Trying to build a fail safe circuit, I would like to know what happens when the cathode connection gets interrupted.
When anode supply gets interrupted, nothing happens, but what if the cathode connection is not there with full supply on the anode side.
In a simulation with 900V supply and grid to -85V, a setting that will normally allow ca. 110ma idle current to flow, when interrupting the cathode connection the sim tells that only a couple of nA will flow, if so, totally harmless.
The question is though, what happens in real life when the cathode connection gets interrupted, how reliable is this simulation result ?
Hans
When anode supply gets interrupted, nothing happens, but what if the cathode connection is not there with full supply on the anode side.
In a simulation with 900V supply and grid to -85V, a setting that will normally allow ca. 110ma idle current to flow, when interrupting the cathode connection the sim tells that only a couple of nA will flow, if so, totally harmless.
The question is though, what happens in real life when the cathode connection gets interrupted, how reliable is this simulation result ?
Hans
Depends of the layout.
If the GM70 is cathode biased and cathode resistor breaks, the grid will work like (low-grade) cathode.
If the tube is cathode biased, the current flow trough the anode-grid diode and grid leak resistor. If the grid leak resistor value is enough large, the current will be maximum few mA.
If the tube is DC coupled to CF or SF, the negative bias voltage is added to anode voltage (minus anode-grid diode loss) and current limiting -at the first time- depends of CF/SF.
After the first transient, the grid voltage will be tend near to the anode (1kV!), and CF/SF output will be much more positive, than its positive supply.
Probably CF tube will survive, but I'm not sure that SF FET too.
I'm not sure, that GM70 thin grid wire will remain intact.
In case of CF/SF the -very small power- 10-100R resistor from CF/CF output series to the grid probably burn first, than grid wire.
BTW I always use -in case of CF/SF drive output tube- few ten kOhm resistor from output tube grid to negative power supply to grant more negative voltage than actual bias if CF/SF does not work.
If the GM70 is cathode biased and cathode resistor breaks, the grid will work like (low-grade) cathode.
If the tube is cathode biased, the current flow trough the anode-grid diode and grid leak resistor. If the grid leak resistor value is enough large, the current will be maximum few mA.
If the tube is DC coupled to CF or SF, the negative bias voltage is added to anode voltage (minus anode-grid diode loss) and current limiting -at the first time- depends of CF/SF.
After the first transient, the grid voltage will be tend near to the anode (1kV!), and CF/SF output will be much more positive, than its positive supply.
Probably CF tube will survive, but I'm not sure that SF FET too.
I'm not sure, that GM70 thin grid wire will remain intact.
In case of CF/SF the -very small power- 10-100R resistor from CF/CF output series to the grid probably burn first, than grid wire.
BTW I always use -in case of CF/SF drive output tube- few ten kOhm resistor from output tube grid to negative power supply to grant more negative voltage than actual bias if CF/SF does not work.
Euro, again thx a lot for replying so fast.
So with a grid leak resistor large enough, not much will happen with only a few mA, true ?
Hans
So with a grid leak resistor large enough, not much will happen with only a few mA, true ?
Hans