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Old 28th December 2008, 07:50 AM   #1
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Default Is 6V DC too low for heater filament?

I am trying to use 7806 as regulator for my tube preamp. Is 6V DC too low? What is the disadvantages of using a lower voltage for filament?
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Old 28th December 2008, 08:08 AM   #2
ericj is offline ericj  United States
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it will work as 6.0v is still within tolerances.

however, we should ask, why do you want to do this?

If you have 6.3vac available, twist the wires tightly and use AC heaters. properly installed AC heat isn't noisy and increases the lifespan of the filament.

if you have only DC available, sometimes you can get better performance by using an lm317 configured as a current source.

even if was sure i wanted voltage regulated heaters, i find that an lm317t is more durable and performs better than a 78 series fixed regulator, so i just use those instead. it helps that i have a large assortment of resistors on hand to come up with the resistor networks for the 317, i admit.
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Old 28th December 2008, 08:25 AM   #3
ja2dhc is offline ja2dhc  Japan
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Default Re: Is 6V DC too low for heater filament?

Quote:
Originally posted by Sunsun22
I am trying to use 7806 as regulator for my tube preamp. Is 6V DC too low? What is the disadvantages of using a lower voltage for filament?
The specs of most of In-Direct heating tubes specify +/- 10%
for voltage tolerance and therefore DC 6V is within spec while
Direct heating' are +/- 5% as is the lower limit if DC6V is used.

A far as I experienced, the lower voltage within the spec effects
good sonic. So I usually apply AC 6V for In-Direct tubes.
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Old 28th December 2008, 09:18 AM   #4
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Default Re: Is 6V DC too low for heater filament?

Quote:
Originally posted by Sunsun22
I am trying to use 7806 as regulator for my tube preamp. Is 6V DC too low?
Probably not, but if you are concerned, put a diode that drops 0.3V between the ground pin of the regulator and ground.
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Old 29th December 2008, 05:03 AM   #5
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It would take a miracle for me to find the relevant article that appeared in either GlassAudio or AudioExpress, but I believe some theorizing took place showing actual improvements in running low filament voltage. I believe there was lower distortion. And as I recall the voltage was significantly lower than design center. Try googling...
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Old 29th December 2008, 05:06 AM   #6
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I searched Mouser but it seems there is no diode that can have a drop of 0.3V. Do you have anything in mind?
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Old 29th December 2008, 05:59 AM   #7
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Quote:
Originally posted by Sparky OR
It would take a miracle for me to find the relevant article
It was on Steve Bench's site. The original site is gone, but it is mirrored at http://greygum.net/sbench/sbench101/

IIRC, this is more with DH than IDH tubes.

Quote:
Originally posted by Sunsun22
I searched Mouser but it seems there is no diode that can have a drop of 0.3V. Do you have anything in mind?
You're probably right. How about a 5V reg with a 1.3V diode
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Old 29th December 2008, 08:18 AM   #8
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Heater voltages affect tube lifetime quite a bit, and noise as well.
Some designs (e.g. Dynaco phono pre, Neumann U47 mic) lower the voltage to decrease noise (and grid current).
Linearity is affected to differing degrees, depending on cathode construction and emission reserve.
Teddy (Tetsu Kimura) did some research years ago on a few popular tubes.
The page is in Japanese, but the graphs speak for themselves.

http://www2.famille.ne.jp/~teddy/datalib/heater.htm
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Old 29th December 2008, 08:45 AM   #9
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Dear all,

Thank you for the inputs. I think I am feeling easy now to use 6V. I am just lazy to save two resistors by not using 317.

Cheers.
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