Biasing issue with Audio Research VT100 Mk II

@edward78 said:

my multimeter is new and is from KAIWEETS.

Model number?

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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.

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!

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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.

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.

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@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

Wow, this is great insight and advice.

@jea48 

Model number?

The multimeter is KAIWEETS ‎HT118A.

Did the amp come to you with all the new tubes installed in the tube sockets already?

The amp was sent with the tubes packed separately and numbered for each tube socket in the amp, which I of course followed.

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!

The repair shop and dealer are the same company and is the authorised ARC dealer/repair shop for Germany.

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.

I have no idea if the tubes were pre-burned in. I can ask the repair shop/dealer.

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.

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.

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.

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.

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.

Thanks. That's how I've been doing it so far.

@jjss49 

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

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.

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

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.

@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!!!

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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!!!

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