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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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).
 
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.
 
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.
 
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.
 
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.
 
When a data sheet says 6.3V 1.9A it is the 6.3V which has to be maintained.

That is in the boundaries of the tolerance, which isn't specified for those tubes.

Some data sheets specify whether the heater is suitable for series (current) or parallel (voltage) supply.

Most tubes datasheets don't mention it.

Again, I don't see any possible harm caused by wiring 2 tubes in series, feeding it with constant current. Yes, possibly one tube's heater will run hotter than the other, but that doesn't seem to make the tubes operate off spec. With AC heaters supply, the heaters' voltage fluctuates anyway, with the mains voltage flactuations.
 
Data sheets vary in their usefulness. Mullard/Philips ones are generally more informative than some others.

The E in EL156 means 6.3V, so the valve is intended for voltage heater supply unless otherwise stated. The 1.9A mentioned in the data sheet is intended to help you know how much current may be drawn. It does not necessarily mean that every EL156 will draw 1.9A, although most will not be far off.

Assume a batch of valves are made with heaters which are 5% high in resistance. Then on a voltage supply the power drawn will be perhaps 3% too low. On a current supply the power will be perhaps 7% high. This is because the change in resistance with temperature works to correct the problem on a voltage supply and increase the problem on a current supply. That is why valves intended for a current supply often have specially-designed heaters.
 
Data sheets vary in their usefulness. Mullard/Philips ones are generally more informative than some others.

The E in EL156 means 6.3V, so the valve is intended for voltage heater supply unless otherwise stated. The 1.9A mentioned in the data sheet is intended to help you know how much current may be drawn. It does not necessarily mean that every EL156 will draw 1.9A, although most will not be far off.

Assume a batch of valves are made with heaters which are 5% high in resistance. Then on a voltage supply the power drawn will be perhaps 3% too low. On a current supply the power will be perhaps 7% high. This is because the change in resistance with temperature works to correct the problem on a voltage supply and increase the problem on a current supply. That is why valves intended for a current supply often have specially-designed heaters.

I can work only with the datasheets I have.
For EL156 I have only Telefunken datasheet.
What are the voltage and current tolerances for EL156?
When the current tolerance doesn't exceed the voltage tolerance, supplying the heaters with constant current wouldn't drive the heaters' voltage out of spec.
 
Current tolerance is always tighter than voltage tolerance, because of temperature effect. I don't think you have grasped this. As a rough guide, current supply needs to be twice as accurate as voltage supply, even for valves designed for current supply.

In the circuit I posted, by modifying the resistor, I have control on the current supplied. I can adjust the current so that the voltage on one of the tubes, at least, will be 6.3V +/- 0.1% or even better. Should there will be more than 0.5% difference in voltage between the two tubes, I can supply each with its' own constant current regulator.

Anyhow, so far no one replied my initial question: "will this circuit operate properly"?
 

tomchr

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2009-02-11 12:58 am
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Updated schematic is attached.

I don't see any reason why that should not work. The output current will depend on the temperature of the pass transistor, though, as the LM317 will maintain 1.25 V across the Vbe + the resistor. As the temperature of the NPN rises, the Vbe will drop (-2 mV/deg C), hence the current will increase slightly. If this is acceptable to you, then great. If not, I suggest looking at an op-amp based solution. One op-amp + NPN + 1~2 resistors = current source (see attached).

Note that the resistor in your new circuit should be calculated from R = (1.25 - Vbe)/I and not the equation in the LM317 datasheet. So it's more like R = 0.6/I or 315 mOhm in your case.

I tested constant current vs constant voltage on 6J5 and 300B and was not able to tell a difference. Some people swear by the constant current regulators, though. For me, personally, I'd opt for a voltage regulator with soft-start.

~Tom
 

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Thank you very much, Tom.

I don't see any reason why that should not work. The output current will depend on the temperature of the pass transistor, though, as the LM317 will maintain 1.25 V across the Vbe + the resistor. As the temperature of the NPN rises, the Vbe will drop (-2 mV/deg C), hence the current will increase slightly. If this is acceptable to you, then great.


I believe the transistor's temperature will be stabilized after a while, probably after about 10 minutes, or so. I can adjust the current and voltage when the temperature is stable. Unless the transistor's temperature doesn't stabilize in reasonable time.


If not, I suggest looking at an op-amp based solution. One op-amp + NPN + 1~2 resistors = current source (see attached).


A viable solution. It takes a stabilized voltage feeding the op-amp.


Note that the resistor in your new circuit should be calculated from R = (1.25 - Vbe)/I and not the equation in the LM317 datasheet. So it's more like R = 0.6/I or 315 mOhm in your case.


Thanks, that's obvious. 3 1Ohm resistors in parallel plus another one for accurate adjustment should do it.

I tested constant current vs constant voltage on 6J5 and 300B and was not able to tell a difference. Some people swear by the constant current regulators, though. For me, personally, I'd opt for a voltage regulator with soft-start.

A schematic for soft start voltage regulator would be appreciated. I'd like the voltage getting it's final value in about 1 minute.