Fender Super Bassman Overdrive Ch audible Oscillation via Gain Control

[nevetslab]

I have this Super Bassman back together, after going thru three different Auto Bias PCB assemblies, the last (and oldest Version code from 2012) installed, it's able to bias up the tubes, adjust bias by changing the range from Normal thru Cool, thru Warm and reset, even settles after coming out of Standby. Will wonders ever cease. So this morning, I removed the power amp chassis again, unsoldering the power tube cathode sense cables, then put the chassis back into place and fastened it down. Powered up the amp, to give a listen again. This time, I found I had distortion that I hadn't heard before, having plugged in thru the Preamp's Normal Ch. I unplugged from there and into the Power Amp input, and also found similar distortion when I increased my signal, so swapped out the driver tube. Some difference, but still distorting. Replaced the Preamp tube of the Power amp circuit, and that seemed to cure it. So, left that tube in place, turned the chassis around so I could see all of the preamp controls. Plugged my bass in, and all sounds solid and fine in the Normal Ch. So, switched to the Overdrive Channel. Turning up the Gain control, I am now getting howling-type oscillation, and the pitch is a function of the Gain Control setting, with the Volume and Balance controls only controlling how much of it you hear. Seems to oscillate above half-way up on the control.

I don't have any service documentation on these Super Bassman amps. Right now, I have my small dual 120mm fan box blowing thru the front to cool off the power tubes as there are two cabinet roof screws holding the back of the preamp in place. The rest are up front, thru the top of the cabinet.

Anyone have experience with this issue on the Super Bassman?

Block Diagram is listed in this Owner's Manual Pg 11.

SuperBassman_OM_Rrev-A.pdf





I've replaced V2 tube, associated with the Overdrive circuits, no difference. Restored that tube, then replaced V1...Normal Ch circuits, with no difference either. Now pulling this preamp assembly out to scratch my head and ponder.

[nevetslab]

This morning, still waiting for TL072 BiFET op amps to arrive (seems to be a shortage of those with delivery times in 2022 posted, along with other semi's), I removed V2 from the Preamp and installed a vintage 12AY7 tube in place of the 12AX7 tube. Audible oscillation/howling gone. Pulled that, plugged in a new ECC83S tube, oscillation back. So, drove down the street to our Guitar Dept to fetch a new 12AY7 and plugged that in. Oscillation gone again. While this tube has less gain than a 12AX7, and probably does affect the circuit design for that extra gain of the tube, it does yield a working amp. Lacking the service doc's on the Preamp and amp sections of the Super Bassman, I think I'll overlook that aspect and put this amp back into service.

Meanwhile, I'll see if we can't get a replacement Preamp assembly from Fender, and send this one back to them.

[nevetslab]

I just love Corporate America. Our Product Manager who oversees all of our rental departments and inventory control, etc has contacted Fender about obtaining a replacement new/working Preamp Chassis assembly for their Fender Super Bassman. Only item in the product Bill of Materials in the Final Product Assembly. Shouldn't even have a separate P/N for Line Voltage, since that is set by the Power Amp chassis. Preamp just plugs into that, and it's the ONLY one in their Service Manual as well as the Engineering Dept docs that list everything in every product in the associated P/N tree. Fender's front line defense stopping us at this moment is requesting the Part Number for the Complete Preamp Chassis assembly.

I looked thru several Service Manuals I have to see how they list items. Fender Twin Reverb 65 Reissue, Hot Rod DeVille, Princeton Reverb. Only the Princeton Reverb had an End Item heading in their Part Number system that gave P/N's for the sub-assemblies that integrated into the final product....Chassis assy (p/n' per line voltage), Cabinet, speaker baffle assy, handle, corner hardware, etc. The 65 Reissue Twin Reverb did give Chassis Assy complete/line voltage. There wasn't one in the Hot Rod DeVille Svc Manual. Each manual was different, so obviously Fender changes these details over time, and I don't have any Service Manual for current products, other than the 65 Twin Reverb, Hot Rod Deville and Princeton Reverb, and they are different from each other with their structure.

Now, as this amp needing a replacement Preamp Chassis assembly (complete, working) to restore THEIR amplifier to working status without Howling when you turn up the Gain Control in Overdrive....seems like a simple request, since we (CenterStaging) have qualified technical service on site.