Seen a few of these recently and worth sharing some problems and interesting solutions that I have found.
Fender no sound or fading sound
If your amp produces no output and the lights on the back are flashing about then you might have a faulty autobias unit.
The auto bias unit fitted to these amps, as well as some Fender Bassmans I believe dynamically control each valve’s grid bias thus ensuring that your amp’s valves stay perfectly adjusted no matter what the wear level or matching level is with the tubes. The board does this by measuring the bias current and comparing it with a reference signal and adjusting the grid bias accordingly in fact it does this thousands of time a second thanks to an innovative little chip. You can adjust the control a little as well between cold, medium and hot levels which is nice.
Unfortunately this innovation seems to be a little unreliable. The two issues I have observed are either: 1), unreliable cables connecting the auto bias unit on the amp’s rear panel to the main PCB, or 2) a faulty autobias unit which seems to either permanently fail or work for a moment at start on and then fail with flashing lights and no sound. Fortunately the board fails safe and pushes the power valves bias way into cut-off silencing the amp but protecting those precious tubes. So, what is the solution to this annoying issue?
Well you could just replace the auto-bias unit and hope that it works out ok. However the amps that I have seen with this fault are just a few years old and so I wonder if replacing the autobias unit could be a just a temporary short term solution.
What about another approach such as down grading the amp to a simple bias pot and accepting that you need to always use well matched valves. Well that is an option and just what one of clients recently asked for as he was fed up having an unreliable amp.
In theory this should be quite a simple mod but the complication lies in the fact that this amp has two operating levels: club and arena and this means two different bias levels rather like the Famous Fender The Twin (or evil twin) from the late 80s.
With my mod you preserve the club and arena feature by automatically switching between two bias levels as you move between the two modes.
The mod consists of removing the auto-bias PCB completely and replacing it with a tag strip of parts, a bias pot away from the hot power valves. The bias pot is located on the underside of the amp near the effects loop well out of the way of the power valves and does not add significant noise levels. The result is an old school amp and one that is much more reliable.
This seems like a downgrade but really it is copying the design of the Fender twin which has been successfully powering amps that belong to some of the world’s most successful musicians. So why not?
Fender Supersonic Crackling Noise
Another problem with these amps is excessive cracking noise. This sounds a bit like crackling of old plate resistors or more descriptively the same as sizzling bacon in a frying pan; just a little louder and really annoying. It certainly does not have any vintage amp charm. None at all. In the case of the most recent amp it was impacted by the reverb control which can be helpful to know when trying to fault find such an issue.
What I have observed is issues with internal layers in the PCB multi layer board. The layers have broken down and make am unreliable connection and that gives you a crackle through your speaker.
If you have been affected by these issues leave a note on this page or get in touch to book yourself in for repair.
