Audio Research VT100 Mk1, Mk3; VS115

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. 

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

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

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.

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.