2. amps-preamps
  3. 183712-178484

Biasing issue with Audio Research VT100 Mk II

Hi Everyone, I recently had an Audio Research VT100 Mk II repaired and retubed by the Audio Research authorised dealer here in Germany. Upon checking the bias after the repair, I noticed that the output tubes were biased at 115m V, whereas they should be at 130m V (2x 65m V) according to the manual and printing on the circuit board. For the left channel, I adjusted  the trim pot to increase the bias to 130m V. For the right channel, I could only get to 119m V before the trim pot wouldn't turn anymore, i.e. the trim pot is turned as far as it will go and it's still not enough. I'm guessing that running the amp at 119m V should be no problem but I also suspect that I might sacrifice some sound quality. I also just want it to meet the specs since I just paid a lot of money for the repair. Any ideas on how to resolve this? I don't want to send it back if possible since it's such a beast to transport. Would switching the left and right output tubes help (they are JJ 6550s)? The left channel tubes have  "27 mA" written on their boxes, whereas the right channel tubes have "23 mA" written on their boxes.

edward78
rhg3
217 posts
 

What does the dealer say? This doesn’t sound like a quality repair. And it doesn’t appear that the dealer attempted to bias the tubes prior to returning the amp. Not something I’d expect from an authorized ARC dealer.

jjss49's avatar
Ag insider logo xs@2x
jjss49
6,612 posts
 

or the output tubes placed in the amp are not a good set and won’t accept the designed level of biasing? if the tubes are confirmed to be fresh and a good set then there is likely still something wrong with the amp, despite the service

definitely swap the tubes side to side and see if the problem stays on the same side (amp problem) or travels with the tubes (tube problem) - when you do this turn DOWN the bias on both sides first, then SLOWLY raise the bias adjustment pot up to the recommended target of 65 ma per 6550 pair (or 130 ma per side through the top mounted bias resistor)

all this said, running at 119-120 ma is pertty close to the target bias... so no problem for now... but i would worry that as the tubes age as you use the amp, the tubes on that side will require more bias current to run at the designed operating point, and the amp will not be able to supply it if it as already maxed out currently on that side of the amp

edward78 OP
6 posts
 

Yeah, I contacted the dealer and they said they are contacting ARC.

That's a good idea with swapping to see if the problem travels with tubes or stays (with the amp). Will give it a try, also raising the bias slowly.

Thank you!

More to discover