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In Reply to: RE: How can this Happen :( posted by Nickel Core on October 23, 2014 at 11:41:53
83s can take pretty ridiculous short-term overloads without damage. As both your 83 and 300B are open, it appears that some humongous voltage got on the filaments.
Or (guessing) your power supply discharged through the secondary, inducing a huge voltage on the 5V winding? I guess that could happen if the 83 shorted.
Follow Ups:
Hi,
I use separate transformers for the filament voltages, so your second scenario could not happen.
NC
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
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.
--------------------------
Buy Chinese. Bury freedom.
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