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Having another Fender Super Bassman in the shop with the traditional non-functioning Auto-Bias circuit, even though the amp does power up and work, you can't adjust the bias on it, and like others I've been unable to cure, once you enter Standby (after it had powered up, came out of Standby and settled, while NOT showing the normal bias adjust routine before it stops flashing), switching back into Standby, and later back out, the LED's never stop flashing. I've been tack-soldering two test cables across the cathode resistors of the output stage and let them pass under the rear panel to plug my DMM probes into (power amp chassis not fastened...only sitting while all connected up).
I'm now toying with the idea of modifying the chassis on another Super Bassman I have whose owner was a former CenterStaging Guitar Dept manager/bass player, who inherited that amp from Fender as their technical staff could never figure out what was wrong with it. I haven't yet had it opened up to see what I find, but no doubt it too has this same misbehaving Auto-Bias nonsense. So, now looking at the mechanics of what I have to work with. Here's the schematics I have of a Fender Super Sonic 100, which uses this same Auto-Bias board to control its' power tubes.
Super-Sonic_100_2011_schematic_Rev-B.pdf
Looking at the diagram for the Auto-Bias circuit board, as well as that of the output stage of that amp, the 20 conductor ribbon cable between the power amp PCB on this Super Bassman and the Auto-Bias board, there's only two bias source transistors installed (Q1, Q2...MPSW92 PNP's), along with four discrete Cathode Sense lines for V1, V2, V3 and V4 (as called out on the Auto-Bias schematic) but for V5 and V6, it's shared with bias voltage for V3 and V4. On the schematic for the power tube circuit, there are steering diodes to prevent the cathode voltage (33mVDC nominal) from connecting to the grid of the power tubes. The Bias voltage (-51VDC nominal), comes in via 220k resistors. So, it looks like the basics are present on the ribbon cable of this Power Amp PCB.
I removed that Auto-Bias board for closer inspection, as well as took some close-up photos of the chassis with it out of the way, and a close-up of the rear panel view of the Auto-Bias display/adjustment soft-key buttons:
Then, the Auto-Bias board itself:
The LED's we see thru the rear panel soft 'keypad' assembly can be pried off the rear panel. On the back side of the PCB, we see the six SMD bi-color LED's that make up the six power tube status indicators, and three vertical SMD LED's that provide the ' RED, GRN and AMBER LED colors for WARM, NORMAL and COOL bias status. So, there are nine small holes thru the chassis for the light source to pass thru to this soft panel. I haven't stopped to look at what the spacing is. I do use Pin Jacks in some of my test fixtures, but, those are panel mount with a sizeable mechanical foot print. Not sure at this point whether any of the existing holes in place where this assembly resides could be used. If I used just two bias pots, then perhaps I could have their centers common with two of those six holes. I'd want to have insulated terminals to accommodate standard DMM test lead probes so one can measure across the cathode resistors relative to ground....so seven test points needed there.
I do like the RED/GRN Window Comparator Bias LED circuit used in the Ampeg SVT-CL. So, that's a possibility. Can I come up with a PCB having two reliable bias pots, seven insulated test points and two pairs of RED/GRN Window Comparator LED meters that would mount in the same space as this Auto-Bias board? Possibly. I'm not restricted to the height of this existing Auto-Bias board....only the internal height constraint inside the chassis.
If I can get all this to fit and work, might be a small market for a retro-fit package to sell and basically plug-into the existing Auto-Bias ribbon cable. User or Shop mechanical work required, so some skill level required to make the conversion work.
Any idea out there?
I'm now toying with the idea of modifying the chassis on another Super Bassman I have whose owner was a former CenterStaging Guitar Dept manager/bass player, who inherited that amp from Fender as their technical staff could never figure out what was wrong with it. I haven't yet had it opened up to see what I find, but no doubt it too has this same misbehaving Auto-Bias nonsense. So, now looking at the mechanics of what I have to work with. Here's the schematics I have of a Fender Super Sonic 100, which uses this same Auto-Bias board to control its' power tubes.
Super-Sonic_100_2011_schematic_Rev-B.pdf
Looking at the diagram for the Auto-Bias circuit board, as well as that of the output stage of that amp, the 20 conductor ribbon cable between the power amp PCB on this Super Bassman and the Auto-Bias board, there's only two bias source transistors installed (Q1, Q2...MPSW92 PNP's), along with four discrete Cathode Sense lines for V1, V2, V3 and V4 (as called out on the Auto-Bias schematic) but for V5 and V6, it's shared with bias voltage for V3 and V4. On the schematic for the power tube circuit, there are steering diodes to prevent the cathode voltage (33mVDC nominal) from connecting to the grid of the power tubes. The Bias voltage (-51VDC nominal), comes in via 220k resistors. So, it looks like the basics are present on the ribbon cable of this Power Amp PCB.
I removed that Auto-Bias board for closer inspection, as well as took some close-up photos of the chassis with it out of the way, and a close-up of the rear panel view of the Auto-Bias display/adjustment soft-key buttons:
Then, the Auto-Bias board itself:
The LED's we see thru the rear panel soft 'keypad' assembly can be pried off the rear panel. On the back side of the PCB, we see the six SMD bi-color LED's that make up the six power tube status indicators, and three vertical SMD LED's that provide the ' RED, GRN and AMBER LED colors for WARM, NORMAL and COOL bias status. So, there are nine small holes thru the chassis for the light source to pass thru to this soft panel. I haven't stopped to look at what the spacing is. I do use Pin Jacks in some of my test fixtures, but, those are panel mount with a sizeable mechanical foot print. Not sure at this point whether any of the existing holes in place where this assembly resides could be used. If I used just two bias pots, then perhaps I could have their centers common with two of those six holes. I'd want to have insulated terminals to accommodate standard DMM test lead probes so one can measure across the cathode resistors relative to ground....so seven test points needed there.
I do like the RED/GRN Window Comparator Bias LED circuit used in the Ampeg SVT-CL. So, that's a possibility. Can I come up with a PCB having two reliable bias pots, seven insulated test points and two pairs of RED/GRN Window Comparator LED meters that would mount in the same space as this Auto-Bias board? Possibly. I'm not restricted to the height of this existing Auto-Bias board....only the internal height constraint inside the chassis.
If I can get all this to fit and work, might be a small market for a retro-fit package to sell and basically plug-into the existing Auto-Bias ribbon cable. User or Shop mechanical work required, so some skill level required to make the conversion work.
Any idea out there?
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