What is the best way to connect DC filaments? - diyAudio
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Old 9th December 2012, 04:08 AM   #1
cotdt is offline cotdt  United States
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Default What is the best way to connect DC filaments?

I use fixed bias so I just connect one end of the DC supply to ground, and it works fine. However, I heard this was a bad way to do it. What is the correct way?
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Old 9th December 2012, 01:45 PM   #2
DF96 is offline DF96  England
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Read the valve datasheet? For directly-heated valves with a filament the data sheet will often say how best to provide DC heating. For indirectly-heated valves with a heater it doesn't matter very much.
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Old 9th December 2012, 04:14 PM   #3
cotdt is offline cotdt  United States
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Quote:
Originally Posted by DF96 View Post
Read the valve datasheet? For directly-heated valves with a filament the data sheet will often say how best to provide DC heating. For indirectly-heated valves with a heater it doesn't matter very much.
The problem is that the 300B datasheets tell me to do it the incorrect way, to connect the grid and plate returns to the DC filament supply's ground.
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Old 9th December 2012, 04:23 PM   #4
DF96 is offline DF96  England
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If you know the datesheet is incorrect, then you obviously know a better way to do it. Why ask?
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Old 9th December 2012, 04:28 PM   #5
cotdt is offline cotdt  United States
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Quote:
Originally Posted by DF96 View Post
If you know the datesheet is incorrect, then you obviously know a better way to do it. Why ask?
I don't know the best way to do it. A very knowledgeable expert told me that I was doing it wrong, but refused to tell me the correct way. So it's been annoying me ever since and I come to ask the people here.
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Old 9th December 2012, 04:34 PM   #6
DF96 is offline DF96  England
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Genuine 'very knowledgeable experts' will normally tell you how to do it, and why exactly they disagree with the manufacturer's datasheet. You can then weigh up their argument, and ask others to consider it too.

However, be patient. Someone may pop up here with good advice.

It may help if a mod changes the thread title to 300B DC filaments, as some people may just assume that you are asking about DC heaters for indirectly-heated valves. In the UK we distinguish between filaments and heaters, but elsewhere that distinction is not always maintained.
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Old 9th December 2012, 04:41 PM   #7
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Hi!

The way you did it is the best way how to connect DC filaments if your filamet supply is very clean. That's how I do it as well.

A hum buck potentiometer is only useful with DC filaments if your supply has some ripple left. Then you can tune that partially out with a um buck. However a good, clean DC supply does not need that.

Just be aware that bias voltages are usually given for the mid point of the filament voltage in the datasheet of DHTs, so you need to take this into account for the bias voltage.

Best regards

Thomas
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Old 9th December 2012, 06:03 PM   #8
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You may enjoy reading Rod Coleman's efforts to remove the filament's power supply from the signal path. There are several relevant threads here in DIYAudio.

All good fortune,
Chris
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Old 9th December 2012, 06:35 PM   #9
cotdt is offline cotdt  United States
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Rod Coleman has a lot of posts to dig through! Thanks for the heads up, I will look over them over some coffee. I already built my filament regulators with LM338 voltage source supply (one for each tube), would Rod Coleman's ideas still be relevant? I would prefer not to use new supply when what I already have works fine.

I just want to be able to decouple any noise generated by the filament supply. The 300B is for a preamp and needs to run very quiet.
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Old 9th December 2012, 07:09 PM   #10
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A preamp? Cool. Certainly hum can be kept arbitrarily low with regulation, and excess noise will vary with tube/valve construction. The Rod Coleman circuit is a high impedance current source feed to the filament, with the intent of removing the power supply from the signal path. All low impedance power supplies are inherently in series with signal to ground.

All good fortune,
Chris
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