Fender Super Bassman Auto-Bias Circuit Issue

[Ferengo]

This amp, which I've never had in the shop until now, ended up having an open fuse for V7/V8 screen supply (T100mA). Only one of those tubes (V7) was bad, showing noticeable darkened soot on the plate. I then went thru a lot of 6550 Sovtek pulls to find one that matched the nominal plate/screen current of the remaining tubes, as measured on my Cathode Resistor Sense cables tacked in for screening all of the tubes. I then let this amp burn in, plate current holding, AND, was working every time I went to S/B and back out, as well as being able to manually adjust the plate current levels.

After turning it off, and before removing the cathode sense cables, I removed that Autobias PCB assy, which was a Rev 1.3, and installed a different Rev 1.3 that I had removed from the other Super Bassman that still won't monitor the tubes once Standby has been entered and left. This replacement PCB also works fine in this amp, as far as the Autobiasing goes. Now, when I applied signal to the Power Amp Input jack, something else was wrong with the amp, it now pulling 700-850W from the wall, and remained there after removing the test signal (burst pink noise). I replaced the Driver tube, though that wasn't it. Replaced the 12AX7 on the power amp chassis, and that cured it. So now, this amp is working normal, and watching the plate current levels thru any of the tubes thru the sense cables all track each other.

SO....this DOES point to the problem NOT being in the Autobias PCB assembly on the other Super Bassman....where this thread began. That pursuit will follow shortly.....no idea what nor where the problem is. What fun awaits.

I'm curious to know if you managed to find the cause? I currently have a Bassman that after being in stand by will run fine for a minute or two then no sound. Won't even switch channels. Turn it off then back on and it's good to go (not going back into stand by). It's had all the boards checked over, Auto bias replaced and a full new set of valves all over. Same issue. I'm wondering since it appears to be related to the stand by issue that if you solved that then it might be a place for me to also look at?

[Enzo]

Hi Ferengo, may I suggest you start a new thread for your amp so we won't get confused about things like whose voltage readings are whose, and like that.

[nevetslab]

I'm curious to know if you managed to find the cause? I currently have a Bassman that after being in stand by will run fine for a minute or two then no sound. Won't even switch channels. Turn it off then back on and it's good to go (not going back into stand by). It's had all the boards checked over, Auto bias replaced and a full new set of valves all over. Same issue. I'm wondering since it appears to be related to the stand by issue that if you solved that then it might be a place for me to also look at?

That Super Bassman I had started this thread about was sent to another shop in Burbank...Valley Sound Music Technologies, and I had spoken with Stretch, the owner, about that amp and the issues I was having with it. He's worked on a number of them, but never had encountered that problem. I never heard what the outcome was, as it was being handled by our then-Guitar Dept Mgr, who has since left the company. I'll have to do a search on that Asset number with our systems manager to see if it ever had a conclusion or was replaced. I believe that was part of Fender's inventory that we rent out. A lot of the gear we have is supplied to us from Amp mfgrs for our visibility with major clients. We both concluded it WASN'T the Auto Bias PCB assy, as I had swapped it out with a new one, and think he may also have done the same.

Meanwhile, Chuch, the Guitar Dept mgr at that time ended up with a free giveaway of a White Fender Super Bassman that nobody had been able to sort out. That amp is still sitting at the bottom of a short stack of Ampeg SVT-CL amps, waiting for me to tackle it. No clue what it's ailments are yet. And, another Super Bassman was sent over from our Guitar Dept for ailments yet unknown. Now that I'm back in my small original shop, where there are NO CenterStaging staff members around, I have to wait until there's someone gullible enough to lend a hand and heft it up onto my check-out bench from the road case. At 71 yrs old, just too bloody heavy to tempt injury in lifting it up from the floor.

I'll follow up on these latest two Super Bassman amps.

I still don't have any Service Documentation from Fender. Closest I have is from their Super Sonic 100, having the same Auto-Bias Computer module. Just what everybody wants in their amp....a bloody computer to 'make things better'!

[nevetslab]

I just heard back from our Equipment Manager here at CenterStaging, LLC on the outcome of that Super Bassman I sent to Stretch at Valley Sound Music Technologies:

Stretch was unable to fix it so he sent it to Fender for a replacement unit.

Here's the notes:

1. Initial test on bench confirms auto bias function rarely stabilizes
2. Remove rear cover for access – note customer output tubes evidencing overheating – remove and install new matched sextet 6550 output tubes – no change to auto bias status
3. Substitute drive/phase splitter tubes in output stage – no change
4. Suspect auto bias PCB likely defective – order replacement. Upon receipt of part – remove original and replace. Re-test function – find some improvement when unit first switched out of standby. However, if put back into standby status (or speaker mute switch activated) and then returned to operational status, auto bias function does not stabilize without a reset
5. Re-test unit with a second set of 6550 tubes – no change
6. Contact Fender Tech, over a number of days, perform multiple tests and measurements, conferring - no conclusive results obtained
7. Per Fender suggestion, order and replace ribbon cable between main circuit and auto bias PCB, Install – no change
8. After further discussions with manufacturer – unit deemed uneconomical to repair and to be replaced
9. Upon receipt of new unit – perform full QC to verify all functions satisfactory – all OK. Original unit and parts to be returned to Fender

Meanwhile, prior to emailing this inquiry to our Equipment manager, I tried to log onto the Fender Forum to see if there had been any development with this Auto-Bias issue. My password was rejected, so I tried to request what it was (knowing it's the same user and P/W as I use logging in here), and was then informed my email address is not valid. I next went to re-register, and was denied access by their administrator. I guess I'm no longer welcome by Fender. Just sent their administrator an email to find out WTF that is all about. Heaven forbid an engineer/technician post anything wrong with their technology to alert the world.

[Ferengo]

I just heard back from our Equipment Manager here at CenterStaging, LLC on the outcome of that Super Bassman I sent to Stretch at Valley Sound Music Technologies:

Stretch was unable to fix it so he sent it to Fender for a replacement unit.

Here's the notes:

1. Initial test on bench confirms auto bias function rarely stabilizes
2. Remove rear cover for access – note customer output tubes evidencing overheating – remove and install new matched sextet 6550 output tubes – no change to auto bias status
3. Substitute drive/phase splitter tubes in output stage – no change
4. Suspect auto bias PCB likely defective – order replacement. Upon receipt of part – remove original and replace. Re-test function – find some improvement when unit first switched out of standby. However, if put back into standby status (or speaker mute switch activated) and then returned to operational status, auto bias function does not stabilize without a reset
5. Re-test unit with a second set of 6550 tubes – no change
6. Contact Fender Tech, over a number of days, perform multiple tests and measurements, conferring - no conclusive results obtained
7. Per Fender suggestion, order and replace ribbon cable between main circuit and auto bias PCB, Install – no change
8. After further discussions with manufacturer – unit deemed uneconomical to repair and to be replaced
9. Upon receipt of new unit – perform full QC to verify all functions satisfactory – all OK. Original unit and parts to be returned to Fender

Meanwhile, prior to emailing this inquiry to our Equipment manager, I tried to log onto the Fender Forum to see if there had been any development with this Auto-Bias issue. My password was rejected, so I tried to request what it was (knowing it's the same user and P/W as I use logging in here), and was then informed my email address is not valid. I next went to re-register, and was denied access by their administrator. I guess I'm no longer welcome by Fender. Just sent their administrator an email to find out WTF that is all about. Heaven forbid an engineer/technician post anything wrong with their technology to alert the world.

Thanks for the replies.

I'll start a thread once I gather the information from th local store regarding what's been tried thus far. I have requested this earlier today.

I figured since you mentioned it was linked to stand by then it might have been worth asking you at least what the conclusion was as a place to start with mine (not that I have the same issue just it appears to possibly be linked to stand by)

You're right though, computers tend to make things like this so much harder.

Once I get the info I'll start a thread. Maybe someone can help.

Thanks again.

[Enzo]

May I add: it MAY be useful to know what has already been done to an amp. But a good technician never relies on that. "Oh, we checked all the solder connections (or caps, or resistors or whatever)" does not mean you can assume they were OK. What if they overlooked something? Or skipped a few parts because they were inaccessible? Maybe they were on the wrong pin or the wrong IC when they made a reading? They installed all new caps? Maybe one was bad right out of the box. You never know. If they put a new transformer in it, I want to know that, but I won't assume it is good. And I won't assume it is the correct part without checking.

The best course of action is to start from scratch. Isolate the problem. Apply a steady signal of some sort (I just use the radio) and the sound dies after a brief run? SO leave it that way. Are all power supply rails present? Find out where signal is and is not within the amp. If you have FX loops or pre out and power amp in jacks, use those to determine how far sound gets.

[Ferengo]

May I add: it MAY be useful to know what has already been done to an amp. But a good technician never relies on that. "Oh, we checked all the solder connections (or caps, or resistors or whatever)" does not mean you can assume they were OK. What if they overlooked something? Or skipped a few parts because they were inaccessible? Maybe they were on the wrong pin or the wrong IC when they made a reading? They installed all new caps? Maybe one was bad right out of the box. You never know. If they put a new transformer in it, I want to know that, but I won't assume it is good. And I won't assume it is the correct part without checking.

The best course of action is to start from scratch. Isolate the problem. Apply a steady signal of some sort (I just use the radio) and the sound dies after a brief run? SO leave it that way. Are all power supply rails present? Find out where signal is and is not within the amp. If you have FX loops or pre out and power amp in jacks, use those to determine how far sound gets.

That's actually really good advice. I'll start a new thread once I pick up with Fender. I spoke with them and they're going to help me directly with this. To start from scratch will be the best way to go. Thanks.

[nevetslab]

Another Super Bassman was sent over from our Guitar Dept. No note on it, so another mystery novel. Don't tell me, let me guess. Auto-Bias doesn't stop blinking. Bells and lights ring and flash! I win the booby prize for guessing the right answer.

The amp does work....I can plug bass into it, works like a normal amp. Just doesn't pay any attention to the computer in it's tiny brain. LED's just keep flashing. I can push the UP/DOWN arrows and make the LED's go from GRN (normal), to RED (HOT) or to AMBER (COLD), but, with my bias probes soldered onto the 1 ohm cathode resistors inside the power amp chassis, nothing changes.

The nominal cathode currents I measured are:

V5 29.6mA
V6 26.0mA
V7 25.0mA
V8 27.8mA
V9 30.3mA
V10 33.3mA

That averages out to: V5-V7-V9 Avg 28.3mA, and V6-V8-V10 Avg 29.0mA So, it is a wider spread than I'm used to on Ampeg SVT-CL's. If the Auto-Bias computer worked, it would probably balance these tubes out. The last time I worked on this amp was October 2020, when I replaced the ribbon cable between the power tube board and the Augo-Bias board. That restored the computer. Back then.

On this amp today, I had reseated all of the cables inside, including that ribbon cable. No difference afterwards.

I also discovered why Fender's Forum rejected my attempt to log in. On March 20, 2020, they terminated their forums. That date was when California Shut DOWN the industries due to the Covid 19 Pandemic. So, no more Technical Forum with Fender.

I have one remaining Auto-Bias board, acquired from the amp that started this thread back in November 2019. It's the 'pull' from that amp that didn't care which or what 'working' Auto-Bias board you install....it just wouldn't work as designed. I have one more Fender Super Bassman sitting on the floor, that belongs to CenterStaging's previous Guitar Dept mgr...a freebe from Fender since nobody in their technical department could figure it out.

Well, I do need labor hours to rack up, though it's only being applied to the 100 hrs of labor owed for 5 months of shop space. Can't get to billable hours with CenterStaging until that 'debt' is zero'd out. I never stopped to put together any test fixtures for these Super Bassman amps. As such, they're a PITA to work on. Not that SVT-CL's are fun, but they can be managed.

[nevetslab]

This morning, starting again with this amp whose Auto-Bias board wouldn't stop flashing, I manged to find my two remaining Auto-Bias PCB's pulled from the first amp that went thru this routine, and swapping out it's board with a new one Fender sent me, that amp finally was sent to Stretch at Valley Sound Music Technologies over in Burbank, CA, and he too never succeeded in curing it. You no doubt read the outcome from that...amp ended up going back to Fender and replaced with another one.

I removed the Auto-Bias board from this particular amp, and swapped it with the latest board I had, it having a Rev code V1.3.1.2017, and removed this board not responding V1.20.6.2014. After letting it warm up for the prescribed 1 minute minimum, I switched out of Standby, and never did see it go thru the pairs bias adjustment I'm used to seeing (when all is right), but it did finally stop flashing after under a minute. The balance wasn't any better than before, and actually now running higher idle current. Switched to Standby, waited around a minute, then back out of Standby. It never stopped flashing. Fender's chart in the owner's manual cites that behavior as the power tubes are tired, and need to be replaced. I doubt it, not finding repeatable behavior with it.

I found I hadn't taken any internal photos showing the simple two cathode sense cables I've been tack-soldering across the cathode resistors and Ground so I can measure the individual tube's cathode current at idle, as well as under load:



After looking at the only schematics I have on their Auto-Bias circuit, for the Super Sonic 100, and puzzling over the absence of more than two PNP xstrs to feed the six power tubes their required bias voltage as dictated by the sense computer, I started thinking about what would it take to rip this stupid circuit out of the chassis and replace it with a manual bias adjust circuit. There are six sense lines off of the six cathode resistors that go out the ribbon cable to the Auto Bias board. And, looking at the pin-out on the Bias Board's ribbon cable connector, they do show four of those lines sharing a Bias Source with Cathode Sense. On the schematic for the output stage, there are steering diodes to prevent the negative voltage via 220k source resistors from seeing the nominal 33mV cathode voltage of the tube (and likewise preventing that voltage from appearing at the grids), seems like the basic needs are provided on that ribbon cable.

This amp, as is, just isn't going to fly, and I never have figured out how to correct their circuits when they behave this way. Meanwhile, the other Super Bassman that's been in my possession since April of this year no doubt has similar problems. As it doesn't work as is, perhaps this can be the Guiney Pig for a non-computer controlled bias circuit. I'll start a new post on this subject shortly.

[clintronics]

Any schematics on this amp yet? I got one of these sent to me from another tech that couldnt get it to work.

[clintronics]

Output section looks like the Bassman300srvc2

[clintronics]

Turned out a pin was pushed in on the OT secondary connector. Once that was corrected the auto bias started working 100%

[Masherbike]

Hi, I have just joined this forum and I have the same super bassman flashing green light problem. Can you explain exactly which pin was pushed in. What is the OT secondary connector? Sorry I'm not an amp tech. Cheers, Andy