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

@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.

judsauce

You will get some great advice here, and many of us use tube preamps in front of our tube and solid state amplifiers......the combination can be a beautiful pairing.

I'd recommend reaching out to some of the best tube preamp manufacturers and have a conversation with them; most of them will give freely of their time.....as Lynn has done already. Ralph at Atma-Sphere and Aric Kimball of Aric Audio sometimes post here as Lynn has.

Some of those that I'd recommend are: Bob at Backert Labs (Rhumba Extreme & Rhythm preamps), Lynn Olson/Don Sachs (Raven preamp), Mick at Supratek, (several great preamps), Ralph at Atmosphere (MP-1 and MP-3 preamps), and Aric Kimball at Aric Audio (Motherlode II and XL and Super 6SN7 preamps).

I spoke with each of them before I purchased my Aric Audio Motherlode XL preamp, and they all were very giving of their time.....and you cannot go wrong with any of those preamps