Biasing issue with Audio Research VT100 Mk II

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!

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.

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.