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

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.

Interesting read. I used a tube preamp with a SS amp for a year without any issues, but reading all this, I will definitely stick to SS for both from now on. Tubes was not my thing. Too many updates temptations.

Hey @judsauce -

If a tube swap fixed your issues you have your answer. Any leakage at the outputs would not change based on the tubes, AFAIK. In either case, this problem is super easy to measure. Get a multimeter that measures at least down to 0.01 V and put it across the preamp outputs. You should measure very little, if any, DC.

I could imagine it is possible that swapping tubes caused oscillation though, which is a different issue than DC.  That's when the pre or an amp starts outputing very high frequencies (the very opposite of DC) which are inaudible, but still heat up the amp and voice coils.

There are some good reasons for mixing tube pre/solid state amp, such as wanting to drive very difficult speakers. Electro-statics for instance can really shine this way.