Biasing issue with Audio Research VT100 Mk II

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

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!

@edward78 said:

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.

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

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.

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?

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Courtesy of Abe Collins.

http://mycollins.net/audio/artube1.html

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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

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.