Stupid Tube Question ??? What is Bias Adjustment?

wayne3burk
wayne3burk Posts: 939
edited February 2010 in 2 Channel Audio
I keep reading about Bias Adjustment in regards to tube amps.

I'm kinda curious what it is and how is it set?

- Wayne
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  • doctor r
    doctor r Posts: 837
    edited February 2010
    To answer your question I would suggest the following website:
    http://www.aikenamps.com/
    Once at the site click on "Tech Info" on the left side, then "Introductory", and at the bottom of the list is a FAQ labeled "What is biasing"
    Good information applicable to any tube amp.

    Rick
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  • heiney9
    heiney9 Posts: 25,165
    edited February 2010
    Perfect explanation!

    In a fixed bias amp the manufacturer should provide you with a bias range. On an amp I have that uses 6V6GT power tubes the recommended bias is 200-250mV per tube. So it's not a guessing game. Too high or too low and you will damage the tube and/or amp. I ran another set of tubes for a short time, not bothering to re-bias and I ruined one of the tubes. Upon it's demise I checked the bias for that one tube and it was 360mV. The idiot who sold me the amp never even knew it had to be biased when you roll different power tubes into the amp.

    H9
    "Appreciation of audio is a completely subjective human experience. Measurements can provide a measure of insight, but are no substitute for human judgment. Why are we looking to reduce a subjective experience to objective criteria anyway? The subtleties of music and audio reproduction are for those who appreciate it. Differentiation by numbers is for those who do not".--Nelson Pass Pass Labs XA25 | EE Avant Pre | EE Mini Max Supreme DAC | MIT Shotgun S1 | Pangea AC14SE MKII | Legend L600 | BlueSound Node 3 - Tubes add soul!
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  • HB27
    HB27 Posts: 1,518
    edited February 2010
    Some solid state amps also need biasing.
    Harry
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  • rubin
    rubin Posts: 565
    edited February 2010
    SS bias? I don't understand
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  • HB27
    HB27 Posts: 1,518
    edited February 2010
    Same principals. Some of my big Sansuis will fry eggs and run in the 160+ degree range If I don't bias them every couple of years. I have to get out the service manuals and set them up.
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  • hearingimpared
    hearingimpared Posts: 21,137
    edited February 2010
    Doctor R wrote: »
    To answer your question I would suggest the following website:
    http://www.aikenamps.com/
    Once at the site click on "Tech Info" on the left side, then "Introductory", and at the bottom of the list is a FAQ labeled "What is biasing"
    Good information applicable to any tube amp.

    Rick

    Awesome info in that link! Now I REALLY know why I don't have tube power amps!:D Can't be bothered these days, I just want to set it and forget it. I'm gettin' old!:(
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  • heiney9
    heiney9 Posts: 25,165
    edited February 2010
    hearingimpared wrote: »
    Awesome info in that link! Now I REALLY know why I don't have tube power amps!:D Can't be bothered these days, I just want to set it and forget it. I'm gettin' old!:(

    Joe, to be fair that's exactly what you do, unless you like to roll a lot of power tubes. Sure as tubes wear it is prudent to check bias say a couple times a year, but mostly it's a set it and forget procedure. It's not like the bias varies all that much, unless you start rolling all kinds of different power tubes. Most amps make it very easy to bias the power tubes with either a built in meter or convenient user accesible testing points and bias pots.

    My integrated is a bit of a PITA simply because the pots are on the bottom under the cover plate and the amp has to be turned upside down to bias but once it's set and I have the tubes I like I won't need to test it for quite sometime.

    bias1.jpg
    bias2.jpg

    FWIW

    H9
    "Appreciation of audio is a completely subjective human experience. Measurements can provide a measure of insight, but are no substitute for human judgment. Why are we looking to reduce a subjective experience to objective criteria anyway? The subtleties of music and audio reproduction are for those who appreciate it. Differentiation by numbers is for those who do not".--Nelson Pass Pass Labs XA25 | EE Avant Pre | EE Mini Max Supreme DAC | MIT Shotgun S1 | Pangea AC14SE MKII | Legend L600 | BlueSound Node 3 - Tubes add soul!
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  • thsmith
    thsmith Posts: 6,082
    edited February 2010
    HB27 wrote: »
    Some solid state amps also need biasing.
    Harry

    I believe the Carver TFM-45 does unless you replace pot with fixed resisters.
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  • hearingimpared
    hearingimpared Posts: 21,137
    edited February 2010
    LOL Brock. I saw that pic of your brother in another thread biasing that piece on his hands and knees and all I can say is :eek::eek::eek:

    Serioulsy though, I've heard some awesome tube amps at Quest for Sound and it really is alluring. For now I am happy with my hybrid integrated but my experience in life is to "never say never!":)
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  • heiney9
    heiney9 Posts: 25,165
    edited February 2010
    hearingimpared wrote: »
    LOL Brock. I saw that pic of your brother in another thread biasing that piece on his hands and knees and all I can say is :eek::eek::eek:

    Serioulsy though, I've heard some awesome tube amps at Quest for Sound and it really is alluring. For now I am happy with my hybrid integrated but my experience in life is to "never say never!":)

    That was ME on my hands and knees biasing the amp -lol.

    H9
    "Appreciation of audio is a completely subjective human experience. Measurements can provide a measure of insight, but are no substitute for human judgment. Why are we looking to reduce a subjective experience to objective criteria anyway? The subtleties of music and audio reproduction are for those who appreciate it. Differentiation by numbers is for those who do not".--Nelson Pass Pass Labs XA25 | EE Avant Pre | EE Mini Max Supreme DAC | MIT Shotgun S1 | Pangea AC14SE MKII | Legend L600 | BlueSound Node 3 - Tubes add soul!
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  • heiney9
    heiney9 Posts: 25,165
    edited February 2010
    thsmith wrote: »
    I believe the Carver TFM-45 does unless you replace pot with fixed resisters.

    Almost all SS amps have bias adjustments but they aren't user friendly like on a tube amp where if you have a fixed bias amp you MUST reset the bias if using different tubes. In SS the bias can drift over a very long period of time as parts wear out, but generally once the bias is set from the factory it shouldn't be touched unless there is an issue.

    H9
    "Appreciation of audio is a completely subjective human experience. Measurements can provide a measure of insight, but are no substitute for human judgment. Why are we looking to reduce a subjective experience to objective criteria anyway? The subtleties of music and audio reproduction are for those who appreciate it. Differentiation by numbers is for those who do not".--Nelson Pass Pass Labs XA25 | EE Avant Pre | EE Mini Max Supreme DAC | MIT Shotgun S1 | Pangea AC14SE MKII | Legend L600 | BlueSound Node 3 - Tubes add soul!
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  • hearingimpared
    hearingimpared Posts: 21,137
    edited February 2010
    heiney9 wrote: »
    That was ME on my hands and knees biasing the amp -lol.

    H9

    You must be brothers then as you have the same butts!:D
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