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Old 21st December 2012, 06:53 AM   #1
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Default Simple DC heating question

Hello,
I have an easy question, I would like to regulate an amplifier filaments, so if they are at 6.3vac, and I rectify them I get 8.8vdc. Can I use a linear regulator to take it down to 6.3vdc and be OK? Am I wrong in my thoughts?

Thank you,
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Old 21st December 2012, 07:41 AM   #2
cotdt is offline cotdt  United States
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Yes, you can use an LT1084 to power all your filaments. With the older regulators you can lose 3V or sometimes more.
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Old 21st December 2012, 08:20 AM   #3
palmas is offline palmas  Portugal
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You could, but unless it is a direct heated tube or experiencing some humm, there is nothing to gain from it
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Old 21st December 2012, 08:33 AM   #4
AJT is offline AJT  Philippines
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yes, if you are having hum issues, going dc on the heaters does not automatically cure the problem, you have to find out why your circuit is humming...
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Old 21st December 2012, 09:27 AM   #5
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I ask this as a question..is it possible to get 6.3 vdc regulated from 6.3v AC after factoring in 1.4v drop in the bridge rectifier, perhaps 1.5v in the low drop out regulator and 10% droop on the 100hz raw input ? Wouldn't you need to start with approx 8v ac ?

Last edited by hugo_zair; 21st December 2012 at 09:36 AM.
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Old 21st December 2012, 09:45 AM   #6
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Quote:
Originally Posted by palmas View Post
... there is nothing to gain from it

There can be something else to gain from it: a soft start is very easy to implement with a linear regulator. As the filaments fail mostly at power on because of the inrush current, this increases the life of the tube.
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Old 21st December 2012, 09:57 AM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by hugo_zair View Post
I ask this as a question..is it possible to get 6.3 vdc regulated from 6.3v AC after factoring in 1.4v drop in the bridge rectifier, perhaps 1.5v in the low drop out regulator and 10% droop on the 100hz raw input ? Wouldn't you need to start with approx 8v ac ?
Possible, but painfully so: the power factor of rectifier + filter cap is in the region of 0.5, compared to 1 for direct connection, and this will be more demanding on the winding.
If you use a standard diode bridge, you will most likely be too short of some hundreds of mV.
You can do it by using germanium or schottky diodes, or even better synchronous rectification.

I have described one or two such solutions in the PSU section.
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Old 21st December 2012, 10:03 AM   #8
DF96 is offline DF96  England
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Quote:
Originally Posted by Vincent77
There can be something else to gain from it: a soft start is very easy to implement with a linear regulator. As the filaments fail mostly at power on because of the inrush current, this increases the life of the tube.
Some form of soft start will also reduce the risk of killing the regulator, which seems to be quite common. DC heaters can be much harder to get right than AC heaters. Better for people to learn how to do AC wiring properly. Phono preamp inputs are the only valves which may need a DC heater.
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Old 21st December 2012, 10:46 AM   #9
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Hi!

Quote:
Originally Posted by Vincent77 View Post
There can be something else to gain from it: a soft start is very easy to implement with a linear regulator. As the filaments fail mostly at power on because of the inrush current, this increases the life of the tube.
I never had a heater failure on an indirectly heated tube due to inrush current.

Different story for directly heated tubes, especially those with thoriated tungsten filaments there a soft watrt will help to avoid filament damage

Thomas
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Old 21st December 2012, 10:49 AM   #10
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Originally Posted by Elvee View Post
Possible, but painfully so: the power factor of rectifier + filter cap is in the region of 0.5, compared to 1 for direct connection, and this will be more demanding on the winding.
Absolutely right!
You will draw more than 1.6x average DC current from the transformer, in short ripple bursts that can reach 5 times the DC average!

So if you need DC heating, make sure that the transformer winding is rated at 2x the DC current you need with enough voltage headroom, and the capacitor withstands the ripple current, or else you will get *a lot* of unwanted heat.
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