I wanted to share a successful troubleshooting session I had with my #Marshall DSL 401 amp and the first phase inverter incident I have ever had. If you are not in the mood to spend 3 minutes reading, the bottom line is that a faulty phase inverter tube could cause your amp power to drop significantly. IF that happens, try another phase inverter tube.
The Longer Version
Today while playing my #Marshall DSL 401 I experienced random and very significant changes in volume for a couple of seconds and then everything got back to normal and I pretended nothing ever happened and kept playing. After a while I put the amp to standby and left the room for a while. When I came back and tried to play again, there was no sound. After playing with the knobs, I realised that there is actually sound, but the volume is very low, even when the master volume is cranked.
Troubleshooting Step 1
I tried to switch channels and it was the same. This is a good indication that the preamp tubes are probably not the problem. If a preamp tube was the problem, switching to another channel would have fixed it.
Troubleshooting Step 2
I tried to connect the guitar directly to the amp (no pedals, no nothing!) with two different cables, one at a time, to eliminate cable fault.
Troubleshooting Step 3
I tried another guitar.
Troubleshooting Step 4
Ok, it's time to check the power tubes! I turned off the amp, disconnected the power cable and left the amp to cool down for a while. When the amp was cool, I carefully removed all four EL84s and replaced them with a new quartet. Connected everything back, turned the amp back on and... no... still low volume...
At this point I realized that my amp probably needs to be serviced by a pro. What a drag... but then I realized that there is one tube I skipped! Yes, that phase inverter tube that seems to just sit there doing nothing for over fourteen years shamelessly calling itself a tube... Well, what's to lose, right?
Troubleshooting Step 5
I swapped the phase inverter tube with one of the tubes from the second channel, turned the amp back on, switched to the first channel and guess what? My amp is back to life!!! Yessssssssssss!!!
So, note, a faulty phase inverter tube could make your amp sound like music played through headphones!
Cheers.
An accessible explanation about phase inverters:
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