Audio Research VT100 Mk1, Mk3; VS115

Richard, 

I used to own the VS115.  It was a fine amp.  I can't say how the VT100 model compares to the VS 115.  But I suggest you call Evan at ARC and get his sense of the comparison between the two models.  Just a footnote , ...  Evan is somewhat new to ARC.  He replaces Kal who replaced Lenard. So if he doesn't know, Warren Gehl might be able to weigh in.  Warren is the ears of ARC.   

Here's another thought, …. ask Evan if ARC is still mod'ing the VT 100 Mk I version to upgraded versions of the amp.  It may be a cost effective way to move up.

Good luck.

BIF

P.S. I never thought the VS 115 sounded solid state'ish.       

Perhaps I should just stay with the Mk1 and be prepared to fix it when it breaks down.

When was the last time you totally retubed the VT100 amp? Specifically the 6922 signal tubes? It’s not for the novice and can be quite expensive. The 6922 input and driver tubes must be biased. Especially the driver tubes.

Next thing to consider is the age of the amp. 21, 22, years old? The power supply electrolytic caps may need to be replaced.

It may cost more to retube the amp than it’s worth. Add new caps it will definitely drive up the cost more than the amp is worth.

VT100 Mk3

Internally, the circuitry of the VT100MKIII is derived from the VTM200. Like its more powerful sibling, the MKIII’s input stage utilizes direct-coupled JFETS for ultra low noise, and is followed by a 6H30 driver/gain stage, capacitor coupled to a 6H30 follower, driving two matched pairs of 6550C output tubes per channel. The conservative 6H30 driver-follower circuit will help prolong the life of the 6550Cs, and the 6H30 is rated to handle higher currents than the previously used 6922, so it has lots of safety margin for even better reliability. (The 6H30 twin triode is a Russian military tube in current production, with an estimated service life of up to 10,000 hours.) There are now four internal bias adjustments-up from the previous two- to allow more accurate DC balance adjustments for the output tubes. The line fuse has been increased to a 7 amp in 120V units (6.3 amp in high voltage units), allowing the MKIII to handle musical dynamics with greater ease.

Simply put, the VT100MKIII is a major sonic advance. As the original VT100 was greatly improved upon by the MKII, the MKIII eclipses it again by the same margin. Resolution and dynamics are startlingly better: suddenly you can pick out each individual voice in a choir, you can hear the air gently modulated by a hundred diaphragms singing in unison in a great cathedral. Small and large details stand out in bolder relief, tightly focused, wonderfully holographic. Bass is tighter and more dynamic. It sounds as though the power has been increased, with a more effortless quality. The MKIII is easily recognized as having been borne of the VTM200. Lastly, the single-ended RCA input is almost indistinguishable sonically from the balanced-XLR input.

http://www.arcdb.ws/VT100/VT100.html
The signal tubes of VT100 Mklll do not require biasing when retubing them like the VT100 Mk 1 and 2 do.

Complete retube of the VT100 Mk 1 and 2.
http://www.audioresearch.com/ContentsFiles/VT100_BiasAdjust_Schem.pdf

Step by step how to bias the 6922 signal tubes.
Courtesy of Abe Collins
http://mycollins.net/audio/artube1.html.

.

Thanks for the link to the tutorial.   The ARC instructions for rebiasing the input tubes by themselves are useless. 

 I just replaced the 8 input tubes without rebiasing them.  I learned about the need to rebias them by accident,  it's not mentioned in the manual,  only on the ARC website.  The amp sounds fine the way it is.   If I get bored someday maybe I will set the bias correctly.   I did properly bias the output tubes, an almost trivial procedure compared with the input tubes. I hope I don't burn down my house because of a dangerous condition caused by improper biasing of my amplifier. 

I'm hoping to replace the amp before it needs new caps.  If I sell it I will definitely rebias everything. 

@richardp01

DO NOT USE THE AMP WITHOUT BIASING THE SIGNAL TUBES! Especially the driver tubes. Failure to bias the 6922 tubes will lead to damage of the amp. It may sound fine now but there is a good chance it is eating up the life of the new 6922 tubes. Also there is a good chance you are over driving a 6550 tube in each push pull circuit of a pair.

If you are lucky you will only blow a screen resistor/s. Of course because ARC installed the screen resistors against circuit traces there is the chance when the resistor/s blow the heat will damage the traces. At that point the amp is pretty much junk. You may also cause a driver tube/s to short which will cause it to take out a 6550 power tube/s.

Where did you buy the tubes? Each section of the 6922 tubes must be closely match. If Not you will spend hours moving the tubes around and may still never be able to bias the tubes to meet ARC design specs.


Best regards,
Jim.

Thanks Jim, 

You just ruined my evening.  The boxes the tubes came in had numbers for triodes1 and 2.  They were all marked either 80 and 85 or 85 and 80 for triodes 1 and 2.  I guess I have a plan for what to do tomorrow.

Rick