A Fender Blues Jr Mk III arrived in the workshop. It was dead. The amp was opened up.
One EL84 output valve was completely destroyed. The other had most of the getter gone in the valve , suggesting long hours of use. The white material inside the valve indicated a slow gas leak that eventually killed the valve. The EL84 failure had blown one of the screen resistors on the main pcb and this resulted in a total amp strip down in order to replace the resistor. The other EL84 had most of the getter gone in the valve , suggesting long hours of use and also a possible valve gas leak.
The amp is probably around 10 years old and the valves were Groove tubes dating from 2011 so probably had been in the amp since new.
The downside of not replacing EL84 output valves when you start to hear “noises ” coming from your amp, and the valves are old, is that collateral damage is done.
The EL84 failure had blown one of the screen resistors on the main pcb and this resulted in a total amp strip down in order to replace the resistor. we replaced both screen resistors with higher wattage types.
The amp dismantled:
Screen resistors replaced ( the blue resistors near the ribbon cables):
You will see that this amp has the infamous Illinois grey Electrolytic capacitors. They were not leaking and their ESR was measured and still well within specification. ESR is the effective series resistance and as a capacitor ages this resistance value rises. This leads to increased mains hum from the amp. THey were left in place and when the amp was finally audio tested there was zero hum from the amp.
The pre-amp valves were tested and two were found to be totally worn. These were also replaced. The remaining good GT12AX7 valve was a balanced one so this was put in the phase splitter position . This ensures both of the output valves are driven evenly. Ensures even wear on the output valves, and improves tonal quality as it minimises nasty sounding cross over distortion.
A common issue on modern Fender valve amps is that the Tolex lifts possibly due to the heat from the amp. I usually glue this back down so the amp looks smart again.
The amp was cleaned up, PAT tested and then finally audio tested. It now looks nice and new and more importantly has a proper Fender tone.
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