Floating heaters off of SE Power Tube Cathode voltgage question

[goldstache]

I've floated the heaters off the Power tube cathode bias resistor voltage before and it worked great.  Through 100-220ohm resistors to filament secondaries.

Then I got to thinking, usually a bad idea for me :), if I have a cathode bypass cap there wouldn't my AC filament voltage just head to DC ground through said bypass cap? 

I'm certain I've done this without error, blindly or not.  Just wondering why it would or wouldn't.

Thanks.

[jjasilli]

Good question.  But, no:  http://www.valvewizard.co.uk/heater.html


The resistance in the heater supply center tap is lifting the AC in the main lines from draining to ground.  If an actual CT is used, the resistance is in the internal windings.  If a virtual CT, the resistors do the lifting while providing a ground reference.


N.B.: The elevation circuit really is not relevant.  With no elevation the CT is connected directly to ground. 

[goldstache]

Thanks for the response.  I was particularly interested in application where there is no CT, just virtual.  Makes sense, and it always seemed to work, but I couldn't wrap my head around it. 

[PRR]

> wouldn't my AC filament voltage just head to DC ground through said bypass cap?

Why would it do that??

"Ground" has no magic attraction.

Circuit electricity always goes back to where it came from.

In this case, the other end of the heater winding.

*Lightning* goes to (or from) Ground/Dirt, because the circuit is earth to cloud and back.

Your home lamps take power from one power wire and it returns to the other power wire, a loop with the power generator. Yes, one of those wires is normally "grounded" but that's got nothing to do with the power flow. A flashlight works with no ground. Large power systems are grounded for lightning (and other) protection, not to make the regular power go round.

[HotBluePlates]

I've floated the heaters off the Power tube cathode bias resistor voltage before and it worked great.  Through 100-220ohm resistors to filament secondaries.

... if I have a cathode bypass cap there wouldn't my AC filament voltage just head to DC ground through said bypass cap? ...

Building on what PRR said:

"Ground" comes in 2 varieties:
1.  Actual "ground" (dirt, the Earth, "Earth ground")
2.  A reference point for the circuit; all other points of the circuit typically measure their voltage in relative to this "ground reference".

Normally your heater CT (or artificial center-tap, as you had with the resistors) would be referenced "to ground" or the 0v reference for the amp circuit.  But instead you're stacking that CT on a different voltage, to refer the heaters to a different voltage (often positive of the tube cathodes).

That's it, that's all you're doing.

Experientially, you already know the AC voltage doesn't "head to ground" in either case:  both work the same except for the impact of the changed reference voltage.  And if you measured a heater pin with respect to ground, you already know you've been getting ~3.15vac at either pin, because the ground reference was halfway between them at the center-tap.

Had you not used the center tap but grounded an end of the heater winding, you'd measure 0v to ground at one end and 6.3vac to ground at the other end.  In all cases, end-to-end voltage for the heater winding remains 6.3vac.  Tubes light up no matter if the reference is at one end of the heater winding, at the center-tap, or at any d.c. voltage relative to ground.