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In Reply to: RE: How can this Happen :( posted by Nickel Core on October 23, 2014 at 14:02:55
The fuse that tripped was part of the B+ supply suggesting that a large current flowed though the B+ supply and not through the filment supply.
NC
Follow Ups:
What is the mA capability of your B+ supply? Is it enough to burn out the filament if the tube saturates from positive grid voltage? I know that seems extremely unlikely, but I have to ask.
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Buy Chinese. Bury freedom.
HI,That must be 10A easily, for surges maybe more...
I specified the B+ transformer to have very low DCR which can only be achieved by using thick wires so a large current capability.
I also feel the grid has become positive at some point. But how? I use fixed bias via again a separate supply.
NC
Edits: 10/24/14
10A B+?? Holy moley! Well, it sounds like whatever was output from the pre when you switched it off was high enough amplitude and duration to push the grid of the 300B positive. Wow, I never would have imagined a B+ supply could burn out a DHT filament. If this is heated with a CT filament transformer, you might consider installing a small fuse in the connection between the CT and ground (or between the hum pot/cathode resistor and ground). I would also scope the output of the pre when its connected to the amplifier to see if it's oscillating during power-down.
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Buy Chinese. Bury freedom.
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