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Post subject: Super Bassman Auto-Bias Circuit issue
Posted: Mon Nov 11, 2019 10:36 am
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I have a Super Bassman on my bench with a biasing problem. Having just replaced the sextet of matched power tubes, it powers up just fine. During warm-up, the Auto-Bias circuit can be seen scrolling thru all six tubes, with the usual single GRN LED's lighting at roughly 1.5 sec per full cycle. Switching out of Standby, it will begin with around 16mA per power tube, all six GRN LED's flashing at around 4X per second. It settles in for steady reading with around 22mA per tube initially, will cycle again as it increases the plate current, finally settling in around 24-26mA per tube, with AC mains drawing 2.3A/231W @ 120VAC. All stable and steady at this point.

Switching back to Standby for any duration of time, and later back out of Standby, the six GRN LED's will flash steadily at around 4X per second, and NEVER stop. I can also use the UP/DOWN arrows to attempt to both decrease or increase without seeing ANY change in plate/screen current. (I have a monitor cable tacked in across the 1 ohm cathode resistors of the output stage during this procedure).

I CAN push the UP/DOWN arrows together, and it will return back to Factory Default Bias. So, the arrow keys are, at least, doing that correctly. BUT, that is the only condition to which they respond.

I've already replaced the V1.3 Auto-Bias PCB assy with a spare I had on hand, V1.2, and it behaves the same way. Auto-Bias ONLY works from a cold power-up. It will NOT make adjustments once it has achieved the nominal factory default bias. The amp works just fine, and under drive conditions, I can see cathode current vary under drive on all six tubes. I just don't see the normal behavior on this amp with the two bias boards I've tried, while other Super Bassman in our Rental Inventory behave normal, and DO make the periodic adjustments during use, as well as resume that behavior coming back out of Standby.

I've re-seated all of the connections within the power amp chassis, though I haven't yet attempted to configure the power amp chassis in a way to monitor the internal power supply levels. NOT so serviceable in this design.

What am I missing here?


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Post subject: Re: Super Bassman Auto-Bias Circuit issue
Posted: Sat Nov 16, 2019 11:04 am
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You're lucky to have a spare amps and auto-bias board to compare with.
Looking at the Super bassman 300 schematic, they don't show the auto-bias in the schematic, even though the side notes refer to it. The schematic shows a manual bias adjust with an additional control for balance. I guess I'm not surprised.
But the interesting thing is that the bias circuits are also tied into the cathode circuits of V4 and V3. There are also a number of high resistance 5-watt resistors associated with the bias circuit as it starts with a B+ that needs 450 volt capacitors that have to drop to 60 volts.
I would start by replacing V3 and V4 and resoldering the large resistors.


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Post subject: Re: Super Bassman Auto-Bias Circuit issue
Posted: Tue Nov 19, 2019 10:49 am
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TimsAudio wrote:
You're lucky to have a spare amps and auto-bias board to compare with.
Looking at the Super bassman 300 schematic, they don't show the auto-bias in the schematic, even though the side notes refer to it. The schematic shows a manual bias adjust with an additional control for balance. I guess I'm not surprised.
But the interesting thing is that the bias circuits are also tied into the cathode circuits of V4 and V3. There are also a number of high resistance 5-watt resistors associated with the bias circuit as it starts with a B+ that needs 450 volt capacitors that have to drop to 60 volts.
I would start by replacing V3 and V4 and resoldering the large resistors.


I already have replaced V3 12AX7 and V4 12AT7 driver. Now, since the Autobias circuit WILL set up the bias from a cold power-up, (as well as identify any of the power tubes if not installed during warm-up scanning), it DOES incrementally adjust the bias for all six power tubes to the normal Plate/Screen current, which repeatedly showed a nominal 26mA per tube. Switching back to Standby, as one would do taking a break, then switching back out of Standby to resume playing, THAT is when these two Autobias PCB assemblies I have fail to monitor the plate current, disallow manual adjusting of the bias with the UP/DOWN arrows. It WILL allow resetting to factory default pressing both arrow buttons. What you're seeing is the GRN LED's flashing 4X a second forever, until you switch back to S/B, or turn it off.

So, we're not dealing with component issues or solder joint fractures.

NOW, Fender is insisting the amp be taking up the street to Audio Design & Service in Burbank for a second opinion. I think instead, I'll have it sent over to Stretch at Valley Sound Music Technologies in Burbank........someone whom I trust. He's also an authorized Fender service center.


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Post subject: Re: Super Bassman Auto-Bias Circuit issue
Posted: Tue Nov 19, 2019 1:29 pm
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Two authorized servicers in your town? You really are lucky. That's your best bet at this point. If you have one module that is acting up with a specific symptom, that's one thing. If you have two modules that have the same failure symptom, the odds go up astronomically that they are the root cause of the failure. Only if they are installed in one of your working amps can you pinpoint that it is or isn't the module.
That's why I think its in the amp, not the module. But not having a schematic, I can't be more specific. Good luck. Let us know what you find.


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Post subject: Re: Super Bassman Auto-Bias Circuit issue
Posted: Thu Jan 02, 2020 7:41 pm
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Hey, nevetslab2!

Did you get your Super fixed? What happened? Did you get the Auto Bias v.2 Update?

Enquiring minds, you know, are nosier than the other kind. ;-)
.


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