biasing problem....

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hobbes1, Sep 4, 2006.

  1. hobbes1

    hobbes1 Member

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    i have a 1988 fender the twin (red knob). i recently retubed with matched quad JJ 6L6GC's and i biased and blanced per manual, via the external test points and my multimeter. no problems.

    after borrowing two of the 6L6's to use for a couple of hours in another, new amp, i replaced them into the twin and thought i'd recheck bias. so, when powered up and taken out of standby, i immediately saw one tube (not one of the "borrowed" ones) begin flourescing wiht all sorts of fireworks inside the tube and there was a loud hum out of the amp. i immediately went back into standby and powered off.
    next, i swapped the affected tube from one socked to another and tried again. the same tube flouresced some more and it seemed there was a bluish glow in the power tubes. i ran the bias back by eye rather than multimeter and the glow went away. so i powered off.

    i got a replacement, matched JJ 6L6Gc to swap for the affected tube and powered up. now, there was no hum, no fireworks and all the power tubes are looking to be glowing normally. the problem is that the amp will no longer bias or balance to manual specs of bias = 0.040 and balance = 0 volts. what i am reading on the mulimeter is a bias that won't go below about 1.25 and balance that won't go below about 10 volts.
    any ideas on whatmight be causing these readings? i'm afraid to play the amp until this is resolved. i have no problem taking it to an amp tech but would like to have some idea, if possible, as to what is going on.
    any advice appreciated.
     
  2. John Phillips

    John Phillips Member

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    Those readings are so far out (assuming that you do mean volts, not mV), that it looks like the shorting tube has fried the 1-ohm bias metering resistor on one side.

    There's no way 1.25V is possible if the resistor is OK - that would mean a tube current of 1.25 AMPS, and a plate dissipation for the tubes on that side of around 600W :).

    I would also get your tech to check the screen resistors - Fender used carbon-comps in that amp, and they tend to burn out under serious short conditions too.

    And no, don't play the amp until you've had it checked out.
     
  3. TheAmpNerd

    TheAmpNerd Member

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    Aye,

    Now there my friends is a tech experienced with musicians.

    Cheers
     
  4. hobbes1

    hobbes1 Member

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    hi john. thanks for the info. i was using the most senstive volts scale on my multimeter to do the bias/balance adjustment. it's reading at that setting would be to 3 decimal places ( 0.000). for the balance i had to shift up to the next level to get a reading at all (00.00).
    thanks for the advice...i will have that checked out.
     
  5. hobbes1

    hobbes1 Member

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    john--any recommendations on what brand of replacement parts to use or not use for the items you mentioned? thanks
     

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