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 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