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

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.

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.

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.

Ps, recapping updates power caps, rarely signal caps.

May be worth putting this into perspective.  Your typical SS pre may have +- 15V rails.  You could put your tongue on that and survive.

A tube preamp, like an old TV or amp, has potentially lethal voltages.

To help explain, the maximum voltage in an amp is around 8V for pro audio gear, and around 2 I think for consumer. So even a few millivolts offset can cause a pop.

The issue isn't just the offset it's a step.  So consider a preamp that's off, but normally has a few milliamps of offset.  When you turn the preamp on, if it has no delay relays, you go from 0V to say 0.010 V, and that sudden step is the popping you hear. A cap in the way still passes the step, but over time blocks the 0.010V.  So if you looked at the wave form on the other side of the cap you'd see a spike, then decline to zero, even though the preamp circuitry may still have 0.010V on the output.  Of course, some of it may just be the random nature of the amp getting powered up even without a final offset.

@immatthewj  - It's usually a little DC.  DC = direct current which can happen from leakage or a preamp which doesn't settle at exactly 0 volts.

Easy and save enough to measure.  Any multimeter will let you know how much DC offset there might be in your preamp.  A few millivolts is fine.

OP:  I should mention that most leakage is in the low V range, so your average input coupling cap will handle it.  You can always measure the DC yourself.

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.

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.