A couple of questions regarding elevated heaters for a 12AU7 Cathode Follower. If I'm using an old radio PT that had the single terminal 6.3volt output and earth as the heater connection , is there any way to elevate the heaters? Also , in the case of using a B+ voltage divider to elevate as in the schematic below , the 6.3volt winding appears to be isolated from the winding that supplies the B+ , so a difference in potential should not result in any current flow , or am I being dumb ? (I must be , but I can't work out quite how)
The only problem is that you would need to elevate all the heaters by the same amount. You can float the whole 6.3 volts up with some voltage divider off B+. With only one 6.3 volt winding it is all or nothing. Maybe some compromise between the cathode voltage of the follower and zero. Two 100 ohm resistors makes the quasi center tap for the 6.3 volts where you hook it to ground - make it a pot and you can have a hum balancer.
Yes you can elevate the heaters like in the schematic you posted . The two 100ohm resistors are to be connected across the 6,3V winding that is shown floating . The current doesn't flow it's just elevating the heaters winding potential above ground , where it sits in a simple design that you mentioned , with one winding end directly tied to earth/ground .
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Thanks for the reply , this a PT from an old radio of the kind where they used earth as one side of the heaters and ran a wire from the heater back to an earth shared with the B+ winding (or on the very low-cost models used the chassis as a earth return).I suppose my thinking was that with one side of the heater going directly to earth then a high potential on the other side would fry it , and a high potential connected to both sides would be connecting high voltage direct to earth.
Sorry , I should have made it clearer that I was asking two questions. The first is "won't there be a problem if one end of the heater filament is at earth potential and a high dc voltage is applied to it when that dc voltage has the same earth reference?" , and the other question , "if I had a set up as in the schematic , why doesn't the fact that the heater winding is isolated from the winding that supplies the B+ mean there couldn't be a problem of the heater cathode potential being too high as they don't reference the same ground?" The reason I add that question is that I wondered if I could put a little isolating transformer in between my heater winding and the valves to isolate it from the B+ , then I thought " wait a minute , it's isolated in the standard "elevated heaters" solution with a voltage divider on the B+ , so if isolating it doesn't do anything there to protect the heaters / cathodes it's no use me pursuing that." But then I couldn't help wondering WHY the isolation isn't enough protection on it's own , hence the second question.
If there were no heater emission, I think the insulation of the transformer winding in itself should be sufficient to prevent cathode-heater breakdown. Cathode-heater breakdown is caused by some slow electrolysis process at the places where the insulated filament touches the inside of the cathode tube (the hot and quite thin layer of alumina heater insulation doesn't insulate perfectly). When you block the current, there will be no electrolysis.
However, one of the reasons why heaters are sometimes elevated is unintended thermionic emission from the heater. If there are electrons emitted from the heater, you could again get a compensating DC current flowing through the heater-cathode insulation, which means there is current for the electrolysis process that slowly breaks down the heater-cathode insulation. Connecting the heater winding centre tap to a suitable voltage provides an alternative path for the compensating current.
Besides, connecting the centre of the heater winding (or the centre of two resistors across the winding) to some defined potential makes the voltages in the circuit more predictable, and engineers love predictability.
However, one of the reasons why heaters are sometimes elevated is unintended thermionic emission from the heater. If there are electrons emitted from the heater, you could again get a compensating DC current flowing through the heater-cathode insulation, which means there is current for the electrolysis process that slowly breaks down the heater-cathode insulation. Connecting the heater winding centre tap to a suitable voltage provides an alternative path for the compensating current.
Besides, connecting the centre of the heater winding (or the centre of two resistors across the winding) to some defined potential makes the voltages in the circuit more predictable, and engineers love predictability.
Thanks for your reply MarcelvdG , I can see the reasons for cathode/heater breakdown are more complicated than I realised . I'm still not sure quite why the potential difference would matter if the windings providing them were isolated from each other , but I shall go on reseaching - I suspect I simply don't have the required in-depth understanding of how valves work.
I'm no expert either, just someone who likes to read old articles.
What I think could happen is this:
Electrons jump off the heater via thermionic emission and flow to some electrode that's positive with respect to the heater.
The external circuit provides a conducting path from that electrode to the cathode.
They go from the cathode to the heater via the electrolysis process that in the long run damages the insulation.
What I think could happen is this:
Electrons jump off the heater via thermionic emission and flow to some electrode that's positive with respect to the heater.
The external circuit provides a conducting path from that electrode to the cathode.
They go from the cathode to the heater via the electrolysis process that in the long run damages the insulation.