I have elevated heaters (60V off the first B+ node) and I have a B+ fuse right after the rectifier. Right now the elevation divider/ cap network comes after the fuse.
Thinking about it I'm not sure what happens if the HT fuse blows. Pretty sure the heaters would loose their ground reference, and not too sure if that would be a problem as the amp would be pretty much shut down anyway. Not sure what powered heaters sitting around with no ground reference would result in.
People here can see the big picture better than me and might explain if it matters if the elevation is taken before or after the B+ fuse.
BTW what happens if there's a catastrophic power tube failure, eg heater to plate, or flyback arc to the heater pin with tube elevation? Does the elevation make the heater closer to the plate V and lessen the chance of an arc?
Heater Elevation & B+ Fuse Question
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Malcolm Irving
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Re: Heater Elevation & B+ Fuse Question
If the heaters lost their ground reference it wouldn’t be a big issue, it’s only there to prevent hum and buzz in the audio.
I would put the elevation after the B+ fuse, just so that the elevation circuit, and the heater circuit connected to, it are protected by the fuse.
The plate voltages of the power tubes swing up to almost twice B+ (thanks to the output transformer). The elevation of 60V would reduce the chance of flash-over, but only slightly.
I would put the elevation after the B+ fuse, just so that the elevation circuit, and the heater circuit connected to, it are protected by the fuse.
The plate voltages of the power tubes swing up to almost twice B+ (thanks to the output transformer). The elevation of 60V would reduce the chance of flash-over, but only slightly.
Re: Heater Elevation & B+ Fuse Question
With your arrangement, if the HT fuse blew, I think the heaters would still have a reference to the circuit 0V (via the resistive potential divider), just that it would have higher resistance to 0V than normal.
The key thing is to prevent the heater circuit floating off beyond the heater - cathode voltage limit, or being pulled up to VHT by a tube short.
The key thing is to prevent the heater circuit floating off beyond the heater - cathode voltage limit, or being pulled up to VHT by a tube short.
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Re: Heater Elevation & B+ Fuse Question
As pdf64 indicates, a blown fuse does not disconnect the heater reference to ground (ie. via the B+ divider).
Other failure modes are complex. If an anode to heater flashover occurred at the valve socket, the heater elevation divider bottom resistor and capacitor would be pulled up, or down, depending on the polarity of the anode (they can fly negative below 0V !!).
If there was enough energy in the arcover to change the elevated supply capacitor voltage, then the cap could be damaged, and the divider resistor(s) could be damaged. If those parts are damaged then the heater is then likely to float, but not cause a fuse to blow, unless a heater-cathode then went sufficiently short circuit, and a cathode was grounded (eg. fixed bias output stage). Arc tracking can make the situation worse. This is best managed with overvoltage protection on the output transformer windings imho.
Other failure modes are complex. If an anode to heater flashover occurred at the valve socket, the heater elevation divider bottom resistor and capacitor would be pulled up, or down, depending on the polarity of the anode (they can fly negative below 0V !!).
If there was enough energy in the arcover to change the elevated supply capacitor voltage, then the cap could be damaged, and the divider resistor(s) could be damaged. If those parts are damaged then the heater is then likely to float, but not cause a fuse to blow, unless a heater-cathode then went sufficiently short circuit, and a cathode was grounded (eg. fixed bias output stage). Arc tracking can make the situation worse. This is best managed with overvoltage protection on the output transformer windings imho.