VAC Signature MK2a w/phono users

A superb tube preamp - congratulations. The manual says "150 hours", but count on 250 hours for 95% of peak performance, and, like most very high-end two channel gear, 1,000 hours for it to fully open up in a highly resolving system.

Kevin Hayes, VAC's principal, is easy to talk to - feel free to pose your questions to him. I believe that the power supply is solid-state, meaning you should be best off leaving the preamp powered up 24/7, but confirm with Kevin (he is one of the few who understands that small-signal tube gear is generally best left powered up 24/7, for purposes of both extending tube life and maximizing sound quality).

Raquel, thx for the responce. Do you have one of these? Yes I read the same in the manual "150 hours" also but was just curious from users and the sonic differences, can't seem to find any discussions about the phono part specifically in this pre.

I'll have to ask Kevin, I wonder if the unit is on and put on mute "stand bye" are the tubes disabled in the pre and phono sections, if they aren't my thinking would be by leaving it on 24/7 would lession the tube life hours which I would not want to do but after reading what you wrote could possibly differ. thx I'll check into this.

Dear Dev:

No, I have never owned a VAC preamp. I have, however, owned a VAC Renaissance 70/70 Mk. III amp or Renaissance 140 Mk. III monoblock amps for the last ten years, and have known Kevin for all of those years. I have friends who are quite familiar with VAC equipment, having sold it (or sell it), and feel that I have a pretty good understanding of it myself. Your preamp is one of the few preamps that is all point-to-point (hand) wired (no circuit boards), and has other qualities that tend to lead to top-quality sound: superb parts quality, extensive voicing (i.e., the designer spent many hundreds of hours carefully listening to the preamp to maximize its performance), a triode circuit, and no negative feedback.

Regarding your question about leaving the preamp powered on 24/7 and the effect it would have on tube life, the studies done on tube life in the 1950's determined that small-signal tubes (the little tubes used in preamps, DAC's, etc.) generally last longer when left on 24/7. I wrote the following (and quoted my VAC amp manual) in another Audiogon thread:

"24/7 operation EXTENDS the life of small-signal tubes in most preamp and DAC circuits. Consider the following from the "TIPS & ADVICE" section of the owner's manual to my VAC Rennaisance 140/140 Mk. III tube amps:

'How long should tubes last? It has long been known in professional circles (and probably now forgotten) that a tube such as the 12AX7 will display BETTER performance characteristics after TWO YEARS of CONTINUAL operation than when it was new. In normal use it is not unusual for a low level [small-signal] tube to last 5 years or longer. Output tubes [i.e., power tubes used in tube power amps] are another story, as they are continuously providing significant amounts of current.' (Emphasis original).

Small-signal tubes pass little current in most preamp and DAC circuits ... . If left on 24/7, they either die within the first 250 hours or so due to "infant mortality" or they basically last forever. What kills small-signal tubes is two things: one, the thermal cycles that result from turning equipment on and off, and two, the voltage rush they see at power on. This occurs if they are in a component with solid-state rectification (i.e., solid-state power supplies), which is almost always the case. The rare component with tube rectification can be a different matter because the tube power supply functions like a variac to apply current gradually (for the same reason, many tube components have "soft start" circuits so as not to decapitate the tubes at power on). This, not 24/7 operation and heat, is the main culprit of tube death. The Colossus computers used in World War II to decipher enemy radio transmissions used thousands of small-signal tubes. The Wikipedia entry for "vacuum tube" has this to say about operation of the Colossus:

'The Colossus computer's designer, Dr Tommy Flowers, had a theory that most of the unreliability was caused during power down and (mainly) power up. Once Colossus was built and installed, it was switched on and left switched on running from dual redundant diesel generators (the wartime mains supply being considered too unreliable). The only time it was switched off was for conversion to the Colossus Mk2 and the addition of another 500 or so tubes. Another 9 Colossus Mk2s were built, and all 10 machines ran with a surprising degree of reliability. The 10 Colossi consumed 15 kilowatts of power each, 24 hours a day, 365 days a year—nearly all of it for the tube heaters.'

The Wikipedia entry for the Colossus emphasizes this point:

'Colossus used state-of-the-art vacuum tubes (thermionic valves), thyratrons and photomultipliers to optically read a paper tape and then applied a programmable logical function to every character, counting how often this function returned "true". Although machines with many valves were known to have high failure rates, it was recognised that valve failures occurred most frequently with the current surge at power on, so the Colossus machines, once turned on, were never powered down unless they malfunctioned.'

In summary, turn a tube power amp on and off because the output tubes pass a lot of current and will wear out if left on. With regard to most equipment using small-signal tubes, leave it on. For the record, I ran three tube preamps (CAT, Hovland and Jadis) 24/7 for a combined eight years, a DAC 24/7 for five years, and my current CD player, which has six-tubes in its output stage, 24/7 for one year [two and a half years now] - I have never lost a single tube or had any tube problems. I finally swapped out the tube in my DAC after many years just for the hell of it and it tested like a new tube. People lose small-signal tubes because they turn their equipment on and off."

+1 to Raquel. Hey, sometimes Audiogon actually does contain useful information!

I want to add two other points that should have been included in the above post.

First, people running small-tube gear with vacuum tube power supplies (rectification) typically should not run such equipment 24/7 because the tubes in the power supply are often run hard (Jadis and ARC, for example, use power amp output tubes in their preamp power supplies).

Second, there is the occasional preamp circuit that runs small signal tubes hard.

In both cases, the tubes are passing a lot of current, and getting hot, relative to their design limits, and just like the output tubes in a power amp (or the tires on a race car), they will wear out and need periodic replacement.

That said, in most preamp and DAC's, the small signal tubes in the circuit path (as opposed to the tubes in the power supplies if the unit has a tube power supply) are cruising along at voltages well below their design limits. When not in use, the preamp's volume pot should simply be turned down and the mute switch (if the unit has one) engaged - this prevents your system from amplifying any pops or surges that may occur from refrigerator motors or other junk that may be on a shared line.

Aside from my tube amp, the only time I power down my equipment is in the event of an electrical storm (in the event of a storm, all equipment must be powered down and UNPLUGGED immediately).

Raquel, thx for all of that detailed info. I won't need to contact Kevin now. Always learning and appreciate the time you spent replying it was very helpful and educational.