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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

Showing 3 responses by jjss49

jjss49's avatar
jjss49
6,612 posts
 

@edward78

my suggestion to you is to insist that the one quad tube set that is not fully biasing to 130 ma be immediately replaced with a fresh set that can bias properly by the repair shop ... as @jea48 said, the tubes will definitely need more bias current over time as you use the amp going forward, it is not good that as ’new’ tubes they already are asking the amplifier to supply 100% of available bias current - this is not good for the amp

that set of jj tubes is simply not strong enough, not working to proper spec for this amp, and if you just paid for service and fresh tubes, that particular set of tubes that were sold to you are inadequate

in my experience with this amp, using various russian 6550 or kt88 tube sets (sovtek, svetlana. gold lion, e-h), to obtain proper bias current into the power tubes, the bias trim pot is only at 40-50% of its adjustment range when a fresh set of tubes are installed

jjss49's avatar
jjss49
6,612 posts
 

yes yes @jea48 is absolutely right!

i was talking about the quads of power tubes only (not the small signal tubes)!

i should have been clearer on that point, my bad

jjss49's avatar
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

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