The pilot lamp is going dim because there is not enough power for it to burn brightly. This is most likely caused by a weak power transformer, although loose solder joints on the filament circuit, or corrosion in the pilot-lamp socket could also be a possible cause. Let’s look at them one at a time.
First, inspect the power transformer. This is the big transformer on the end of the amp chassis opposite the input jacks. Are the transformer laminates rusty? If so, this could be your problem. A transformer is made with many thin steel laminates that are supposed to be insulated from each other electrically to prevent conductivity from one laminate to the next and thus stop losses through eddy currents. When the laminates rust they sometimes become conductive, creating eddy currents that rob power from the power supply. To test rusty laminates for conductivity, use an ohmmeter set for a low setting (perhaps 100 ohms or so). Touch one meter lead to the edge of a laminate on one end of the transformer and touch the other meter lead to a laminate on the opposite end. If there is any continuity at all, then the transformer will have eddy currents and the power in all circuits will be compromised. In this case, I would recommend getting a new power transformer.
If the laminates are not conductive, inspect all the filament heater wiring and associated solder joints—especially the solder joints on the pilot lamp. The filament heater circuit is the pair of green wires that go from the power transformer to the pilot lamp, and from the pilot lamp to pins #2 and #7 on the power tubes, and then from the power tubes to the preamp tubes. Each of these solder connections should be re-cooked with a soldering iron just to make sure the joints are good.
Lastly, sometimes the base of the pilot lamp or the innards of the pilot lamp socket can become corroded. You could try cleaning them with some DeOxit spray. In extreme cases of corrosion, you may need to use some 220 grit sandpaper to clean the base of the lamp and the inside of the lamp socket.—Gerald Weber, Kendrick Amplifiers
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