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


I have an older Krell KAV amp that has been recapped and refreshed.  A technician that I respect very much said do NOT use a vacuum tube preamp with my KAV 250, or any other Krell amp for that matter.  Can someone with more technical knowledge than myself tell me why I should not be using a vacuum tube preamp with my Krell?  Are there some technical specs that I should be aware of when pairing?

Thanks

judsauce

@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.

I know zero about the technical issues being discussed in this thread. I can speak only of my own experience. I ran a Krell KAV 250a with an Audible Illusions Modulus 3A preamp for over 20 years….without a single technical problem. And it sounded great. YMMV

@immatthewj   you're too funny. Nothing to get off the pot on. Check my virtual system. It's the preamp that I have that's in question.  It has the best music coming out of my system that I have ever heard. That's why I'm scratching my head.

@lynn_olson  thanks so much for that information. It was very detailed and much appreciated. A big wealth of knowledge that I hope others studied as well.