Experienced performance degradation in R-390-F unit#2, in service for a year or so, continuous. IMD and MDS thresholds were off about ten DB... Apparently, because all standard ocsillator designs draw grid current over part of their operating cycle, the grids become contaminated fairly quickly, about one year of continuous operation. This contamination is expressed on a tube tester as grid emission and generates oscillator noise that limits receiver performance, both weak signal and IMD. Replacing the defective oscillator tubes with tubes without grid emission restored performance, even though drive levels remained the same.

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