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

  • #2
    If you have the schematic, please post it.
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

    Comment


    • #3
      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
      Logic is an organized way of going wrong with confidence

      Comment


      • #4
        "If you have the schematic, please post it."

        Sorry, I thought I had done so.

        http://www.fmicassets.com/Damroot/Or..._schematic.pdf
        It's weird, because it WAS working fine.....

        Comment


        • #5
          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

          Comment


          • #6
            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

            Comment


            • #7
              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?
              Last edited by Randall; 03-01-2019, 11:50 PM.
              It's weird, because it WAS working fine.....

              Comment


              • #8
                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.
                Last edited by Randall; 03-02-2019, 06:49 PM.
                It's weird, because it WAS working fine.....

                Comment


                • #9
                  Fender Auto Bias - Bassman 100T - Issues RESOLVED

                  Hello all - I realize this is an old thread but I have found a solution to the autobias issues related to the Fender Series of Amplifiers with this feature. The amp model that came to my shop was a Fender 100T Bassman - an excellent sounding amp and well designed WHEN IT WORKED. My customer bought this amp second hand - so out of warranty (no original receipt & past warranty expiration anyway). I mention this because my experience dealing with Fender Tech support was not at all good - I am not an authorized service center so I didn't expect the best support but the support I did receive was awful. In essence, exploring my options - I was told I could buy the bias board assembly if need be and to my surprise it was listed on the Fender website parts list @ what I thought was a reasonable price of around $60. But when I tried to put it in my checkout basket message popped up that it was no longer available - after several calls and emails back and forth I was told that they removed it from on-line parts and ONLY authorized service centers could purchase it - when I asked for some help determining if it was in fact the cause of the problem (had my voltages and component readings all ready) they wouldn't help me - and when I asked what they did with others who had this problem which is common - they told me that if the amp was in warranty they would just replace it if not customer was out of luck. More to come....

                  Anyway - the problem was similar to what is described here - one of 4 6L6s would bias up then eventually most times within 5 minutes - one of that pair would turn LED orange, then entire pair would fail. Swapping know good tubes would not fix problem moving suspect "bad" tubes to other sockets did not have problem follow tube - it stay with socket - I spent hours monitoring voltages and checking every component (except ICs on bias board) in the bias circuit - most of the resistors that are feeding the bias control circuit are high quality precision resistors and were for the most-part well within tolerances for that component.

                  Resetting the bias-board cleared the problem - the first time it cleared after I re-tubed the amp - I thought that the board was just glitched and that this corrected the issue - I let the amp burn in (outside the case) for 3 days straight - last 36 hours with 1K tone running through the amp / speaker cab just to be sure - then powered up & down several times - ran another 24 hours and called customer to pick up amp. After putting amp back in cabinet and having customer test amp while he was there picking it up - it failed - very embarrassing indeed!!! - What changed putting it back in cabinet????? First thing I thought was heat - and why wasn't this cooling fan ever turning on was this the problem. - After reading temps with infrared HQ temp gun - no... Fan I later reasoned out was not for general cooling of the amp - it was for the big power resistors which are only used when amp is in silent mode (headphone or pugging head into mixing board) - that's when these large power resistors are used to simulate speaker load - the sensor did work at the rated temperature (tested that with heat gun and temp reader - & it operated as designed) - although as an added bonus I installed an LED lit 2 way mini toggle so that customer can bypass thermostat and power fan on anytime - in the off position is reverted back to factory spec using thermostat (dual color LED - Red =on, blue = factory/off).

                  So what was problem - well there are these ribbon cables connected to the bias daughterboard and main motherboard providing the PCBs operating voltages as well as the voltages from the cathodes of each output tube and to the output tube connection where the negative bias voltage would be applied to bias the tube (based on the cathode current of that tube in idle). I noticed that the cabinet gently pushed that cable down - so I started moving the cable around while monitoring the tube bias LEDs - (BTW Fender used cable ties on these ribbon cables crunching them and causing tension on the connectors - these are computer grade cables that are not designed for that kind of abuse) - low and behold if the cable was moved or squeezed the amp would lose bias or bias voltage causing one or more of the bias LEDs to go crazy. So I called Fender for a part number so I could replace the cable - again they said they couldn't or didn't know or it came as part of a kit with the board - no help at all - after some trial and error and educating my self on the different type of ribbon cables (pitch, pin count and the various different connector head sizes and types) I found only one supplier that had the exact replacement for this cable - MOUSER (note that the cable connector on one end is standard but on the other end is a low profile head - two low profile heads will work). I am certain that considering the number of failures with this bias circuit that it was either a bad batch of ribbon cables or it was the poor installation method used (cable ties that literally pulled one or more wires off the head especially those wires on either end by taking a flat cable and making it round with a too tight cable tie. I verified the fix 100% and after 3 months - customer uses his Bassman 100T almost everyday with no return of the bias fail problem. Mouser Part # 200-TCSD10D05.0001N Mfg. Part # - Type IDC Ribbon Cable - TCSD-10-D-05.00-01-N.

                  NOW KEEP IN MIND - this was determined after traditional amp tech troubleshooting - eliminating possible other causes - like loose or dirty output tube socket pins, loose or broken output tube solder connections to board, all resistors / caps from all pins on output tubes checked for continuity, values, bad or broken solder joints, all other cables ribbon and other types criss-crossing the board - sockets, pins and loose connections. After all that using the "chop-stick" test - SAFETY FIRST - with amp out of chassis and speaker load connected - power on and using a non-conductive probe (e.g. wood chopstick) monitor voltages listen to speaker output - touch wires, connectors apply gentle pressure, touch key components in output tube circuit feeding bias board to see if anything is loose or intermittent -

                  If the ribbon cable is proven to be suspect turn off power and discharge filter caps - carefully remove glue used to hold cable head on motherboard may need to use an exacto knife or similar - CAREFUL not to cut or damage circuit board or other components / wires - IU found problem to be in ribbon cable head connected to motherboard - the one connected to the bias board was OK. No need to remove either the bias board or the motherboard to replace cable - clean area around connector where u needed to remove glue install new cable gently don't bend pins - and then test amp before gluing down ribbon cable. I use E-6000 for gluing components to PCBs (e.g. capacitors etc..) and also for loose fitting cables like these ribbon cables - they do not have any kind of locking mechanism and must be secured using glue. You can use silicone (softer, longer cure, not as strong) or this self leveling E-6000 - very strong use just a small amount - ensure cable head is fully seated all around and perfectly flat to the board not cockeyed - this stuff dries quickly and the odor is strong but completely gone after it cures for about 24 hours. I don't recall if both ends were glued or just the connector on the motherboard (which required that mini head to fit) Just base this decision on how old cable is glued.

                  Besides the fan bypass on toggle DPDT w/3mm dual color LED mod, and securing cables in a non-damaging way - the only other mod I Strongly recommend for this amp is changing the lobster claw tube retainers for the 4 output tubes with the spring type Marshall style tube retainers because they hold those upside down heavy tubes better and keep them from shifting especially if you are a working musician moving the amp around = shifting can cause bad pin connections and arcing - which can quickly ruin your tubes - and BTW output tube selection can make a big difference in tone - I personally like the Tung Sol Brand 6L6-STR & KT66 for this amp although I also discovered and tried The Tube Store's Premium line 6L6GC - a bit pricier but I was floored by the tone - much like the now defunct Winged "C" 6L6GCs - see link https://www.thetubestore.com/preferred-series-6l6gc . Tung Sol is my favorite for both the 6L6 & KT66. The Kt66 does require a different Spring Retainer than does the 6L6/EL34 - the spring length is the same but the end "cup" that secures the rounded end of the tube is wider on the retainers for the KT66 because the tube is much fatter than either the 6L6 or EL34.

                  So I repair any new or Vintage tube amplifier any pedals especially the vintage ones - I stock many vintage / period correct parts for both amps and pedals and save my NOS tubes for vintage amps as I see no reason to install vintage tubes on any new production amps - 20 years ago maybe - but today there are so many great choice in new stock - if you have a favorite I can get them but I stock Tung Sol and JJ and now a couple of sets of these Premium 6L6s (which look and sound like Blackplate RCAs - haven't noticed any reproductions that actually look like an RCA tube. - I can be reached via text @ 908 400 2760 - I am located in New Jersey. - Here is a pic of the LED/Fan mod.


                  Chris


                  [IMG]D:\From Nora PC 1_25_20\Chris Stuff new Laptop\Amp_Schematics\Fender\Bassman_100T\20190902__Bass man_100T_Fan_LED_Switch_Blue_On[/IMG]


                  OBVIOUSLY I haven't figured out how to post a picture from a file on my PC - I don't understand why we have to use these HTML programing codes to upload a picture or file - (like a schematic or drawing) - maybe I am missing something..
                  Last edited by Chriscolt; 02-21-2020, 08:04 AM. Reason: apology for not knowing how to post a picture from my PC

                  Comment


                  • #10
                    Great post Chris, and welcome to the forum.
                    Not sure how you tried to attach the image? If you click on the 'insert image' icon, you should be able to select your file, then click on 'upload'. Or you can select 'go advanced' below the reply box, then use 'manage attachments' to attach your file.
                    Lastly, it's possible you may not have sufficient privileges yet as you only have a couple posts.
                    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                    Comment


                    • #11
                      Or upload photos to an external hosting site, eg imgbb, and provide a link.
                      My band:- http://www.youtube.com/user/RedwingBand

                      Comment


                      • #12
                        Great job, and clever fix, but at some point shouldn't we be asking ourselves at which point is this entirely too much work and time for what we are paid? I know I do, especially when it keeps me from working on other amps and generating billable time.
                        It's weird, because it WAS working fine.....

                        Comment


                        • #13
                          It is like any complex procedure. Will we be seeing it, or was it a one-off? My first PV Classic 30 was a pain, and I spent too long on it. But once done, the next one took just a few minutes. My local friend was struggling with one recently, and I came over and demonstrated how to get it apart simply. SO if we struggle with this Fender mightily and discover some cure that will be used often. next time we know it.


                          I am concerned that I see more and more people deciding to reengineer circuits instead of repairing them.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • #14
                            Originally posted by Randall View Post
                            Great job, and clever fix, but at some point shouldn't we be asking ourselves at which point is this entirely too much work and time for what we are paid? I know I do, especially when it keeps me from working on other amps and generating billable time.
                            Fender way over-engineered the bias system on this amp, then depended on cheezy ribbon cable & accompanying connectors to make it work. Good thing they're not bio-engineering, we'd have people with seven elbows on each arm, and three knees on each leg. To the "engineers" I say "DECAF boyz, DECAF! Stop fixing problems that don't exist. Keep It Simple, Stupid!"

                            It's been making my head spin, reading this thread. Thanks Chriscolt for popping in and detailing the fix. I hope nobody shows up at my place with one of these. "It's got tubes in it, you're s'posed to be able to fix tube amps, right?" Grrrrr.....
                            Enjoy. Every. Sandwich.

                            Comment


                            • #15
                              "It's got tubes in it, you're s'posed to be able to fix tube amps, right?"
                              Well, the internal combustion engine is all the same from the beginning but they keep adding all kind of electronics around it and I don't see anyone to object their new cars so maybe we'll have to live with it.
                              I was wondering if anyone could kindly save the software from the uCU (if it's readable and not locked)?

                              Comment

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