Audio Research VT100 Mk1, Mk3; VS115

I own an ARC VS115 amp. The original  yellow Rel-cap coupling caps ( two for each channel) were replaced with Stealth gold 2.0 uf 450 V caps. These 2.0 uf Stealth caps are drop in replacements on the circuit board replacing the stock  2.0 uf 450V Rel-caps. These Stealth caps are the identical caps which ARC uses in all their current reference products. Bypasses ( used with the stock Rel-cap coupling caps) are no longer needed with these 2.0 uf Stealths. The stock ARC Ref 75SE also has identical Stealth gold caps without bypasses utilized. The stock 6550 tubes were replaced with Tung-Sol KT120's. My VS 115 amp sounded excellent stock but with these upgrades it sounds significantly better! Night and day really. Very musical amp with outstanding dynamics and tonal balance. An even better three dimensional sound stage with the Stealth upgrade.  Will be upgrading the four (4)  diodes in the rectifier stage soon with  C6D10065A  Silicon Carbide Diodes.  I am considering upgrading the VS- 115  to KT150 tubes. The output transformers will probably be  capable of handling  the increased plate current of the  KT150's but need to do more research on this first. 

Jea48,
Thanks for your concern.  I seem to be blessed with regards to my VT100.  I ran it for a month on new input and driver tubes without even checking the bias.  When I did do the biasing I found the 160 volt measuring points to be within the specified tolerance of +/- 20 volts of 160 volts and 20 volts of each other before I began to adjust anything and they remained that way after all was done. So I didn't have to swap the tubes around willy nilly. The tubes I ordered were not matched but the two sections were very close to each other for each tube.  It took me over 2 hours of iterations on the resister pots to get everything within spec.  I didn't measure any points not mentioned in the biasing instructions.  After I biased the power tubes I went back and checked the voltages for the input and driver tubes again.  The 60 volt points had drifted to 61.5 v.  I turned them down with just one pot in each channel and the other voltages stayed within tolerance.  I finally checked the power tube biases again.  You can see why it took such a long time.

In spite of everything being within tolerance things can still fail.  That is not only the nature of tube equipment but virtually any machine created by man.  Tube gear just fails more frequently.  I bought the amp 6 years ago and it is at least 20 years old.  I figure I've had a good run with it.  If something fails before I sell it I'm prepared to spend some addition money to replace it.

According to Alice Trillin, wife of famous humorist Calvin Trillin, “Money not spent on a luxury once considered briefly is the equivalent of a windfall income and should be spent accordingly.”  I'm in her camp.

@richardp01

Are you 100% sure you have everything adjusted correctly?

I ran across an old Agon thread that may be of interest.

Quote from thread.

-> there is another measurement that you should check (before the power tubes are in) and that is only listed on the actual schematic. It refers to the grid #1 bias. If it as MUCH more positive, say -30 or -20 vdc, then you put that tube in a condition for excessive bias current. Since you blew both the 1 ohm plate resistor and the 100 ohm grid #2 resistors and the traces under the resistors that tube was either put in that state by excessive positive bias on pin 5, and/or it was faulty and simply arced and took everything out (likely the case since you dialed in the input tubes so well)

https://forum.audiogon.com/discussions/audio-research-vt100-mkii-retubing-blew-resistors

I just ran through my set of test tracks to check the new sound of the VT100 with proper biasing.  One is always uncertain in making comparisons without A-B switching but I detected less silkiness in the highs after properly biasing the input tubes.  Just a touch more sibilance in female voices.  Darn!  Maria Callas singing Norma is still quite acceptable; her crescendos are not too brittle.  Well not any more when compared to the old tubes.  The amp has a slightly more "tubey" sound with the proper biasing; and more than with the old tubes.  How does that work?  More highs and warmer midrange?

I plan to live with the unit for a while longer to get my money's worth out of the new tubes.  I'll probably go for a VS115 by the end of the year.

Jim,
I owe you a big chunk of gratitude for turning me on to that tutorial and scaring me into doing the bias adjustments to the VT100.  I just finished the procedure which indeed took about four hours.  I lucked out in that my new tubes did not have to be rotated to obtain the specified tolerance between the downstream voltages.  I won't bore you with all the details I just want to say I couldn't have done it without the tutorial.  The tutorial is for a Mark II and mine is a Mark I (actually, no mark) but I muddled through, locating the measuring points that changed between models.

For those who recommended against doing the biasing myself I will say your advice is well taken.  I know just enough about electronics to get myself into serious trouble although I've always managed to recover from my boo boo's.  One great piece of information I took from the tutorial was to attach alligator clips to my digital multimeter leads so that I didn't need three hands to do the adjustments and measurements.

I would chide the people at ARC into placing information in their manual about replacing the input tubes, even if it just says, "Don't try this at home."  They should also include the link to the above tutorial with their biasing instructions.  That would generate more maintenance business for them than they would lose when people get informed about the actual work involved in replacing tubes. Also it would be nice if they would reduce the number of screws holding down their equipment covers--twenty screws per panel--come on!


For everyone's information I replaced the power tubes about a year and a half ago at a cost of about $300 for NOS Winged C's.  (These are no longer available.)  I decided to replace the input tubes about a month ago at a cost of about $240 for EH 6922 gold pins selected for low noise and microphonics by Tube Depot.  The original tubes were just standard Electro Harmonics 6922's.  I found that the new tubes sound silkier than the old ones even without proper biasing.  It will be interesting to see if rebiasing caused any SQ change.

Thanks to all for your communications.

Rick

Richard, in your original post, you mentioned upgrade, am assuming you mean upgrade from Mk1 to Mk2 or Mk3?

If that is the case, you would be better off selling your amp and buying a Mk2 or Mk3 on the used market. You would never get the return on the money spent upgrading your Mk1 to a Mk2 or 3.

I have both a Mk1 and Mk2. Have heard the Mk3, but do not own one.

The newer ARC gear seems to have less of that tube sound.

would suggest the following with your Mk1 amp; try a set of Shuguang Nature sound KT-88s in the amp you currently have and bias the amp for the tubes. Think you'll be surprised at the sonic improvement over the stock ARC tubes.

best 

jea48

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 ...

That’s absurd. The amplifier will not be "pretty much junk" and can be readily repaired by a competent tech.

Rich, … I think Jim is giving you good advice.  I would like to reiterate what I posted.  Call ARC and talk to Evan.  Ask him what the best course of action is.  If Jim's assessment is correct, this is not a do-it-your-self'er.  

I can't make the economic call for you about whether you should movie up to the VS-115, but if your VT 100 MK I is long in the tooth and needs a retube (and possibly other work), I would call Evan and ask him if this is something you should attempt.  

If Evan tells you that retubing the VT 100 is like doing brain surgery on yourself, then an old Yoga Bera'ism comes to mind, "when you come to a fork in the road, take it."  Either send the VT 100 back to ARC (or an ARC authorized service station) or swap it for something else.

Been there and done it with many ARC products, including the VS 100, the VS-115, the Ref 150 SE and so forth.  ARC customer service is first rate.

BIF  

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

@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 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. 

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.

.

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.