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In Reply to: RE: filament transformer CT grounding posted by DAK on January 29, 2016 at 08:23:25:
A "filament" is only found in a direct heated tube (45, 2a3, 300b, etc.).
A "filament" is both heater and cathode combined.
A "heater" is found in a indirectly heated tube.
A indirectly heated tube has a cathode sleeve that is normally not connected to the heater. (the cathode is connected to the heater in rectifiers but not in triodes)
The cathode sleeve is the cathode point of the tube. It normally connects to ground through a cathode resistor and has only the audio current flowing through it.
The heater is not part of the audio path of the tube. It just provides heat for the cathode.
If we use AC for the heater on a indirectly heated tube, floating the heated by using some of the B+ will stop the heater from conducting to the cathode, something that it's not meant to do but can happen.
With the potential of the heater higher than the cathode, the cathode might conduct to the heater (we don't care about that) but the heater can't conduct to the cathode causing AC hum in the audio.
With a DHT, if we heat the filament with AC, all of that AC is present on the cathode and it will get amplified by the tube. (AC on the cathode of a tube might as well be on the control grid)
A hum pot on a DHT has the heater current and the audio current flowing through it (just like the filament). The wiper of the hum pot is the cathode point of the tube. It normally connects to ground through a cathode resistor.
The hum pot's purpose is to adjust the center point (the point that will go to ground through the cathode resistor) of the filament so that equal, but out of phase, AC heating current is present within the tube on either side of that center point.
When equal and out of phase AC heating current is present on either side of the center point within the tube, that AC cancels itself out.
So "floating a heater on a indirectly heated tube" and "using a hum pot on a directly heated tube" are two totally different things.
Note to the technical types, I'm just trying to explain some concepts here.
Tre'
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Follow Ups
- RE: filament transformer CT grounding - Tre' 09:50:15 01/29/16 (2)
- RE: filament transformer CT grounding - vinnie2 11:32:37 01/29/16 (0)
- RE: filament transformer CT grounding - DAK 10:30:22 01/29/16 (0)
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