Tube advice

How does one check the bias on a vt100 mk II?  Is it also necessary to check bias on the LS-25?

Preamplifiers, both linestage and phono stage, generally do not require any bias adjustments.  Nor do the tubes in the input and driver stage of any amplifier I have ever known about.  It's the output power tubes that generally do or do not need occasional checking for bias.  (I say "do not", because some amplifiers have an autobias circuit that takes care of this issue.)

I am not sure I agree that one really "needs" a tube tester to get by with tube equipment.  If you keep rough track of the hours of use, and if you pay attention to how your stuff sounds from day to day, the question of when to re-tube is typically based on subjective judgement of those two aspects.  If a tube tests bad on a tester, it will sound really lousy.  However, tubes can test "weak", but within spec, and still sound fine.  There is no need to change them out unless the sound has gone noticeably off.  The problem with a tube tester, even some of the best ones, is that they test tubes at currents, voltages, and bias settings that may not at all apply to the operating conditions in your amplifier.  This can result in a false sense of security or insecurity, depending upon what the meter says.

Mark, Don't you have an owners manual for your VT100?  If not, it seems unlikely that you could not acquire one for free (check on line for a pdf file) or for a pittance from ARC. 

lewm and marktomaras you might want to read up on this. 

http://mycollins.net/audio/artube1.html

As far as a tube tester goes I bet Upscale Audio and ARC use a tester before they send out tubes so saying a tester isn't an important necessity for a tube user is questionable especially if one likes to roll tubes often. I agree to keep track of your hours but a tester does much more than just test the strength of a tube. Just curious if you own a tester - lewn? The driver tubes in an ARC VT100 do require biasing I believe.

ARC VT 50, 100 MK I, II and VT200 amps are difficult to bias. If you don't have highly matched tubes it is almost impossible to bias one of these which likely you will if you're lucky only lose a tube and a resistor in the process. If you're not knowledgeable on basic electronics, know how to read a schematic and comfortable with a soldering iron I suggest not tackling one of these amps as you'll likely send it in to ARC to get it fix. The VT100 is a great amp and this is the reason they're on the used market so often, or were several years back. For what it will cost to send a VT100 in to ARC to get retubed and biased by them you couldn't buy a new amp for the money to replace it. If you don't know what you are doing I'd send it in to ARC. 

Dear ADG, Yes, I own a Hickok tube tester, and the number of times it has served me to detect truly defective tubes is near to zero, because such tubes gave away their condition just by listening to them.  It is not that useful for tube matching, either, because it tests all tubes at the same plate voltage and current, which is typically very different from the in-circuit parameters.  Tubes that "match" at one voltage/current setting very often do not match at other settings for voltage, current, grid bias.  Some of the best Hickok testers (e.g., 539B and C) do offer a choice of some of these parameters; those models are to be preferred and are both scarce and very expensive these days.  Further, if you own one, it needs to be calibrated and kept that way.  As an alternative and perhaps to your point, there are available modern tube testers that can actually trace curves.  (I forget the brand and model names, but the internet will tell what they are.) That's the valid way to match tubes. Those are the types used by reputable pros who sell tubes, like Jim McShane and Kevin Deal.  If you want to spend a few thousand bucks on such a tester, that's in a different league from the vintage testers, but are you seriously suggesting that we all need to go out and buy such a device?