I know my tubes (VT100 mkII) are going when...

Power tubes are usually good on average for 1500 to 2000 hrs.
Life of small 6922 signal tubes can vary but can last as many as 10,000 hours. Though not that high on new production tubes made today.

Replacing the power tubes is fairly simple. Just follow the ARC bias instructions.
Nothing wrong with buying your new power tubes from the The Tube Store. Just make sure you tell them they are for a ARC VT100 MKll.

When you had the amp retubed in 2015 did they replace the 6922 input and driver tubes? If so I would think they still have plenty of life in them.

Biasing the input and driver tubes can be a real pita. And if you do not know what you are doing you can not only damage your new tubes you can blow screen resistors and or damage power traces on the amp’s channel circuit boards.

YOU MUST buy closely, tightly, matched triode sections 6922 tubes. Especially the driver tubes. I mean really tightly closely matched sections. If you do not you will never bias the amp’s input and driver tubes to within ARC specs. Especially the driver tubes. One reason I would recommend buying the 6922 tubes from ARC.

If you buy them from The Tube Store make sure you tell them close wont get it.

http://www.audioresearch.com/ContentsFiles/VT100%20MKII_BiasAdjust.pdf
You will also need a wiring schematic diagram for the amp. I don’t know why ARC no longer furnishes it on their website.

How to totally retube the VT100 MKll amp, courtesy of Abe Collins.
http://mycollins.net/audio/artube1.html

As for you thinking the amp doesn’t sound as loud as it used too. I guess that could be the power tubes. It can also be due low AC mains voltage feeding the amp. Is the amp on a dedicated 20 amp circuit? Or is it plugged into a 15 amp convenience outlet circuit? If yes have you added any loads, plugged in additional loads, to the same circuit causing a voltage drop at the outlet feeding the amp?

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Mark,

I would plug the VT100 amp directly into the wall outlet, jmo.....

I do not have any experience or knowledge with regards to the use of KT120 power tubes in the VT100 MK2.
You might want to contact ARC service at the factory in Minnesota.

I found a copy of the schematic wiring diagram for the VT100 MK2.
http://www.arcdb.ws/VT100/VT100.html
Scroll down the page until you come to the two schematics shown.
The first one, on the left, is for one channel of the VT100. The second drawing is for the power supplies. Note the right hand drawing says for the VT100MK2.

I noticed you posted another thread where you say the tubes have more hours on them than you previously thought. You may want to do a complete retube now.
(Unless you have a tube tester and can test the 6922 tubes doing a life test. If you can test the 6922 tubes they must go back in the same tube socket they were removed from.)

Biasing the input and driver 6922 tubes is not for the novice. Lethal DC voltage of around 450VDC will be present where you will be working. You will need 2 meters. Make sure you have clip on test connectors for the meter probes. Side panels on each side of the amp will need to be removed for access to the tube socket contact points where you will attach the meters probes. This is also where the trim pots are located that you will be adjusting.
I suggest you go through Abe Collins’ how to retube the VT100 MK2 a few times before you actually try it yourself, live power.

I can not over state the importance of buying really tight matched 6922 tubes for the VT100 amp.

Jim

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If I want to practice the connections with the two multimeters and agitator clips in a dry run with no power in the amp, how do I ensure that there is no energy in the amplifier?  Should I have music playing and then turn off the power amp so it is still supplying output to the speakers until it fades out?  Or just unplug it and wait a certain amount of time?

Just power down the amp as you normally do. Wait about a 1/2 hour - one hour and then disconnect the speakers, ICs. Remove the amp, and put it on the table where you will working on it.

Remove the top cover.
Set meter to VDC auto or above 450VDC.
Check for DC voltage from B+ to chassis on both left and right channels.
Remove the side panels.

agitator clips

I got the ones I use at Radio Shack.
They look something like this.
  https://www.digikey.com/product-detail/en/pomona-electronics/4233-0/501-1342-ND/736757
You want as much insulation as possible on the probe/s.

3.  Do I need to adjust bias in the LS25 as well?

No.

LS25
http://www.audioresearch.com/ContentsFiles/LS25LS25II_Manual.pdf

http://www.audioresearch.com/en-us/download

Mark,

 Note the tubes in the LS25 are only good for around 4000 hours.

I wonder what the average life expectancy hours are for the 6922 tubes in the VT 100 MK2?