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Thread: Bias after pulling out two power tubes?

  1. #1
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    Bias after pulling out two power tubes?

    I have a 94 Fender Twin with external Bias test. I have pulled the two inner 6L6 tubes to lower the wattage down to 60 watts per manual instructions. The manual says adjust bias to .08 volts but when I turn the adjustment screw all the way I only get .055
    So I tap on the power tubes, one is a bit harmonic the other is not. So I am thinking I need to replace my power tubes. My main question is do I still set the bias to .08 when running only two tubes? or do I set the Bias for half of .08 that being .04?

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    is this the evil twin?
    anyway,2 tubes means half current,so 40 mA each,and you measure 40 mVdc.
    you have to use a load of double impedance of what you have on the amp,i guess 4 ohms,so the speaker must be 8 ohms,if you have 2 in parallel i'd disconnect one.
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    Evil Twin

    Quote Originally Posted by alexradium View Post
    is this the evil twin?
    anyway,2 tubes means half current,so 40 mA each,and you measure 40 mVdc.
    you have to use a load of double impedance of what you have on the amp,i guess 4 ohms,so the speaker must be 8 ohms,if you have 2 in parallel i'd disconnect one.
    Yes this is an Evil Twin also referred to as a 94. It seem there is a lot of confusion online about the name Evil Twin.
    Also a follow up question when tapping on the two power tubes is it normal for one to be a bit harmonic and the other NOT harmonic?

    Thanks for your help.

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    If you look closer at the manual, you will see it says .04V for optimum tube life, .08V for optimum performance.
    So, somewhere in the range of .04 to .08V.
    Now, when you pull 2 tubes, you cut that in half, like alexradium said.
    So somewhere between .02 and .04V, then adjust impedance setting or disconnect a speaker like he said.
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    Quote Originally Posted by Pennypicker View Post
    Yes this is an Evil Twin also referred to as a 94. It seem there is a lot of confusion online about the name Evil Twin.
    Also a follow up question when tapping on the two power tubes is it normal for one to be a bit harmonic and the other NOT harmonic?

    Thanks for your help.
    what do you mean with harmonic,rattling,buzzing?
    if you play it at proper volume and don't hear noises from the speaker,leave it like that,otherwise try another set.

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    I think he meant that a tube was microphonic - and those can hum, rattle, screech, or any other sort of sound you can imagine...

    As far as "Evil Twin," I believe this was the moniker for the Blackface-Style "The Twin," which was a part of the "Pro Tube Series" in the 90s, along with the Concert, Prosonic(!), and there was one other one... the earlier incarnation was the "Red-Knob Twin."

    Justin
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    Quote Originally Posted by Justin Thomas View Post
    As far as "Evil Twin," I believe this was the moniker for the Blackface-Style "The Twin," which was a part of the "Pro Tube Series" in the 90s, along with the Concert, Prosonic(!), and there was one other one... the earlier incarnation was the "Red-Knob Twin."
    Evil twin is '94 Twin Amp which is what is says on the nameplate.
    Red knob is the earlier version which is labelled The Twin. There may be versions of this whose knobs are black, but they are pointer style knobs like the red knobs.
    Twin Amp (aka 94 aka Evil twin) uses standard skirted knobs.
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    Ok thanks. Oh I should have asked do I do anything to the balance adjustment? now that there is only one pair of tubes?

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    Yes this all is correct. The 94 has more features then most amps. Two effect loops one of them has a blend knob and each loop has level adjustment, reverb switch for each channel,XLR out, Main Speaker out/Ser/par, 4 8 16 ohms selector, 3 channels, presence etc!

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    Thanks g1, forgot about that one! :P

    Justin
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    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Should the Balance adjustment still be set to Zero?

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    When I tap with my finger on the tube one power tube makes noise. The amp sounds ok but the amp has to much bass even when turned all the way down I already order a set of tubes so I'll see if that fixes the problem.

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    Yes, adjust balance to zero.
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    g1 Thanks for the help!
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    Balance adjustment doesn't need any test equipment but your ears.
    Set any Master Vol or each preamp Vol to 0 so you are not getting the preamp sourced hum into the power amp..
    Then put your ear against the speaker and adjust balance for minimum hum. Mimimum hum will be at the point where the idle currents on each side of the push pull are equal.

    This works regardless of if you run 2 or 4 tubes, I would do it after pulling 2 tubes.

    Cheers,
    Ian
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