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RE: strange behavior of Belvu and S&H 5751 tubes in my EICO ST84 preamp

The tube that hums, when you tested, did you test for shorts? Filament to cathode or grid?

5751 gain is considerably less than 12ax7, but I encountered a no sound situation with 5751's where 12ax7's sounded fine and it was a power supply problem causing the tubes to be biased off the chart for either tube. Suggest you measure DC voltage on cathode, grid and anode on both sections (pins 3,2,1 and 8,7,6) and take a look at the curves for both 5751 and 12ax7.

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