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In Reply to: RE: What would cause filament to burn out on My 300B tubes posted by Palustris on July 17, 2022 at 09:14:09:
I think this is the core of the problem. I believe the amp uses solid state rectifier for B+ because the amount of capacitance needed for a very low noise B+ is more than a slow turnon (indirectly heated) tube rectifier can be reliably used. The easiest way is adding a switch that turns on the filament supply first, and then after some time to warm up, through the second switch. A more foolproof alternative is add a time delay relay for the B+ supply, preferably on the 120 AC input side of the transformer since relays are typically not rated to 600-700VDC switching. If you had room for another tube socket and a 6.3 volt AC filament supply, you can use a damper diode in series with the solid stage rectifier OR on the transformer centertap (or ground from the bridge) to provide a delay due to the slow warmup characteristic of the damper diode. (I did this on a pair of DRD 300B amps I built). Finally, check your filament voltage on the 300B, it needs to be on spec (5V). If the rectified DC voltage is too high, it will result in more current through the 300B filament which will reduce the life of the tube. Just a few thoughts.
David
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Follow Ups
- RE: What would cause filament to burn out on My 300B tubes - David McGown 19:03:39 07/18/22 (1)
- RE: What would cause filament to burn out on My 300B tubes - Palustris 06:44:02 07/19/22 (0)
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