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Biased my Fender 300 / 300T Amp (Confused)

Discussion in 'Amps and Cabs [BG]' started by sincity, Jan 11, 2006.


  1. sincity

    sincity

    Oct 16, 2002
    Connecticut
    So, I went to bias my tubes on my Fender Bassman 300. I think I did everything correctly, but when I turn the bias control screw, my readings do not change, no matter how much I adjust the trim screw. The same goes for the balance adjustment.

    My digital meter has a sensitivity from 0-200 mVDC.


    My reading for the tube bias is: .7 VDC
    My reading for the balance is: 0 VDC


    I could not adjust the bias down to .1 VDC as described below, no matter how far I turned the trim adjustment in either directioin. The same goes for the balance adjustment.

    If I read the directions below, my readings are within the set points, but because the readings would not change with the trim adjustments, I am a little perplexed.

    Here is what the manual says to do:

    1. Turn on your 300T and let it warm up for at least two
    minutes with the STANDBY switch in the ON position. Make
    sure the amplifier is properly connected to a speaker.

    2. Remove the bias controls cover box.

    3. With a digital voltmeter set to its most sensitive DC
    voltage scale, measure the voltage between the BIAS
    100mV test points (test points 1&2) and adjust the BIAS
    control for a reading of .1 VDC (100 mVDC).

    4. Next, connect the digital voltmeter to the BALANCE 0V
    test points (test points 2&3) and adjust the BALANCE control
    for a reading of 0 VDC.

    5. Replace the bias controls cover box.

    Notes

    1. Make bias and balance adjustments ONLY when
    necessary. If you are unsure about setting bias and
    balance, take your amp to the nearest authorized Fender
    Service Center.

    2. For optimum sonic performance, set bias to 100mVDC
    then set balance.

    3. If the output tubes can not be biased to at least
    80mVDC, replace the 6550C output tubes.

    4. If the output tubes can not be balanced, check the bias
    setting and/or replace the 6550C output tubes.

    _______________________________________________

    I was just wondering if this is normal, or should I have a tech check it out further?

    Thanks for your help
     
  2. Passinwind

    Passinwind I Know Nothing Supporting Member

    What kind of load are you using? Speaker? Can you hear the effect of the balance control?
     
  3. sincity

    sincity

    Oct 16, 2002
    Connecticut
    I am using a speaker load. Because the amp has to remain on Standby per the directions, I cannot hear a difference when adjusting the balance.

    I thought about taking the amp off standby, to see it made a difference with the readings.
     
  4. Passinwind

    Passinwind I Know Nothing Supporting Member

    Aha! The directions are wrong (more like incomplete or just plain confusing); the tubes will not bias without plate voltage present, which is the function of turning the standby switch on.:cool: On many amps, the standby ON position means the amp is not in standby though. As in, a choice of STANDBY or ON. I assume that's the case here?

    The major function of the balance control is to adjust it so the amp sounds as quiet as possible at idle. If you can't hear that, it'll be hard to adjust optimally.
     
  5. sincity

    sincity

    Oct 16, 2002
    Connecticut
    So after this instruction from the manual:

    1. Turn on your 300T and let it warm up for at least two
    minutes with the STANDBY switch in the ON position. Make
    sure the amplifier is properly connected to a speaker.

    They should have said, "Take the amp off of standby"

    Correct?
     
  6. Passinwind

    Passinwind I Know Nothing Supporting Member

    Yeah, that's correct. See my edit above for more thoughts on switch labeling though.
     
  7. Horny Toad

    Horny Toad Guest

    Mar 4, 2005
    NJ

    From the 300 PRO manual:

    "STANDBY - When the power switch and this switch
    are ON, your 300 is ready to perform. In the standby
    position, power is supplied only to the tube filaments and
    sound will NOT come out of the amp. Using the standby
    switch during short breaks eliminates tube warm-up time
    and will increase tube life."


    So yes, I'm guessing they mean NOT on standby.

    Hope that helps. Hope I'm correct. ;)




    [​IMG]
     
  8. sincity

    sincity

    Oct 16, 2002
    Connecticut
    Yeah, kind of a play on words that I obviously took the wrong way. Makes perfect sense what you described.

    Well great, I am looking forward to trying this again tonight to see what happens.

    Thanks for your help.
     
  9. Passinwind

    Passinwind I Know Nothing Supporting Member

    Wish they were all that easy!

    BTW, how do you like the amp?
     
  10. sincity

    sincity

    Oct 16, 2002
    Connecticut
    What, now you're going to make me read the manual!:D
     
  11. Horny Toad

    Horny Toad Guest

    Mar 4, 2005
    NJ

    I would never do such a thing! You ask the questions, we'll read it for you. ;) :D


    [​IMG]
     
  12. sincity

    sincity

    Oct 16, 2002
    Connecticut
    Just wanted to let you guys know the bias & balance worked perfectly. Thank you.

    Passinwind, you were right about listening to the sound of the amp. As the amp reached .1 mV for bias, the amp's hiss got very quiet.

    Thanks again.
     
  13. Quick question, would the bias of 0.1VDC and balnce of 0 VDC change depending on the tubes you're using. For example if someone were to use KT88s instead of 6550 tubes would the bias still need to be set at 0.1 and balance at 0 or would it be something different? And does this vary between deifferent tube manufacturers eg. JJ KT88 vs EH KT88?
     
  14. sincity

    sincity

    Oct 16, 2002
    Connecticut
    It does not matter which tube is used. The bias and balance numbers are the same.