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Post subject: Super Bassman Auto-Bias Circuit issue
Posted: Tue Nov 12, 2019 10:50 am
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I have a Super Bassman on the bench with Auto-Bias issues. I just replaced the power tubes with a fresh matched set of Sovtek 6550’s (Groove Tube), and all is well from the initial power-up. The Auto-Bias during warm-up is scanning the six tubes repeatedly at roughly 1.5 sec per full cycle. After a few minutes, I switch out of Standby, and press both UP/DOWN Arrows to initiate Factory Default Bias. That goes normal, with the current starting around 16mA thru the cathode resistors, GRN LED’s all flashing at roughly 4X a second, increases to around 22mA and yields steady reading for a moment, then the GRN LED’s flash some more as the current increases to roughtly 26mA and stop flashing. AC Mains is 2.4A/238W @ 120VAC. I have a set of cables tacked on to the cathode resistors of each tube (upper & lower) so I can monitor the plate/screen current of each tube during this procedure.

If I switch to Standby for any duration of time, then switch back out, the GRN LED’s now flash at the 4X per second, and never stop flashing. Pressing either UP or DOWN arrow to shift the bias manually doesn’t change anything. Only pressing both arrow buttons will respond, resetting the bias to Factory Default setting again.

If I remove a pair of tubes (V7 & V8), then switching out of standby after initial bias WILL stop flashing, AND I can adjust the UP or DOWN arrows to change the plate current, and can see it happen.

The amp had V1.3 Auto-Bias PCB installed. I’ve swapped that out with a V1.2 PCB that I had on hand from a previous Auto-Bias problem a couple years ago.

I’ve no idea why this won’t respond properly after switching to Standby, than back on again, as one would do between sets on stage. The amp works fine. I’ve exercised all the interface connectors inside the power amp chassis (after discharging the supplies, of course), and everything is firmly seated. No bad semiconductors found. And, both Auto-Bias PCB’s are doing the same thing with this non-responsive monitoring of bias after having switched into and out of Standby, after initial bias was set.

I don’t get this behavior on the other Super Bassman amps in our rental inventory.

Any suggestions?


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Post subject: Re: Super Bassman Auto-Bias Circuit issue
Posted: Thu Nov 14, 2019 3:02 am
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Location: Province de Québec, Canada
You may have more answer in Modern Guitar Amplifiers sub forum

I hope this will help you;


Bassman 100T Automatic Bias how it should work properly:

https://www.youtube.com/watch?v=pmg3VplRT0I
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https://music-electronics-forum.com/sho ... hp?t=39663

Super Sonic 100 Auto Bias replacement ; https://forums.fender.com/viewtopic.php?f=13&t=99889

Owner manual ; https://www.fmicassets.com/Damroot/Orig ... al_all.pdf


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Post subject: Re: Super Bassman Auto-Bias Circuit issue
Posted: Thu Nov 14, 2019 10:37 am
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Joined: Sat Nov 09, 2019 6:04 pm
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stratele52 wrote:
You may have more answer in Modern Guitar Amplifiers sub forum

I hope this will help you;


Bassman 100T Automatic Bias how it should work properly:

https://www.youtube.com/watch?v=pmg3VplRT0I
________________________

https://music-electronics-forum.com/sho ... hp?t=39663

This post on Music Electronics Forum was posted by me.

Super Sonic 100 Auto Bias replacement ; https://forums.fender.com/viewtopic.php?f=13&t=99889

Owner manual ; https://www.fmicassets.com/Damroot/Orig ... al_all.pdf


Thus far, none of these have shed any light on the effect switching back to Standby, and later back out of Standby, to find the LED's flashing all GRN, never stopping, so no monitoring and automatic adjustment of the plate current in the power tubes during operation, which is normally occurring with a properly working circuit.

Presently waiting arrival of a replacement Autobias PCB assembly, since ALL of the biasing operation is taking place on that module. We'll see. I did find it odd when only four power tubes installed, the monitoring DOES take place, AND you can alter the NORMAL bias with manually adjusting the UP/DOWN Arrows. It just DOES NOT do that with all six power tubes installed. Difference in plate current with all six tubes installed.......2-3 mA on one tube, the rest within 1mA of each other.


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Post subject: Re: Super Bassman Auto-Bias Circuit issue
Posted: Thu Nov 14, 2019 11:34 am
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TimAudio a old forum member is a great tech folowing Guitar amp sub forum. He talk about some work he do on a SS100 Auto bias circuit replacement, a link I wrote.
IMO Bassman could be also a guitar amp.

If you order complet auto -bias circuit brd issue must be fix.
It always fun to find wich component is wrong.

I never work on this amp


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Post subject: Re: Super Bassman Auto-Bias Circuit issue
Posted: Thu Nov 14, 2019 1:13 pm
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Joined: Sat Nov 09, 2019 6:04 pm
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Location: Burbank, CA
stratele52 wrote:
TimAudio a old forum member is a great tech folowing Guitar amp sub forum. He talk about some work he do on a SS100 Auto bias circuit replacement, a link I wrote.
IMO Bassman could be also a guitar amp.

If you order complet auto -bias circuit brd issue must be fix.
It always fun to find wich component is wrong.

I never work on this amp


I will advise when the latest Autobias PCB assembly arrives. So far, both V1.2 and V1.3 are behaving with the same problem.

I had found two posts on the MusicElectronicsFourm pertaining to the Autobias circuit:

Reply from gingertube 2-28-19 to Randall:
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.

Then, from Randall, a few posts later:

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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