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

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.

Conventional vacuum tube preamps often use a "cathode follower" to assure low output impedance and compatibility with a wide range of power amps ... vacuum tube, transistor, Class D, GANFet, etc.

The plate circuit of the cathode follower is typically at 300 volts DC, and the cathode (the output) is typically at half that, or 150 volts DC. A blocking capacitor is used to remove the high voltage, leaving the output at zero volts DC, but when the preamp is turned on or off, significant transients can make it through the blocking capacitor. The most common strategy to deal with this is a shorting relay with a time-delay circuit ... that’s the faint "click" some preamps make when they are turned on or off.

Although it is good practice to turn on the preamp first, wait a minute for warm-up, then the power amp, and turn off the power amp first, followed by the preamp, this mostly protects the speakers from loud transients. A transistor power amp can be damaged, even if it is turned off, if the input transistors are exposed to more than 20 volts. That’s why a good vacuum tube preamp will have a muting relay as part of the circuit ... but not all tube preamps have a muting relay.

The preamp I designed for Don Sachs uses an output transformer, which are free of turn-on and turn-on transients. But coupling capacitors are much more common than output transformers, and muting relays are pretty much a requirement for that kind of circuit.

Old-school 1950’s preamps almost never had a muting relay, but all power amps used tubes back then, and they aren’t affected by transients (although speakers are). It was OK to simply turn on the whole system at once, since the tubes in the power amp took the longest to warm up, which protected the speakers.

Things got more tricky when DC-coupled transistor amps became the norm in the early Seventies. A mix-n-match of a traditional tube preamp could easily damage a first or second-generation transistor power amp, and maybe take out the speakers, as well. Modern tube preamps typically have a muting relay, which is there to prevent transients or failure-mode DC offsets.

You are not necessarily home free with a transistor preamp. Some use DC servo circuits to automatically zero out DC offsets, and if this servo circuit fails, and presents the power amp with several volts DC, that can take out a DC-coupled power amp, and damage the woofer, as well. Loudspeakers can only tolerate less than a watt of DC offset.

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.