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

I tried calling Krell close to where I am, they're in Connecticut. But when I went to leave a message with tech support I got a nice sounding man who said, "please leave me a message". Then his mailbox promptly told me, "voice mail box full". So, I left a message with sales. 50 bucks says I'll never hear back. Not like the old days, for sure. Wife runs the company now, Dan is off selling, well, you know what. 

@erik_squires  , can you explain what happens that makes that "pop" when a preamp is inadvertantly turned off before the amp?

@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.

Thanks, @erik_squires  ; I always appreciate the tid bits of knowledge that I get from you and @atmasphere  .

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.