Tubes... Check. Bias probe... Check... now WTF do I do?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mohowski, Jan 29, 2008.

  1. Jan 29, 2008 #1
    mohowski

    mohowski Member

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    So i got my new tubes and bias probe in today. The preamp tubes are SWEET! Now, i'd like to put in the power tubes. I've biased my single ended EL84 homebrew before and am familiar with all the concepts... But when it comes to the Fender Prosonic i'm a little lost on account of the three different rectifier/class settings. Can somebody walk me through biasing it so i've got it right in all 3 modes?
     
    mohowski, Jan 29, 2008
    mohowski, Jan 29, 2008
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  2. Jan 29, 2008 #2
    Blue Strat

    Blue Strat Member

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    You won't be able adjust anything for "class A" (which probably really isn't class A but "cathode biased"). The pot will allow you to adjust in class AB.

    I have no idea what the 3rd mode is. What do they call it?

    Before installing new power tubes practice biasing the ones in the amp. See where they're set now, adjust the pot and watch the readings move, and set the pot to the lowest possible current point before installing the new tubes.
     
    Blue Strat, Jan 29, 2008
    Blue Strat, Jan 29, 2008
    #2
  3. Jan 29, 2008 #3
    mohowski

    mohowski Member

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    I believe it is just cathode biased rather than true class A as the switch denotes.

    The three switch settings are 1. Class A/Tube Rectifier, 2. Class AB, Tube Rectifier, 3. Class AB, S.S. Rectifier. When I measure the existing tubes with the switch in the AB/Tube rectifier (position 2), current is at 25ma/tube at a plate voltage of 465V. In position 3 (SS Rectifier), they are running around 20ma. In the class A/tube rectifier position, they measure 72ma with a plate voltage of 380V. I assume i'll have to pull the chassis and measure cathode plate voltage if i want to calculate the power dissipated for the cathode biased mode? Is 72ma too hot for that mode?

    Looking at the schematic on BlueGuitar, it looks like the bias trimpot is connected to both the AB modes (tube and SS rectifier). Which do I bias it in?

    Lastly, when I first turn the amp off of standby with the probe heads installed between tubes and amp, or when i flip the switch on the bias probe between the cathode current and plate voltage mode and the amp is on, i get about half a second of awful hiss, that seems to die out quicly. Is that normal?

    p.s. Thanks for the tubes Mike! The preamp tubes are just what i was looking for to tame the lead channel. It sounds so sweet my roomate was running up the stairs and into the room to see what was making such an awesome ruckus!
     
    mohowski, Jan 29, 2008
    mohowski, Jan 29, 2008
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  4. Jan 30, 2008 #4
    Blue Strat

    Blue Strat Member

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    Happy that you like the tubes!

    For the AB modes, bias for whichever one gives you the highest reading. Somewhere from high 20's to mid 30's will always work for 6L6GC's (the GC suffix is critical to this discussion). What particular power tubes are you using?

    72mA at 380 plate voltage gives an idle power of 27.36 watts. Acceptible if the tubes are rated at 30 watts max. You'll lose a bit due to cathode voltage too so you're probably ok here.

    I can't address the temporary hiss issue. Is this only with the bias device installed?
     
    Blue Strat, Jan 30, 2008
    Blue Strat, Jan 30, 2008
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  5. Jan 30, 2008 #5
    mohowski

    mohowski Member

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    Thanks for your insight. The existing tubes are groove tubes 5881s which look like a relabeled 6l6__ something or other, i cant quite make out the original writing. I'm going to be installing =C= SED 6L6GCs or Ruby 6L6GCMSTRs in their stead.

    Out of curiosity, any idea why the plate voltage drops so much when it's in cathode bias mode?
     
    mohowski, Jan 30, 2008
    mohowski, Jan 30, 2008
    #5
  6. Jan 30, 2008 #6
    Blue Strat

    Blue Strat Member

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    It must be by design. Lower voltage transformer tap or something else in the power supply. They seem to be trying to emulate a tweed circuit, most (or all) of which ran the plates below 400V.
     
    Blue Strat, Jan 30, 2008
    Blue Strat, Jan 30, 2008
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