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

I'll begin by saying that I do realize a lot has been discussed on this subject. I read a lot of threads here.  Here is my situation:

1.  ARC retubed and did some other maintenance on my VT100 mkII in April 2015, so they are 3 years and 3 months old.

2.  I have no Idea how many hours are on my tubes in my power amp or preamp.  Tube timers came in later models. Darn, I want a tube timer! (Side note, anyone know if there is a good external or accessory tube timer to buy?)

3.  I learned to check bias on the power tubes, but have not attempted to measure the signal tubes yet.  I am ready to try. I have the tools (need one more multimeter) and I like to be hands on.  When I checked the power tubes 2 weeks ago, they were perfect, despite not being checked or adjusted since ARC did the service in 3/2015.

4. I could be crazy, but I think my system is not as loud as it once was. I am not sure about a diminished sound quality, but it is possible that has occurred very slightly.  No noisy tubes or bad tube static though.

5.  Loads of people spoke about installing KT120s in this amp. Are people in agreement on this?

6.  Is The Tube Store reliable for well matched tubes?

thank you everyone!
- Mark

Thetubestore.com is a well run outfit and have taken tubes back from me when there were issues no problemo (once I guess, when a NOS preamp tube committed tubicide). Highly recommended. Signal tubes can last a long time but hey…swapping out tubes is interesting just to see what’s different and any self respecting tube freak winds up with a drawer full of tubes (or in the case of "Blackie" something, a NYC amp repair dude, aquariums full of tubes…check out Youtube for the video about him). I use my receipts (in my email) to keep track of tube age. Also, I thought I read here someplace that KT120s were approved by ARC, but they implied you have to get them from THEM so you can pay through the nose because their tubes are Mo Bettah. Note that I tried to find any incidents of transformers melting from heater draw with these tubes and my careful research revealed zero issues ever in the entire world…my research was fear reaching, thorough, and maybe nonsense, but there it is.
Correction: Not "fear" reaching, simply "far" reaching, which, of course, it wasn't really.
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.

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.

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?


Jea48, thank you for the detailed response!

If I decide to keep the ARC supplied input and driver tubes, should I even mess with the bias on those, or should I just install new power tubes, adjust power tube bias, and call it a day?  Obviously that would be easier, but I fear I may be leaving a stone unturned.  The bias on the power tubes was perfect by the way, and I have not checked or touched them since ARC retubed in early 2015.

what do you think of using KT120 tubes in the VT100 mk II? The threads I saw were positive. It seems there is a minor risk of the heaters drawing more power, but I suspect that there is enough headroom in the power supply to handle it.

part of me wants to retube everything, including my preamp. I am devising a plan to build an external tube hour counter, so it would be great to start from scratch with everything, though I also do not want to just throw money away...

i have a dedicated 20 amp breaker with heavy gauge romex power cable in the wall to a single duplex outlet. I use a Shunyata Talos conditioner, and all of my gear is plugged into the Talos, including the power amp.  Do you feel this is ok? Or do you recommend plugging the VT100 directly into the wall?



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.
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, that is excellent advice.  I agree on the full re-tube.  I am planing to retube my power amp and preamp completely, and at the same time, I am planning to design and assemble a tube hour counting device (described here)

Let me ask a few questions:

1.  I found and used collins's tutorial on biasing the power tubes.  I was intimidated by the signal tubes.  However, I am inspired to learn.  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?

2. I spoke to Upscale Audio and Tube Depot on tube matching.  They told me that they match them as best as they know how, and that they do not have a way of offering even tighter matching.  They seemed confidant that their matching is really accurate.  I am leaning towards Upscale Audio, the way that they describe their method and practices seems really perfect.  Do you agree? Are their matched tubes tight enough for the power amp?

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

I was intimidated by the signal tubes.  However, I am inspired to learn. 

Part of learning is the knowledge gained from making mistakes.  It would be great if you could watch someone else bias your 6922 tubes the first time. 

I agree!  Any tube veterans want to visit me in Miami for a biasing session?  I will provide a gourmet meal, fine wine, single malt scotch, and a great listening session on the newly biased tubes!

:-) Mark

PS: This is a serious offer
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.
You want as much insulation as possible on the probe/s.
3.  Do I need to adjust bias in the LS25 as well?



 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?