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Thread: Fender Super Bassman Auto-Bias Circuit Issue

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    Supporting Member nevetslab's Avatar
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    Fender Super Bassman Auto-Bias Circuit Issue

    I have a Fender Super Bassman on the bench which had one failed Sovtek 6550 in itís output tube set. Having exhausted my Sovtek 6550 spare pulls on hand, I ordered a fresh matched Sextet of J/J KT-88 Power Tubes, which came in a couple days ago. I installed them yesterday, and also installed my two cathode resistor sense cables so I could measure the combined plate/screen current thru the six tubes during/after the Auto-Bias does itís adjustment.

    What you normally see when first powering up is the GRN LEDís one by one, left to right, light up while the amp is warming up. When you switch out of Standby, then the six GRN LEDís will flash about around four times a second, and eventually settle to a steady display of all six GRN LEDís lit. This being a new set of tubes, I Pushed the Up/Down Arrows simultaneously after switching out of Standby, and it finally settled. Though watching that sequence, it didnít seem normal, where youíd see pairs highlighted on the display as it adjusted them. I switched back to S/B, waited a minute or two, and switched out of S/B to see if anything changed. While in S/B, the six LEDís now turn on and off about once a seconds. Switched back out of S/B, itís no longer normal.

    What changed, is now the Auto-Bias GRN LEDĒs are just flashing without ever stopping, while the amp is drawing nominal idle current as it was prior to switching to S/B. The only way Iíve found to stop that is to turn the amp off, then back on again. But, for some reason, Standby messes the Auto-Bias circuit up. The amp works, and the current balance and values remain similar to what was there prior to switching to S/B.

    Under drive conditions, having the sense cables installed to monitor the tubes, you can see the increase in plate/screen current just fine, and the amp sounds normal. But, not finding the Auto-Bias behaving as normal, I tried replacing the driver (12AT7) with a new one. No difference.

    A few years ago, when I had a failure in the Auto-Bias in one of our Super Bassman amps, I ordered a replacement PCB along with the short ribbon cable. Problem then turned out to be the ribbon cable, so Iím about to try swapping out the Auto-Bias PCB assy.

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    I still havenít acquired full service documents for the Super Bassman amp. Closest I have is that of the Super Sonic 100_2011, which also has this same Auto-Bias circuit and does detail it in the schematic, though no operational details to chase whatís going on here. Iíve attached that schematic, along with the Auto-Bias instructions page from the Super Bassman Ownerís Manual.

    SuperBassman_Auto-Bias_Inst.pdf
    Super-Sonic_100_2011_schematic_Rev-B.pdf

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  2. #2
    Supporting Member nevetslab's Avatar
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    I've removed the Auto-Bias PCB V1.3 PCB from the inside rear panel of the chassis and installed the V1.2 PCB I had tucked away. Hardest part of changing that board out is getting the tiny flexible circuit from the LED Display assembly to plug back into the mating connector at the bottom edge of the Auto-Bias PCB.

    Powered up the amp, and it behaves just the same as the one I removed. It will bias up from a cold start, but, will NOT return to stable LED status once you've switched back to Stand-By and back on again. I've also found that the Manual Up/Down buttons which allow you to decrease or increase the nominal plate current level from the factory default level. I went and powered back up from scratch, since nothing works with this Auto-Bias circuit once you've entered & left Standby mode. When the amp was thru setting the Automatic Bias from the cold start, the Manual adjustment keys WILL NOT CHANGE the bias level AT ALL.

    I swapped out the 12AX7 input stage tube of the power amp on this T-shaped chassis, but of course, that made no difference either.

    My life feels so much better knowing Fender has installed a computer-controlled bias circuit in these amps!

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by nevetslab View Post
    I've removed the Auto-Bias PCB V1.3 PCB from the inside rear panel of the chassis and installed the V1.2 PCB I had tucked away. Hardest part of changing that board out is getting the tiny flexible circuit from the LED Display assembly to plug back into the mating connector at the bottom edge of the Auto-Bias PCB.

    Powered up the amp, and it behaves just the same as the one I removed. It will bias up from a cold start, but, will NOT return to stable LED status once you've switched back to Stand-By and back on again. I've also found that the Manual Up/Down buttons which allow you to decrease or increase the nominal plate current level from the factory default level. I went and powered back up from scratch, since nothing works with this Auto-Bias circuit once you've entered & left Standby mode. When the amp was thru setting the Automatic Bias from the cold start, the Manual adjustment keys WILL NOT CHANGE the bias level AT ALL.

    I swapped out the 12AX7 input stage tube of the power amp on this T-shaped chassis, but of course, that made no difference either.

    My life feels so much better knowing Fender has installed a computer-controlled bias circuit in these amps!
    I just worked on one of these amps for the first time about 2-3 weeks ago. One thing I did appreciate was they built the chassis around the huge transformers (mounted directly to the cabinet, making it easier to remove the chassis) and service the boards.
    But, I didn't like the way the modular/molex connectors are fit and secured in this amp. They easily become dislodged, and the female connectors do not seem to have much strength in the way they mounted them.
    But the smaller inter-pcb ribbon connectors can slide off the board connectors very easily. Any chance that this, or a connection issue involving these can be causing your problems?
    I haven't looked at the docs you posted yet, but I'm going to take a look now...

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by SoulFetish View Post
    I just worked on one of these amps for the first time about 2-3 weeks ago. One thing I did appreciate was they built the chassis around the huge transformers (mounted directly to the cabinet, making it easier to remove the chassis) and service the boards.
    But, I didn't like the way the modular/molex connectors are fit and secured in this amp. They easily become dislodged, and the female connectors do not seem to have much strength in the way they mounted them.
    But the smaller inter-pcb ribbon connectors can slide off the board connectors very easily. Any chance that this, or a connection issue involving these can be causing your problems?
    I haven't looked at the docs you posted yet, but I'm going to take a look now...
    My last venture back into the chassis was going thru and reseating ALL of the interconnects, including the power tube PCB side of the ribbon cable from the Autobias PCB mounted to the inside surface of the back wall. No difference found.

    I haven't yet tried to position the T-shaped chassis in a way to allow powering up the amp, to verify all of the appropriate voltages ARE present on the AutoBias PCB. I have no reason to doubt all the system voltages are nominal, since the amp does work, and it goes thru the initial scrutiny of the power tubes during warm-up, and goes thru the initial biasing to arrive at a stable condition, where the six LED's are all GRN and no longer flashing. BUT, unlike the other Super Bassman I've worked on, using the UP/DOWN arrow keys of it's display to change the plate current level.....that function isn't working, while pressing both arrow keys together WILL invoke reset to factory default bias.

    Switching to Stand By, then later back to Operate, the LED's....all GRN, continue to flash at the nominal 4X a second rate, AND, it continues to ignore changing the bias level manually. The single small LED associated with the UP/DOWN arrow keys work, but, monitoring the current thru the tubes (with my cathode resistor monitor cables attached), NO change in current occurs.

    I looked thru Fender's Forum for any clues to this Auto-Bias issue, and, of course, nothing found there. I did just post on their Bass Amp Forum the basic details of this problem, but haven't yet even seen it appear (has to be approved by their moderator). I'm thinking I need to order another AutoBias PCB assy and install it. It appears ALL of the biasing circuit is on that small board, though I haven't confirmed that with DMM probing, which I'll do this morning.

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    Last edited by nevetslab; 11-11-2019 at 06:58 PM.
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    Supporting Member nevetslab's Avatar
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    New Findings! I took the previous pulled Sovtek 6550 power tubes from the amp, less the one that failed (frosty white inside the bottle), and it's partner from V7/V8 positions, and installed those in place of the fresh matched Sextet of KT-88's. Powered up from cold start, with the Auto-Bias circuit noting the absence of the middle pair of tubes, now marked RED. Switched out of S/B, and it biased up this set of 4 6550's to about 30 mA per tube, LED's now steady GRN where there are tubes, and RED where the missing tubes are. I switched to Standby, then shortly after, switched back out to Operate. It trimmed up the bias and again gave steady reading, instead of the continual flashing as I've been seeing with the KT-88's. First time I've seen it work properly.

    I used the Down-Arrow key and lowered the plate current to watch and see if it made the adjustment (cathode sense cables still in place, monitored by DMM). It lowered the plate current. Reset it to nominal GRN setting, and it re-balanced, then gave steady reading as it should.

    Well, I'll be buggered. Something in the internal code of the Auto-Bias computer doesn't want to work with the KT-88 power tubes.

    I had also forgotten about the mechanical spacing in the layout of their chassis/tube PCB, which doesn't provide enough clearance for the larger diameter of the KT-88 power tubes. I had ordered the new set without having read my previous notes pertaining to the potential breakage of KT-88's in this chassis!! OPPS!

    So, I have a solution.......I'm waiting for a new set of KT-88/6550 Hold-Down clamps coming in from Antique, and an SVT-CL in our inventory that has those late-generation base clamps. I had to replace one of the 6550 power tubes from that set when I needed a known working sextet of power tubes to check a different SVT-CL, and had replaced the broken tube that resulted from rough handling by our staff......I'll remove THAT working set of 6550's, swap them out with the new matched Sextet of KT-88's, and install the shock-mounted hold-down clamps in the process. That should restore this Super Bassman to normal Auto-Bias behavior, since it seems to be able to monitor and adjust 6550's. Learn something new every day!

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    Don't forget the joker g1's Avatar
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    I'd suspect something funny with the JJ KT88 pin out compared to 6550, but I see eurotubes including them in a tube kit for this amp.
    I do see the JJ datasheet a little funny in that they call out pin1 as g3 but show no connection.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by g1 View Post
    I'd suspect something funny with the JJ KT88 pin out compared to 6550, but I see eurotubes including them in a tube kit for this amp.
    I do see the JJ datasheet a little funny in that they call out pin1 as g3 but show no connection.
    The Super Bassman is using Pin 8 for their Cathode connection. I'll have to look to see if Pin 1 is used for anything.

    After I posted my new findings yesterday afternoon, I found after putting in a used set of Sovtek 6550's, the Auto-Bias circuit works if there's only 4 power tubes installed, but with six tubes, I'm right back to where this started.....entering and leaving Standby after initial biasing.....the circuit just keeps flashing, and never stops, and, again, the UP/DOWN arrows discretely will NOT change the current thru the power tubes.

    After having forgotten about the size issues with the J/J KT88 power tubes, I've removed them from the amp since the likelihood of breakage is very high with the front row/back row of tubes withing 0.050" of each other.

    I had posted this issue on Fender's Bass Amp forum yesterday, never saw the post published (only registered on Saturday), so posted again this morning. Though having browsed their forum, I don't find real technical issues discussed there as we have here on MEF.

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    This amp is getting to be like a bad penny. I gave up, put the whole thing back together, powered back up to give a listen, assuming all still worked. I hadn't yet mounted the rear fan panel, so I plugged the tiny 12V Fan connector into the chassis connection, heard a little snit, and no FAN running. GREAT! Pulled the chassis back out, expecting to find one of the three internal clip-mounted fuses open. All fuses are intact. It couldn't have blown the 10A Mains fuse. NO, that's ok, as are the cathode fuses, having nothing to do with any of this. So, I better check the fan. Tiny female connector, so dug thru my lead trimmings to find the right size lead to make a pair of test points, found and made those, then dialed up 12VDC to try this Sunon 12VDC fan. Fan spun up just fine.

    I DO NOT LIKE WHERE THAT is POINTING!! I don't have service documentation for this amp, and, this amp isn't mechanically designed to be service while powered up. All I wanted to do was to get a new set of power tubes installed and send it back to inventory.................sigh.

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    I have had little luck on working on any of these amps with the auto-bias circuit.

    The first time a few years ago a Super Sonic came in with a bad bias unit. Luckily it was only a bad connection between the keypad/display board and the bias board.

    The problem that I have seen more than once, is the circuit will start out working fine, and then suddenly lock up in a mode where it will send all of the bias voltage to the output tubes, something like -80 volts, which will shut off the power amp. Fender recommends changing the driver tube as a fix for all problems with the auto bias circuit.

    Good luck with it.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by 52 Bill View Post
    I have had little luck on working on any of these amps with the auto-bias circuit.

    The first time a few years ago a Super Sonic came in with a bad bias unit. Luckily it was only a bad connection between the keypad/display board and the bias board.

    The problem that I have seen more than once, is the circuit will start out working fine, and then suddenly lock up in a mode where it will send all of the bias voltage to the output tubes, something like -80 volts, which will shut off the power amp. Fender recommends changing the driver tube as a fix for all problems with the auto bias circuit.

    Good luck with it.
    After I got the fan circuit working again, when I moved on to see if everything was still working, plugged in signal, and immediately it was horribly distorting, and pulling around 900W from the wall. Shut it down, powered back up, this time plugging signal into the Power Amp input jack, and got the same problem. Shut it down, and replaced both the driver tube and the 12AX7 preamp tube for the power amp, and that cured that distortion problem.

    It didn't alter the problem with six 6550 power tubes installed, and for Auto-Bias to continue monitoring & tweaking bias once Standby has been entered at any time. Like you, my success with this Auto-Bias circuit is shameful. I haven't yet encountered that 'Auto-Shutdown, turning off the bias all together. Now waiting for Fender to send another Autobias PCB assy. Turns out these Super Bassman amps in our inventory DO belong to Fender, not CenterStaging. So far, the moderator on Fender's Forum appears unwilling to acknowledge my posts on this problem. What a surprise.

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    Member mhuss's Avatar
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    I'd suspect that the KT88 needs a slightly different bias voltage than the 6550, and it's just on or past the edge of the min or max range for the auto bias circuit.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by mhuss View Post
    I'd suspect that the KT88 needs a slightly different bias voltage than the 6550, and it's just on or past the edge of the min or max range for the auto bias circuit.
    Disregarding the size issue due to the larger diameter, seriously risking tube breakage in the Super Bassman, the KT-88's did bias up just fine, and incremented up from their starting point much like the 6550's do. So, that didn't appear to be an issue. The main issue yet to be resolved is the failure of the current Auto-Bias PCB assemblies I have to continually monitor/adjust the bias on the tubes, after Standby has been entered and left (after initial biasing).

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