In Reply to: Tube Radio sounds great then fades.... posted by MannyE on May 28, 2016 at 12:15:17:
If I had to guess I put my money on the first audio stage. Those were typically biased by Edison effect. There is a very high value carbon comp resistor from the control grid to ground (~4.7 Meg-Ohm). They go open and can cause this. Get an Ohmmeter which can measure that high and check it.
If it is bad replace it with a more modern resistor (metal film) and it will likely outlast you.
As you are guessing if a coupling cap gets leaky, that can also cause this issue. If the output tube plate is glowing, suspect the coupling cap from the first audio stage to the grid of the output tube.
The silver inside the output tube is the "getter flash". It got there when they flashed the getter as the tube was being completed.
Phil
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- RE: Tube Radio sounds great then fades.... - coffee-phil 18:00:03 05/30/16 (0)