2. amps-preamps
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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
jea48
4,424 posts
 

@edward78,

Just a word of warning in case you are not aware... The DC voltage on each side of the resistor you use to set the bias 130mVDC is around 420VDC to the metal chassis of the amp.

If you do not have any of these or something similar I strongly suggest you buy some ASAP before doing anymore testing with the probe leads using the multi meter.

PAGOW 10pcs Multimeter Test Hook Clip, Electrical ...

*** NOTE THE SILVER COLOR PHILLIPS HEAD SCREW JUST ABOVE THE BIAS TEST RESISTOR... That screw is B - chassis ground... If the probe of the multimeter lead touches the resistor lead, either side, and the Phillips head, or the DC B+ rail circuit trace of the Phillips head screw, there will be one big loud bang and a lot of damage to the DC rail circuit trace of the amp where the resistor lead connects... It will blow a hole in it!!!

.

jea48
4,424 posts
 

Correction to my above post.

If the probe of the multimeter lead touches the resistor lead, either side, and the Phillips head, or the B + rail circuit trace, the resistor is soldered to, and the Phillips head screw at the same time, there will be one big loud bang and a lot of damage to the DC rail circuit trace of the amp where the resistor lead connects... It will blow a hole in it!!!

.

litocil
4 posts
 

$2,120 for tube replacement is a little much for that amplifier. Try this store https://www.tubedepot.com/products to source the new, reasonable price tubes.

For what it’s worth I have great results with using either Svetlana or Electro Harmonics tubes on my ARC components.

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