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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
 
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.
jedinite24's avatar
jedinite24
1,503 posts
 
Hi Mceljo

I would just bias your amp at .45v and call it a day. I had similar experiences with my Jolida amp. I at first set the bias to all power tubes to 500mV exactly. Later I found that depending on the voltage from the socket would affect the bias of my amp. The voltage around my neighborhood is pretty bad. There are lots of swings. I would see the red LEDs flicker or darken. After that I just set the bias to 550 or 560mV and never had any issues with the LEDs going out or tube life.
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