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.
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.
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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 said:
Power tubes only! The 6922 tubes are biased for the tube sockets (circuits), they are installed in. https://www.arcdb.ws/Database/VT100/ARC_VT100_bias_adjustment.pdf You won’t find that in the owner manual... https://audioresearch.com/wp-content/uploads/2021/08/VT100II_Manual.pdf
Was it a total retube? Therein 6922 tubes replaced as well? You just don’t install new 6922 tubes in a VT100 II. They have to biased. Did you have any problem setting the bias on the power tubes at 130mV before sending the amp in for service? Are you using a good multimeter? . Courtesy of Abe Collins. http://mycollins.net/audio/artube1.html .
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Yes, thanks, I understood to only swap the output tubes. So I tried swapping the left and right output tubes and slowly raising the bias adjustment and that actually solved the problem. I'm still only able to get to 129m V but that is close enough. The trim pot is now maxed out on the left channel (before it was the right), so it seems that the problem is related to the tubes. Or maybe slowly increasing the bias adjustment pot also helped? When I received the amp, they also sent the original boxes of the tubes (JJ Electronics 6550). The tubes that seem to cause the problem have stickers with "23 mA" attached to them. I'm guessing that's some kind of parameter when you measure the tubes and that these values should all be the same value when matching tubes. It would seem then that the lower mA value of the tube, the more you have to increase the bias adjustment? So I guess going forward I should look for tubes with higher mA values? It's still disappointing that the ARC authorized dealer didn't catch this before sending it to me. It would also be nice to have a bit of headroom on the bias adjustment in case I ever want to go above 129m V for whatever reason. Anyway, they are all new tubes, so I will give it several weeks of listening and then check again. P.S. my multimeter is new and is from KAIWEETS. I never checked the bias before sending it in for repairs. I bought it second hand and just used the amp until two resistors got blown. This is my first tube amp, so still kind of learning the ropes. |
@edward78 said:
Model number? .
Did the amp come to you with all the new tubes installed in the tube sockets already? IMO, you have a problem with the amplifier. Could be just the power tubes??? You need to be able to directly contact the repair shop that fixed and re-tubed the amp. Going through the Dealer and Distributor just doesn’t work! .
Every time the amp, (amplifier) is playing music the power tubes are using up their life. For the VT100 II the average tube life is around 1500 to 2000 hours, going from memory. (YMMV)... The harder you push the amp the shorter the life of the power tubes. Over setting the bias voltage above specs, 130mV, will use up tube life faster. Hopefully the new 6550 tubes were pre burned in, at least 48 hours, 72 hours is better. Bias for the power tubes was then checked, set again before the amp was shipped to you. IF the power tubes were not pre burned in then the bias voltage should be checked after the amp has been playing music for a few to several hours. You should have been advised to check the bias on the power tubes again after hooking up the amp to your speakers. Reason is the repair shop AC mains voltage may be higher or lower than your nominal AC mains voltage. That will have an impact on the bias voltage setting. Depending how often you listen to music, and how hard you push the amp, will determine how often you check the bias on the new power tubes. The bias is not set once and then forgotten... If you listen to music everyday, for say a couple of hours each day, with the new tubes you might want to check the bias again after a week or two. If the bias voltage measures the same without having to adjust it then you might want to check it again in about 4 weeks. If the bias voltage measures good then you might then check it every other month. Your usage of the amp will determine how often you will need to to check the bias. After the tubes get a few hundred hours on them you may find you only need to check the bias every six months. What you will find as the power tubes age, (from playing music), you will need to increase the setting on the blue trim pots to raise the bias voltage to 130mV. Usually near end of life of the power tubes the tubes will not hold a bias. FYI, when checking, setting, the bias for the power tubes the amp should be allowed to stabilize without any signal applied for at least 30 minutes. Therein after first being turned on or after listening to music. . |
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 |
Wow, this is great insight and advice.
The multimeter is KAIWEETS HT118A.
The amp was sent with the tubes packed separately and numbered for each tube socket in the amp, which I of course followed.
The repair shop and dealer are the same company and is the authorised ARC dealer/repair shop for Germany.
I have no idea if the tubes were pre-burned in. I can ask the repair shop/dealer.
OK, I hadn't considered that. Maybe the repair shop did check the bias before sending and it appeared correct to them due to the difference in AC mains voltage.
Understood, but that means that I'm probably going to have issues with these tubes down the road as they start to require more bias voltage. Not good.
Thanks. That's how I've been doing it so far.
Yes, it sounds more and more like it's a tube issue and I should try to have the repair shop replace them. This is also way more convenient than sending back the amp to the repair shop.
The decision to use JJ tubes was actually my decision. The repair shop contacted ARC and they quoted 2,120 USD for a complete set of new tubes for the VT100 Mk II. So I asked the repair shop to look for an alternative. I'm willing to spend a bit more on new tubes, but not 2,120 USD. Even with the JJ tubes, I still spent EUR 1,750 on the repairs and re-tubing, the main part of which was replacing 20 resistors. The resistors were sourced from ARC. |
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!!! . |
Correction to my above post.
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$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. |
