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What is the acceptable range for tube bias?

When I purchased my Yaqin MC-30L EL34 tube amplifier, the local shop owner set it up for me as a courtesy and tonight I finally put my own multimeter on it to check the bias now that it's broken in. The amp was warmed up for a little over two hours.

The manual indicates "about 0.4v" for the spec. I got readings of 0.4, 0.39, 0.39, and 0.38 on the four tubes. I'm assuming that this qualifies as "about" but am wondering at what point I should consider turning the wrench.

In general, how sensitive is the adjustment on a tube amplifier? Should it be possible to easily adjust in 0.01 increments or is it more likely to be in 0.05 increments where you just have to go back and forth trying to find the sweet spot?

As a side note, it has been a while since I last had my speakers hook to this amplifier and the sound of my newest SACD finally sounds pleasing to my ear.
mceljo
brf
1,715 posts
 
Your multi-meter should be set to measure milliamps (ma) not volts. What exactly does your manual say for the bias setting? Bias will drift between tubes over time, which is why it is recommended to periodically check the bias.

You should be able to adjust the bias exactly for each tube(s) to match the recommended setting.
atmasphere's avatar
atmasphere
11,807 posts
 
FWIW if the manual says '0.4V' then we are talking volts not milliamps. So the volts setting is correct.

I usually set the bias slightly low if the amp has just been started up. Then I check it after a few minutes to verify that its OK, finally after 1 hour it should not exceed the readings in the manual.

Bias is often not that critical unless you are too high- in which case it could damage the tubes if left in that condition too long. But if its too low it can affect the sound, although slightly low usually not.
brf
1,715 posts
 
I stand corrected (thanks Atmasphere). None of my tube amps measured bias in volts.
mceljo OP
1,357 posts
 
Yes, the manual clearly states 0.4v and it is also stamped on the amplifieir next to each set of measurement points. I figured that I was on the right track when I got results that were pretty much right on what was expected.

I will probably warm it up for a few hours tomorrow when a friend is over and then we'll try to set all four of them to be identical. It's probably just an exercise in "just for giggles", but we'll see how sensitive the adjustment is.

It will be easier with two people to avoid the hot tube while making the adjustment and also keeping the multimeter on the contacts.
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