12vdc heaters elevated?

[joesatch]

is there any advantage to elevating DC heaters?  Attaching the 12vdc ground to a +75vdc?  What is he doing?

http://diy-fever.com/amps/black-box/

[pdf64]

The snippet seems to explain the case? ie valves have a constraint on the max permissible voltage between a cathode and its heater.

Does your amp have any cathodes at high voltage, eg >90V?

[joesatch]

i guess my question is how do you elevate a dc heater?  Is he connecting the negative lead to a +75vdc ? I dont understand this. I know all about elevating AC heater CT's but a DC heater what exactly is elevated?

[shooter]

there's a missing piece somewhere
when you put 75vdc and 12.6vdc together, ugly things happen

[pdf64]

i guess my question is how do you elevate a dc heater?  Is he connecting the negative lead to a +75vdc ? …

Yes.

[sluckey]

It doesn't matter whether the +75v elevating voltage connects to the plus side or the negative side of the filament circuit. The important thing to remember is ***NO PART OF THE 12V SUPPLY CAN BE CONNECTED TO CHASSIS GROUND. IT MUST BE FLOATED!***

That also includes the PT filament winding.
 

[HotBluePlates]

i guess my question is how do you elevate a dc heater?  Is he connecting the negative lead to a +75vdc ? ... a DC heater what exactly is elevated?

there's a missing piece somewhere
when you put 75vdc and 12.6vdc together, ugly things happen

"12vdc" is the voltage from one side of the heater wiring to the other side.  Or "Pin 4 to Pin 5 of a 12AX7" (where Pin 9 is unused).

If "Ground" is 0v, then we might connect one side of the heater wiring to Ground.  We are referencing the heater voltage to 0vdc.  Let's say for this example that Pin 4 is our reference:  Measuring from Ground we see 0v at Pin 4, 12v at Pin 5.

We could move that reference to any other voltage.  Maybe +75vdc.

Now let's say Pin 4 is connected to our 75vdc reference.  Measuring from Ground, Pin 4 shows 75vdc on our meter.  Pin 5 shows 87vdc on our meter.  If we measure Pin 4 to Pin 5, we see 12vdc.  The heaters feel the same as they always did (except for any voltage-difference to the cathode), we just referenced the heater circuit to 75vdc instead of 0vdc.

[sluckey]

Now let's say Pin 4 is connected to our 75vdc reference.  Measuring from Ground, Pin 4 shows 75vdc on our meter.  Pin 5 shows 87vdc on our meter.

Since the entire heater circuit is floated, I believe you will see 75v between any filament pin (4, 5, or 9) and chassis.
Sorry, did not think that through very well.

[shooter]

no drawing SF!
stick-figure

[pdf64]

It’s akin to ‘elevating’ one 9V battery with another, to create 9 and 18V supplies, or +/-9V, depending on which node we reference to 0V common.

[tubeswell]

is there any advantage to elevating DC heaters?  Attaching the 12vdc ground to a +75vdc?  What is he doing?

http://diy-fever.com/amps/black-box/

He's taking the 12VDC heater supply and raising the -ve side of that to +75VDC. (So, the '+ve' side of the heater supply will be at 87VDC and the '-ve' side of the heater supply will be at 75VDC, which means there will still be 12VDC across the heater filaments in question). What is the advantage of doing that? Avoiding breakdown of the insulation* between the heater filament(s) and the cathode(s) of any tube which might be running a highish cathode voltage, such as a CF. (And it's not worth the bother of setting up if you're not running high cathode voltages).


*H-K insulation is a coating of insulating material, usually some non-conductive oxide, that is coated onto the surface of the filament wire during manufacture. Why is this necessary? You don't want a short between the heater and the cathode in tubes that have indirectly-heated cathodes - where the heater and the cathode are at different voltages and are part of different electrical circuits, because this would bugger-up the PT).  H-K insulation usually comes in specific voltage-insulation ratings, and you can get the manufacturers recommended rating off the tube datasheet - usually specified as a maximum heater-to-cathode voltage.

[joesatch]

thanks for the explanation. I'm thinking i will skip this elevated DC heater and just go with standard 12VDC heater with the neg to chassis ground

[PRR]

I have a house 12 feet tall.

I move it to the top of a 75 foot hill.

Inside is still 12 feet.

Look out the window it is 75 feet. So don't let the tubes look out there.