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Old 24th September 2007, 07:58 PM   #1
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Default Referencing DC heaters to cathode

Very spcifically:

Do I need to reference DC regulated 6VDC heaters to 1/4 B+ in my Aikido preamp?

Wavebourn Labs in one posting suggested that this wasn't necessary.

The center-tap of the secondaries of my 6.3VAC trafo is referenced to 1/4 B+ via 300K and 100K (the 100K bypassed by a 0.1uF cap). When B+ is applied, the 6.3VAC trafo buzzes.

QUESTION: Can the -ve (GND) connection of the regulator circuit be referenced to 1/4B+? This would move the B+ reference to the DC side of the regulator and may take some of the strain of the trafo.

Thanks,
Charlie
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Old 24th September 2007, 09:10 PM   #2
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Default Re: Referencing DC heaters to cathode

Quote:
Originally posted by cbutterworth
The center-tap of the secondaries of my 6.3VAC trafo is referenced to 1/4 B+ via 300K and 100K (the 100K bypassed by a 0.1uF cap). When B+ is applied, the 6.3VAC trafo buzzes.

QUESTION: Can the -ve (GND) connection of the regulator circuit be referenced to 1/4B+? This would move the B+ reference to the DC side of the regulator and may take some of the strain of the trafo.
When you call the -ve connection GND, do you mean you actually grounded it? If you grounded the negative and also connected the voltage divider to the positive side of your heater circuit, that's the problem right there.
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Old 24th September 2007, 11:05 PM   #3
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OK, I have attached an image containing my current configuration and an alternative configuration which would move the 1/4B+ from the trafo to the regulator circuit.

Charlie
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File Type: gif heater-reference.gif (10.8 KB, 645 views)
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Old 24th September 2007, 11:11 PM   #4
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The regulator has a GND on it, although I cannot see how it is actually connected to ground, other than via the tube filaments, if at all. Maybe this is the problem, and that I actually need to connect the "GND" on the DC side to IEC GND. If this is the case, then wouldn't the "Alternative Configuration" in my schematic (see previous message) apply?

Charlie
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Old 25th September 2007, 03:11 AM   #5
drj759 is offline drj759  United States
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Default Re: Referencing DC heaters to cathode

Quote:
Originally posted by cbutterworth
Very spcifically:

Do I need to reference DC regulated 6VDC heaters to 1/4 B+ in my Aikido preamp?

Thanks,
Charlie

Yes, you do need to reference the heaters in the Aikido to prevent heater insulation breakdown resulting in heater to cathode shorts that will kill the tube. This requirement is the same for AC or DC powered heaters for the Aikido. So you are on the right track.

Applying the heater reference on the center tap of the transformer is the usual method for referencing the heaters when running the heaters on AC. But your situation is different because of the regulator.

With the regulator, applying the heater reference on the center tap of your transformer will do nothing for your heaters. All that you accomplished was to apply the heater reference to the input of the regulator. Hopefully the regulator did not fry.

Applying the heater reference to only the -ve terminal as in your second configuration will most likely cause problems too.

You need to create a "virtual center tap" on the output of the regulator by placing two 47R, 0.5W resistors across +ve and -ve. Then connect the heater reference to the node between the 47R resistors. The heater supply +ve and -ve need to "float" so do not connect the +ve or -ve to anything other than the heater circuit. Do not ground the -ve. Ignore the "GND" terminology that Welborne uses in their documentation. There is no ground in this application.

I crudely modified one of your sketches to illustrate the concept

Dave
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File Type: gif heater-reference2.gif (7.8 KB, 644 views)
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Old 25th September 2007, 05:26 AM   #6
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Whew!

I have been running the Aikido for over six months now with the center tap of the transformer connected to 1/4 B+. This likely explains the problem with the buzzing 6.3V trafo. Obviously, it is over-designed and has survived.

What power rating of resistor should I use? I expect that very little actual current will be carried by the resistors. Maybe, I should err on caution and use 2W resistors.

Thank-you for explaining things to me.

Charlie
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Old 25th September 2007, 03:55 PM   #7
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Default Re: Re: Referencing DC heaters to cathode

Quote:
Originally posted by drj759
Applying the heater reference to only the -ve terminal as in your second configuration will most likely cause problems too.
Why?
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Old 26th September 2007, 03:20 AM   #8
drj759 is offline drj759  United States
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Default Re: Re: Re: Referencing DC heaters to cathode

Quote:
Originally posted by leadbelly


Why?
A bit less noise based on previous experimentation.

Couldn't find my old notes so I repeated the experiment tonight of measuring the output of my Aikido (quad 12SN7s) applying the two methods of referencing the heaters (dual 47R resistors vs. no resistors and applied to -ve). LM317 as the regulator for the heaters. I observed a bit less noise (a mV or so) on the scope across a resistor on the output with the inputs shorted.
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Old 26th September 2007, 03:55 AM   #9
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OK,

I'll get some 47R resistors tomorrow. I can only buy 1W locally, will these do? I can also buy 5W power resistors, I'll get a pair of these at 47R each, the power rating will be way lower than 5W.

Why 47R?

I suppose that a quick calculation will give me an idea:

6VDC across 47R and 47R = 0.19W per resistor with 3VDC output?

So 1W resistors should be more than sufficient?

Is the 3VDC at the far and of the divider )ie. 1/2 Vin) the reason for the 47R?

Thanks,
Charlie
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Old 26th September 2007, 08:07 PM   #10
Merlinb is offline Merlinb  United Kingdom
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You can use any resistor up to about 220R with good results. More people use 100R.

Incidentally, I think your buzz just needs a bigger cap on the potential divider!
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