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

Krell does not use coupling caps at the input which can result in problems if DC is leaked in from the preamp (both for the amp and speakers). The upside is better transient response as the signal is not impeded a constant charging and discharging capacitor.

Even though the amp will have some sort of DC protection, it may not be enough to overcome a tube preamp DC spike per lynn_olson’s post above. You should contact that tech again and ask if the amplifier has coupling caps that can be activated in order to use a tube amp.

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.

I think the thing to consider is the DC offset or surge from the preamp. Turning the tube preamp on first & waiting a couple minutes to stabilize usually does the trick. Turning the preamp off last during shutdown of the system is also common knowledge. I run tube preamps with solid state and on a couple of occasions have turned on the tubes not knowing the solid state power amp was already on. One of my amps has protection so it will go into protection mode if I screw up. My other amp blew a fuse & shutdown after being lackadaisical with my on/off process. I initially thought my subwoofer blew up due to the loud thump. When my amp with protection shut down it first sent a loud squealing noise through the speakers. 

I've never had any significant or permanent damage but the risk of it happening is real. It's a scary feeling when you think you fried your gear. Another thing to consider is a quick power outage..that just as quickly power's back on. This can cause the DC effect as well off certain preamps. I don't know the correct terminology or actual effects from an engineering standpoint but I know it's not good. 

With all that said I do enjoy my digital music thru some type of tube gear.. preferably a preamp. So that's what I choose to do. I don't really worry about it & enjoy the music...& give my tube gear time to stabilize before turning other components on. One should always make sure that their tube pre or power amp is up to spec. Buying used vintage tube gear should be looked over before hooking it up to expensive equipment or speakers. 

@lynn_olson   Your input is much appreciated.

HERE IS THE SKINNY:

I have 2 KAV250a's. In May, 2022, I had the oldest one recapped with some new resistors, etc, to the tune of $1,600.

Being a guitar player with several tube amps, I always loved the warmth and body vaccum tubes offered. 

In March of 2023 I bought the Audio GD Vacuum HE1-XLR BALANCED LINE PREAMP.  I used that along with my OPPO BD-95 for cd, and my Sonic Frontiers SFP1 phono pre-amp.  The phono preamp was recapped and reconditioned by Glenn at Parts Connection.  Those sources, along with the pre-amp and amp have logged over 220 hours in my diary with absolutely no issues.  The sound, to me, was fabulous. 

Fast forward to this past week.

THESE ARE THE EXACT CONDITIONS IN PLACE WHEN I HAD A MALFUNCTION:  I had just purchased and installed a pair of NOS Mullards and a pair of Gold Lions in the Sonic Frontiers phono pre-amp.  I fired everything up and put on Dire Straits lp, Leaving Elvis.  Before the song was over, the left channel went silent.  Then about a minute later, followed by a mild and dull low volume "thud", the right channel went out.  No smoke, no smell, just no music.  My trouble shooting revealed that the amp was not working.  So, I swapped in my second KAV250a, immediately put back in place my original tubes in the phono pre-amp, and have only played the OPPO blue ray so far.  And everything is fine.

The Krell has 26.4dB of gain, input sensitivity of 2.15 Vrms, and 100K ohms input impedance.  

The Audio GD Vacuum HE1 XLR LINESTAGE PREAMP has an excellent review by STEREOPHILE, whose link I will attemp to paste below.  I bought it from Walter and Underwood Hi-Fi (whom will be retiring this week, he sold the company.)

The amp is going back to the technician next week.  So, is this a bad combo, in your opinion?  By the way, the malfunction could be independent from the tube swap in the phono preamp, and it could be coincidental.  

All opinions welcomed.  Have a great Holiday Weekend, and pray for our soldiers.

Jud

@mattw73 Thanks for your reminder about the sequence of powering on and off.

I have to admit, I sometimes forget and then end up crossing my fingers.  Not good.

Regarding the power going out or flickering during storms: I have both preamp and amp going straight into fresh/new sockets in the wall.  Whenever there is even the slightest prediction of a storm, I yank the power cables on both until the threat passes.  And if I'm away for a long time, I pull them just as a precaution.  

Like you, I also prefer some tubes in the chain.  The Krell paired with a SS preamp sounds a bit bright for my taste.  And there is nothing like high volume creamy distortion when driving a class D tube guitar amp. Buttery smooth and so fun to play.  But a vacuum preamp, for my taste, rounds out the brittle high end and adds a bit of warmth.