2. amps-preamps
  3. 130817-8622

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


marktomaras
jea48
4,460 posts
 
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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marktomaras OP
544 posts
 
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?

thanks!
jetter
3,035 posts
 
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. 
marktomaras OP
544 posts
 
Jetter,

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
jea48
4,460 posts
 
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.
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