tube pre amp and Krell FPB 600

I agree that it is not likely a preamp turn-on or turn-off thump will damage the power amp. The real point is the damage to the speakers. Krell has good reason to be concerned about this in my opinion.

Since I have been all-tubes for many years, including phono, linestage, power amps and even DAC, I really don't have a dog in this hunt. Whatever Krell does or does not do is of no concern to me.

I would never underestimate, however, the ability of some well-meaning audiophiles to mess things up. I clearly recall a lengthy exchange I had on some forum, probably Audiogon but I am not certain, where the other party was insistent that a preamp should always be turned on after the power amp, never before. (That is not a typo.) He dismissed my concerns about possible damage by saying he had never had a problem or experienced any thump for 20 or more years. Of course, his good experience hardly proves the rule; it just shows the muting relay in his preamp works.

With due respect to Ralph, I think Krell's concern about tube preamps being used with their direct-coupled amps is a valid one. Many tube preamps have a substantial low-frequency pulse on turn-on and turn-off which can cause major problems for a high-powered direct-coupled solid state amp. Note that these pulses occur notwithstanding a coupling capacitor on the output of the preamp. Most modern tube preamps have a muting relay which allows the preamp voltages to stabilize before the amp is connected. However, not all tube preamps have muting relays and Krell is understandably going to err on the side of caution. No manufacturer wants an unhappy customer.

Of course, Krell could have just warned that the power amp should always be turned on after the preamp has warmed up and turned off before the preamp. This simple procedure would also solve the thump problem, but then who wants to count on their customers actually following this advice.