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Old 18th November 2010, 02:26 PM   #11
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Quote:
Originally Posted by DF96 View Post
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.

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Originally Posted by DF96 View Post
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.
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Old 18th November 2010, 06:33 PM   #12
DF96 is offline DF96  England
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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.
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Old 18th November 2010, 06:46 PM   #13
el156 is offline el156  Portugal
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Hi joshua!
May i ask you wich OPTs are you going to use with your EL156?
Thanks
Silvino
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Old 18th November 2010, 06:48 PM   #14
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Quote:
Originally Posted by DF96 View Post
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.
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Old 18th November 2010, 06:49 PM   #15
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Hi joshua!
May i ask you wich OPTs are you going to use with your EL156?
Thanks
Silvino
Either Lundhal or Tamura, I haven't decided yet.
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Old 18th November 2010, 07:42 PM   #16
DF96 is offline DF96  England
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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.
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Old 18th November 2010, 07:51 PM   #17
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Quote:
Originally Posted by DF96 View Post
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"?
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Old 18th November 2010, 07:58 PM   #18
tomchr is offline tomchr  United States
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Originally Posted by Joshua_G View Post
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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Last edited by tomchr; 18th November 2010 at 08:12 PM.
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Old 18th November 2010, 08:31 PM   #19
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Thank you very much, Tom.

Quote:
Originally Posted by tomchr View Post
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.


Quote:
Originally Posted by tomchr View Post
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.


Quote:
Originally Posted by tomchr View Post
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.

Quote:
Originally Posted by tomchr View Post
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.
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