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

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.

The big thing to confirm is the preamp under consideration has an auto-mute circuit, which triggers both for turn-on and turn-off, as well as brief power drop-outs. Not every preamp has a power management circuit that protects against all sources of transients. This is ESSENTIAL if you intend to mix a tube preamp with a transistor power amp.

Voltages over 20 to 50 volts will kill or degrade the input transistors, which are not designed for high voltages. A transistor power amp has no way of defending itself against transients of this magnitude ... I’ve yet to see a transistor power amp with an input relay. For that matter, coupling caps are quite rare in the transistor world. In any event, an input coupling cap will not protect the input section against a transient that large.

By contrast, a tube amp will tolerate a transient of hundreds of volts, even at the input. Not for long, of course, but for a few seconds, yes. To damage a tube amp the plate of the tube actually has to melt down, or more likely, burn up a cathode resistor. That takes time. By contrast, a transistor will fail from over-voltage or over-current in less than a millisecond, too fast to run across the room and turn it off. You’ll see a little puff of smoke emerge from the power amp and that will be it ... next step, a new circuit board with all-new parts. (I’ve seen this happen.)

There is no practical way to protect a transistor amp from what any manufacturer considers abuse (yes, the warranty will be voided, and yes, they can tell when the tech sees a burned circuit board with dead input transistors).

For better or worse, the burden of protection lies with the design of the preamp. Even transistor preamps can be the culprit if it has a split +/- 15 volt supply and one side folds down because it doesn’t feel like working any more (or if a regulator quits). That will yank the preamp output to either plus or minus 15 volts, it will stay there until repaired, and bye-bye power amp.

The real function of muting relay circuits is preventing these disasters. As mentioned earlier, output transformers can also prevent DC from getting through, and DC transients don’t make it through, either, because the transformer rejects all signals below 14 to 20 Hz.

Direct coupling has many advantages, but unfortunately, failure modes can propagate through the entire electronics chain unless protective measures are taken.