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Thread: need help for a fender twin-amp

  1. #1
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    need help for a fender twin-amp

    need help to adjust bias in fender amp.

    --------------------------------------------------------------------------------

    i have a twin-amp fender that doesn' work, it doesn't have any sound at the speakers, in the schematics i was following the signal (voltages) and everything is ok in all the preamp Stage, the problem is the output of the 12AX7 tube (before the first 6L6) because in the schematics says it has to be 18.4VAC and i have almost 36 VAC i don't know why... and i checked the input in the 6L6 and i have almost 36 VAC, but at the output of the 6L6 tube i don't have any voltage... i read i should adjust the bias (Set BIAS: With a DC voltmeter, measure VDC between te “GND” and “V9,V10” test points. Adjust BIAS for .06VDC (60mVDC).
    Set BALANCE: With a DC voltmeter, measure VDC
    between the “V9,V10” and “V11,V12” test points. Adjust BALANCE for zero (0) VDC.)
    but i don't have any voltage in the measure points never.. so i want to know if someone can help me.

  2. #2
    Senior Member mikeboone's Avatar
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    You need to provide a little more information. What exact year and model of Twin do you have? Can you post the schematic?

    There are a few very basic basic steps you can take that can be very telling.

    Turn the amp on and let it warm up for a bit. Observe the tubes when powered up. Make sure they are glowing (but not cherry red). Record DC voltage on all pins on each tube and list them here. Start with V1 and make your way over to the power tubes.

    You mentioned that you have no voltage present when you measure at the bias test point. This indicates that either your bias test point is bad OR that your tubes are not drawing any bias current.

    I am just thinking of how I would start to break it down if it were mine.

  3. #3
    Junior Member
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    ahh thanks for your help, well the amplifier is a twin-amp from 2000 year
    i don't know how to post the schematics, but i did what you tell me (Turn the amp on and let it warm up for a bit. Observe the tubes when powered up. Make sure they are glowing (but not cherry red). Record DC voltage on all pins on each tube and list them here. Start with V1 and make your way over to the power tubes) the tubes are glowing right and all the preamp Stage voltages are OK.. the problem starts when the signal arrives to the power amp Stage... because in the input of the 6L6 tubes i had the signal, but in the output of the 6L6 tubes i don't have any voltaje... but here where i live there's a person who sells tubes, so i took the tubes with him and he tested the 4 6L6.. and they are not working any more, he told me that they have a timelife from 1000 to 3000 hours more less... and they are finished their life.. so i solved my problem.. but thanks for your help any way.

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