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
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.
jea48
4,460 posts
 
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
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