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

@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.

@judsauce , well, we’ve got the long weekend coming up, so maybe that is having an effect on whose there to answer the phone. We’ll hope so anyway. Maybe if you give them a call on Tuesday you will get through. Have you got your eye on a pre that you are thinking you better get crap or get off the pot on? If so, I do know the feeling.