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Old 2nd December 2002, 06:02 PM   #1
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Default Biasing heater supply above ground.

Hi all,

What if I decided to hell with it, I'm going to build a tube amp from scratch. I also decided that I want to bias the heater supply above ground. The heater supply is 6.3 volts AC. If I built a voltage divider from B+ and tied a wire from the voltage divider to the hot of the heater supply would this effectively bias the supply above ground? If so I'm assuming that I would not tie the negative of the heater supply to ground since that would put a hundred volts or so to ground through very little resistance. If I have this wrong please let me know. I don't want to let the smoke out. It's such a pain getting it back in there.

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Old 2nd December 2002, 06:54 PM   #2
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why do you have to float the filaments?
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Old 2nd December 2002, 07:22 PM   #3
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I might be looking at this wrong but If I ground the negative of the filament supply and put 100 volts on the positive of the heater supply to bias it above ground then the only resistance that I would have between the middle of the voltage divider and ground would be the all of the tube heaters in parallel and the filament winding of the power transformer.

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Old 2nd December 2002, 07:31 PM   #4
Joel is offline Joel  United States
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Gavin, yes I think you are looking at it wrong. What is the circuit? Please post a schematic.

General practice is to ground the center-tap of the filament winding. If one isn't provided, two 50 ohm resistors acorss the winding, and grounded, will do the same thing.
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Old 2nd December 2002, 07:37 PM   #5
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Yes normally that would be the way to do it. I'm not sure that you should ground it when biasing the heater supply off of the B+ via a voltage divider. I will post a schematic when I get home in a couple of hours.

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Old 2nd December 2002, 10:02 PM   #6
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I think that this is the right way to bias the heater supply above ground( and the cathode too).:

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Old 2nd December 2002, 10:23 PM   #7
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Default and if you don't have a CT

you can use two equal value (I use 470ohm) resistors from the 45v node to each leg of the transformer filament winding.

Robert Morin
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Old 2nd December 2002, 10:39 PM   #8
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I thought that that might be the way to do it if you don't have a center tapped winding. Thanks.

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Old 3rd December 2002, 01:31 AM   #9
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Default Safe filament to cathode voltage ratings

I thought that I would add this since there may be some beginners may not know the purpose of floating the filament supply at a positive DC voltage above ground.

Normally tube filaments are floated above ground with some positive potential if there is a chance of exceeding the filament to cathode voltage in applications where the cathode to ground voltage is fairly high. Such a problem can occur in totem pole, cathode follower and perhaps a hand full of other circuits. Since many tubes are usually feed from the same filament winding a happy medium must be found in the amount of positive voltage to float the filaments at. A good rule of thumb is about half way between the highest cathode potential and ground. Some tube cathodes will not be to far from ground while others may have cathode voltages substantially higher. The filament to cathode voltage maximum ratings for any tube should never be exceeded and maximum signal swings need also be taken into account when setting the amount of DC voltage.

The circuit diagram posted by G looks fine to me and I have used this exact method in the past. My choice would be to increase the size of the capacitor to get a lower effective ground connection at the center tap of the transformer at 60 Hz. If no center tap is available on the filament transformer then two resistors as mentioned in prior posts can be used to provide a artificial center tap.

Since we are talking about lighting filaments directly from a filament transformer I assume that we are talking about high-level stages. With preamp circuits it pays to run all tube filaments from a well-filtered DC power source. This power source can then be floated at a positive potential using similar techniques.

There may be additional minor benefits for using this float voltage, but tube protection is the primary reason.

John Fassotte
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Old 3rd December 2002, 04:45 AM   #10
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Thank you John for explaining that. I just knew that it reduced noise in the circuit.

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