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Old 17th November 2010, 06:19 PM   #1
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Default Heaters Constant Current Regulator

I need Heaters Constant Current Regulator for EL156, which draws 1.9A.
I can use LM338 regulator, alas, it is quite expensive.
The other option I have in mind is using LM317 regulator, which is limited to 1.5A, with power transistor.
Will it work properly, the way drawn in the attached schematic?

Edit,
In the schematic attached, it should read LM338, instead of LM337.
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File Type: jpg Heater_Options_a.jpg (684.0 KB, 272 views)

Last edited by Joshua_G; 17th November 2010 at 06:23 PM. Reason: Error in schematic
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Old 17th November 2010, 06:25 PM   #2
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Updated schematic is attached.
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Old 17th November 2010, 06:43 PM   #3
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For E series valves it may be better to regulate heater voltage, except for those that are specifically designed to run in a series heater chain too.
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Old 17th November 2010, 07:08 PM   #4
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Quote:
Originally Posted by DF96 View Post
For E series valves it may be better to regulate heater voltage, except for those that are specifically designed to run in a series heater chain too.
Thanks.
Why is it so?
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Old 17th November 2010, 07:22 PM   #5
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any specific reason you want to regulate the EL156 filament?

At least with the Shuguangs I had, there were no hum issues with AC.
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Old 17th November 2010, 07:28 PM   #6
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Quote:
Originally Posted by kstagger View Post
any specific reason you want to regulate the EL156 filament?

At least with the Shuguangs I had, there were no hum issues with AC.
I'm going to use Telefunken EL156. It seems to me that DC heaters voltage is better than AC, even for OP tubes. However, the main reason for using constant current is to let the tubes warm up gradually, and, most of all, to prevent the high current inrush when the heaters are cold - it helps prolonging tubes life. Those Telefunken tubes are rare, and very expansive (for me).
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Old 17th November 2010, 08:52 PM   #7
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Constant current will give a smoother start, apart from that AC is fine for output (and most input too). E series valves are designed for 6.3V, and might vary slightly in current drawn. Similarly, P series are designed for 300mA and might vary in voltage.

You can stop inrush either by including a resistor fed from a slightly higher voltage, or as part of general inrush protection in the transformer primary. Or you could have a DC voltage supply which ramps up gradually to 6.3V.
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Old 17th November 2010, 10:12 PM   #8
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Telefunken EL156 datasheet states that the heater is 6.3V, 1.9A. It doesn't state the heater's tolerance, so I assume keeping either voltage or current constant wouldn't shift the tube off spec - unless there is something I don’t know of.

BTW, I intend to have two tubes' heaters in series. If that is not advised, I can supply each tube with its' own regulator.
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Old 17th November 2010, 10:31 PM   #9
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Quote:
Originally Posted by Joshua_G View Post
However, the main reason for using constant current is to let the tubes warm up gradually, and, most of all, to prevent the high current inrush when the heaters are cold - it helps prolonging tubes life. Those Telefunken tubes are rare, and very expansive (for me).
The transformer supplying the heaters will have a current limit anyway.
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Old 18th November 2010, 11:10 AM   #10
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It is OK to put two valve heaters in series. Up to about four in series is OK. Beyond that you really need heaters which are designed for series operation, although you might get away with it if they are all identical and the constant current supply will help.

When a data sheet says 6.3V 1.9A it is the 6.3V which has to be maintained. A change in internal design might mean that some heaters take a little more or less current at 6.3V, but that does not matter too much. Any such changes are magnified by a constant current supply, as a hotter heater has higher resistance and so dissipates even more power. Voltage supply works the other way. Some data sheets specify whether the heater is suitable for series (current) or parallel (voltage) supply.
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