[Otis]

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[jackman]

I'll take a shot at this...

It's never too early for tubes to crap out.  Could be the tube, even if it's brand new.  The tubes in this design last a LONG time but, being tubes, they can go or start to go at any time.  Just swap the tubes and see if the problem migrates to the other channel.  If it does, you have your answer.

About a year ago, I thought I had a problem with one channel of my preamp, or my tweeter because it kept cutting out.  Turned out to be a bad interconnect.  Swap the IC's and see if you have the same problem.  

My money is on the tube.  AVA gear is pretty bulletproof.  Unfortunately, tubes can go at any time.  The good news is, tubes are cheap!  Unless you use NOS stuff, stock tubes can be had for a song.  I'd keep extras around just in case.  The worst part is unscrewing all those little screws!

J

[Tyson]

It's probably a switch that is dirty.  Luckily the switches on AVA equipment are self-cleaning.  Cycle each switch on/off about 50 times each (except the power switch, you can leave that one alone).  That will probably fix the problem.

[avahifi]

Obviously the problem cannot be a tube, inasmuch as both channels play normally with the unit in the MONO position.  That requires that both active audio channels be in good working order. If the problem occured ONLY on Phono, then it could be a phono tube, as the phono section is ahead of the selector switch and rocker switches.  The way to troubleshoot a single channel problem in a vacuum tube unit is to turn the unit off, swap the tubes from one channel to the other, and see if the problem follows the tube.

In this case, since the problem cannot be the active audio circuits, the only thing ahead of the audio circuits is the switching and control network in the preamp and the source(s) themselves.  If the problem occurs on only one source, that source device would be suspect.  If the problem occurs on all sources, then the control and switching network in the preamp is at fault.

Actually Tyson probably located the problem for you accurately, a signal dropout due to a dirty contact in the Tape-Input switch.  These switches are designed to be self cleaning if used, but in units in which these switches are never used, oxide can build up until a dropout occurs.

The three fix possibliities are:

1.  Cycle the three rocker switches back and forth 50 times or so each with the unit turned off.  This will likely remove built up oxidation and restore normal function. A spray of Caig Labs DeoxIt in the process is useful.

2.  If this won't do it, and you are handly with tools, we will supply you with replacement original type switches at no charge.  Of course if the unit is within the three year factory warranty, we will replace them here in kind at no charge too.

3.  We have made a running specification change to the SL chassis preamps within the past year to a very expensive gold contact logic level switch that will never drop out.  We can upgrade your older AVA SL series preamp with all three switches for a flat fee of $115.00 plus $15 return shipping in the USA.  The labor costs are considerable as the switches all have to be rewired too.

Frank Van Alstine