Friday, November 2, 2012

When to Change Tubes

Many of you know that I am a tube fan and own a Dared MC-7P preamp. After performing several rounds of modifications by pulling out ground loops, changing caps, correcting RIAA errors, replacing the volume potentiometer, and swapping out those cheap tubes, I have grown to enjoy the musicality of this preamp despite its modest initial price. It is light, airy, detailed, silent, and dynamic providing clarity and timbre only the finest transistor designs can match.

But being a tube preamp, once I swap out tubes the clock starts running; one day I will need to replace them.  But the better question would be: when?  NOS tubes take about a week of use to settle in and start to sound the way they will for the next several months (maybe even a bit over a year) and with the price of a good Mullard 12AX7 going from $50-$400 a piece, the timing for replacement can mean the difference between making a wise investment and making a bad decision.

The problem with tube aging is that it is a gradual process and can creep up on you as you become desensitized to its impact.  In other words it is sort of like a frog: toss a frog in boiling watter and it will immediately jump out but put it in cool water and slowly bring it to boil and it will die.  The fact that you listen to your gear every day makes you not notice these symptoms but one day you will become alerted to their telltale degraded signs.

It is interesting to note that tubes that should be changed may still measure good on a tube tester (so what else is new?).  So here are the top 3 hints that let you now from subjective listening when it may be time to swap out those little room heaters (aside from a complete failure meaning that it stopped working entirely). 
  1. Pops, tinks, and other weird spontaneous noise.  This is the number one reason to change a right now tube.  Issues of injected noise or spurious spikes are obvious signs of having to say goodbye to what may be an old and well appreciated friend.  Tubes that age to this point can actually be dangerous to the amp and speakers and should be replaced immediately.
  2. Edginess and sibilance.  This is a deceptive condition where on the one hand the tube starts to emphasize one region of the audio band you may find appealing but believe me, the tube is on its way out.  This is the second most important reason to change a tube since what is hiding behind this symptom can also prove to be disastrous to the rest of your gear.  This is usually an indication that for some reason the tube is breaking into spurious oscillation and with time this symptom will only get worse.  Do yourself a favor: change it now.
  3. Hiss.  When the noise floor starts to creep up causing silent passages to sound like you are listening to music in a  wind storm, the tube is going bad.  This symptom is the most gradual and can be the least noticeable from all of the rest.  Noise can sometimes be discovered on a tester  indicating a "weak" tube.
I recently swapped out the two gain stages in my Dared after noticing a sibilance issue and sure enough both tubes were going south.  I have been listening to the new ones for about two days now and I can hear them mellowing to that lush full body revealing resonances in sound boards, the character of drum membranes, and nuances in woodwind reeds.  Front-to-back imaging at first play took a huge step backwards getting very flat and narrow, but that too is improving as things stabilize.

By next week, things should get back to that pristine sound I enjoy so well.  During this time, I will enjoy the transition period since it is much like swapping out a piece of gear with a new one every day.  After swapping out tubes, you too will hear this mellowing process evolve.  Be patient and allow your new tubes to burn in.  Swapping to a new tube always takes time for you to appreciate what it can do to improve your listening pleasure.

There are other reasons you should swap out your tubes (like quality upgrades and other failure modes), but these are my top three reasons once you settle into a tube choice for that component.  And a note of advice: when you find a tube that you like, buy spares of that exact same kind.  Even new tubes can suffer from infant mortality and last only a few weeks or months.  Each time I buy mine, I always "stock up" on what I consider my current selection for the state of the system.

Yours for higher fidelity,
Philip Rastocny

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