube gear is that internal voltages are often 400V or greater. Most tube preamps use a "coupling capacitor" at the output to block that high DC voltage to nearly zero and let only the music go through.

The problem is on the rare occasion and over time the coupling caps can leak. If the amplifier lacks it’s own coupling cap on the input, or that coupling cap is not rated for the leaked voltage you can push the amp into overdrive, not to mention send your speaker cones ballistic.

Most solid state amps are "ac coupled." Meaning they use a cap on the input to prevent just such a problem. I believe I read somewhere that some Krell amps have an internal jumper in case you are not using a tube pre and want to avoid your music going through it. Worth checking on it.

Actually, OP, after reading @erik_squires  reply, I do now remember someone posting about losing a pair of speakers due to what @americanradiance just described.  That must be what your tech was talking about.

Typically you want the input impedance of your amp to be more than 10 times the output impedance of the preamp for optimal performance.  The input impedance of your amp is 10 kOhms so as long as the preamp is below 1k Ohms (as many are) it should be fine unless your amp is different from almost any other solid state amp.  I’d shoot Krell a call just to make sure, but I highly doubt it’d be a problem to use a tube preamp.  As mentioned above, many people here use a tube pre with a solid state amp with no problem and wonderful results.

@immatthewj 

@invalid 

Thank you both!

@erik_squires 

When you say "over time, the coupling capacitors can leak", what if they were replaced during the reconditioning? And, what would you consider a time frame for malfunction: 50 hours, 150, 250, or over depending on use?

Thanks again, everyone!.

OP:

Tube preamps use high quality film caps that usually have a life span longer than ours, but failures do happen and they may be small (few millivolts) to larger.

The input coupling caps on amps may vary depending on the quality of the amp. Lots of Yammies used bi-polar electrolitic. 40 years or so?

But worth getting into some details. The preamp coupling caps are rated for the voltage in the preamp, so 650V caps are not uncommon. Amp caps though may only be 50 V. Should the preamp cap fail and deliver 500V, it will absolutely break through the amp’s coupling cap and go boom. :)

If your preamp caps have just been replaced with high quality films they’ll last you a hundred years if they last you five. :)

The real guru on this subjecte though is going to be the manufacturer, @atmasphere who will correct me at length if I'm wrong.