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Thread: Fender Auto bias circuit

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    Supporting Member Randall's Avatar
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    Fender Auto bias circuit

    A pretty new Fender Bassman 100T bass amp came to me with their Auto bias circuit, which I have never seen. It allows the user to step between cool, normal, and warm bias setting, and it biases each individual tube separately on an ongoing basis. It has four LEDs, one for each tube, green is good, red is bad, and orange means that tube in the pair with the bad one is shut off. When it came to me, one pair was lit red and orange, and the amp didn't have full power. While I was plaing it, it fizzled out to nothing, which was the complaint. Indeed I discovered all four LEDs were either red or orange. It has the original tubes in it, so wanting to determine if the tubes were actually bad, or if the auto bias circuit was at fault, we opted to start with a fresh quad of 6L6s.

    While I am waiting for that order to arrive I decided to take a look at the circuit, and came to an impasse. I see were raw C- comes in, and I see the white arrows for bias V 4-7, but I don't understand how this circuit works. Does anyone have any experience or insight with this?

    I suspect a problem with the circuit because I swapped the tube that showed a fault with one that showed green, and after several minutes of all 4 showing green, it went back to showing a fault in the same socket as before, and the tube that had showed red is now showing green on the socket it got swapped to.

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    It's weird, because it WAS working fine.....

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    Don't forget the joker g1's Avatar
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    If you have the schematic, please post it.

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by g1 View Post
    If you have the schematic, please post it.
    I have a Super Sonic schematic that has that Automatic Bias circuit

    Super-Sonic_100_2011_schematic_Rev-B.pdf

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    Supporting Member Randall's Avatar
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    "If you have the schematic, please post it."

    Sorry, I thought I had done so.

    http://www.fmicassets.com/Damroot/Or..._schematic.pdf

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    It's weird, because it WAS working fine.....

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    Randall,
    P23 obviously feeds the autobias circuitry but not shown on your posted schematic.


    From the Super_Sonic schematics (assuming they are the same) then the cathode voltages (from the current sense resistors) feed some Opamp circuits which generated sum an difference voltages per output tube push pull pair. These feed analog to Digital Converter inputs on a microcontroller.
    The microcontroller uses Pulse Width Modulation (PWM) outputs (one for each tube) to control the bias. These would be adjusted by the software to get the sum to a selected value and the difference to zero, and set the LED color for that tube appropriately.

    I would suspect one or more bad 6L6s but be sure to check that the current sense resistors (shown as Fp1) ?Fire proof 1 Ohm?, the grid "fuse" resistors (shown as Fr22) ?fire resistant 22 Ohms? and the 470 Ohm screen resistors.


    Cheers,
    Ian

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    Supporting Member Jazz P Bass's Avatar
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    Unfortunately Fender did not include the bias board.
    P23 and the associated circuitry does appear similar to the Super Sonic 100 P11 connector.

    Looks to me as if the whole ball of wax is microprocessor controlled.

    Fender auto bias circuit.pdf

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    Supporting Member Randall's Avatar
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    All the suggested resistors check OK, except for one small issue. The 470R 5 watt 10% screen resistor on V6 measures 456R, well within tolerance, but the farthest away from 470R than the other three. It is also the tube socket that is showing a failed tube with a red LED. I just put in a fresh matched quad of JJ 6L6. The LEDs are indicating the same as with the originl tubes. This is looking like a microprocessor board swap to me at this point, I have no chance of fixing that.

    edit: I have some more data on this. I believe all the components around V6 are good. But I have -44v bias on V6 and -53v on the other three, which are at ~30mA. V6 is at ~ 60mA. Plate and Screen voltages are the same for all four tubes. After a time, the auto bias puts -76v bias on V6 and it mate V5, turning them off.

    SO, which is it, do I have one tube drawing too much current due to another issue, and auto bias is catching this and turning that pair off, or is the auto bias responsible for the lower bias voltage at V6?

    The bias voltages are this way in standby, so I am working in this mode for now. I have -80v RAW C- at one side of R265 and -44v on the other side where BIAS V6 connects. I find that if I measure the bias voltage in standby at V6 pin 5 it is -44v, but if I unplug power to the amp it jumps up to -60v, where the other three are also at, and slowly fades down along with them. So, this suggests to me that auto bias is regulating the bias to V6 incorrectly, and then shutting it down when it detects a too high current draw.

    The question is, how to remedy this without rolling the dice on a replacement smd processor board?

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    Last edited by Randall; 03-02-2019 at 12:50 AM.
    It's weird, because it WAS working fine.....

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    Supporting Member Randall's Avatar
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    After chasing this for a while, I did a reset on the bias board (which, btw I had done before), by pressing both up/down arrows for 3 - 5 secs, and the amp has now stabilized. All green LEDs with V6 pullling the same current as the others, and the bias voltage being the same as well. So, I guess this is not the most reliable circuit. I know I don't care for it. I would not be surprised based on what I have read, for this to happen again. Unfortunately, this will be one of those cases where I will have to tell him there is not much else I can do for him, unless he can get Fender to send him a new board to try. I have read of several cases where Factory authorised shops tried several repairs only to ending up replacing the amps, so Fender is certainly aware of this.

    edit: It ran all green for two hours, so I powered it down and let it cool off. When I powered it back up, after about 10 minutes it indicated V5 red and V6 orange, the opposite of before. So, that points to the microprocessor board.

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    Last edited by Randall; 03-02-2019 at 07:49 PM.
    It's weird, because it WAS working fine.....

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