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Old 31st May 2005, 01:47 PM   #1
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Default Heater Supply using LM3X7 current source

Hello,

I am a tube-noob. And probably asking some stupid question.

I have a 6DJ8 running in a low voltage application. (15V) In this application, heater supply is diverted from the 15V. I am using LM337 in current source setup to give me about 300mA like the tube spec sheet tells me.

I believe I am feeding the tube with that much current should be OK with the tube. But then I checked the voltage drop across the tube heater; which is running around 11.5V or so.

I feel the voltage drop is coming from the heated filament which is compensating for the heat emission to the manageable level. But the I am no specialist. I am pretty stumped at this time.


Thanks in advance,

T
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Old 31st May 2005, 04:00 PM   #2
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Default Re: Heater Supply using LM3X7 current source

Quote:
Originally posted by Tomo
But then I checked the voltage drop across the tube heater; which is running around 11.5V or so.
Hot or cold?
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Old 31st May 2005, 10:51 PM   #3
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I don't understand this. If you genuinely have 11.5V across a 6DJ8, the poor thing should be lit up like a lightbulb. Surely you would notice if this was so instead of a dull orange glow? Are you sure you are measuring across the heater pins with the valve in place?
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Old 31st May 2005, 11:30 PM   #4
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Hi,

The tube is running at 11.5V HOT. But then it is fairly dark glow to me.

The important thing is that there shouldn't be more than 300mA (266mA) across the current source which is connected in series with the tube heater filament. (Voltage drop across the current source is 3.5V. So 15V total.)

I am just wandering (I suppose) if I can power the filament with a current source instead of a voltage source.

TY, T
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Old 31st May 2005, 11:58 PM   #5
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Quote:
Originally posted by Tomo
I am just wandering (I suppose) if I can power the filament with a current source instead of a voltage source.
I am running a LM317 CCS for each of my 6922, set at 300 mA, very much like your 6DJ8's, and all is fine and dandy.

Don't take this the wrong way....are you actually measuring the 11.5V across the filament or just assuming that from your 3.5V measurement? And your current set resistor for the 317 is about 4 ohms?
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Old 1st June 2005, 02:33 AM   #6
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Hey,

Yeah, I did use the RADIOSHACK multimeter to measure both locations. The filament at about 11.5V and current source at about 3.5V. (voltages across them)

I used 4.7ohms with LM337. So 266mA to be exact.

Was I being paranoid?

Tomo
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Old 1st June 2005, 03:19 AM   #7
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The 6DJ8 has a 6.3V heater. If you're measuring 11.5V across the heater, and the poor thing isn't glowing extremely brightly (for a small signal valve) there most certainly is something wrong.
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Old 1st June 2005, 04:23 AM   #8
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Humm,are you sure it's a 6DJ8,and not a 12A_7? (or some other 12V tube?)
Seems like a 6.3V filament wouldn't hold up long at 11+V
I've blown filaments right out of a quad of (NOS!) 807's with 12-18V
*grumble....grumble*
Always double check your heater wiring!
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Old 1st June 2005, 07:01 AM   #9
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Hi,

It ain't glowing up a Halogen tube. Orange. I see Orange. You guys seem to misunderstand. Please forgive me.

The current is fixed to 266mA no matter whatever the voltage drop the tube has. Since LM337 is temperature compensated, I think heater supply is still getting 266mA or so when the heater is hot.

Few of my friends and I have a hypothesis. The resistance across the heater increase as it heats up. The voltage drop will increase due to it, but heat emission is reduced since heat is V^2 / R.

Furthermore, "I" think the amount of the heat emission is what determines whether the tube die or not. So voltage or current alone would not matter. (100V and 10mA should be ok if the heater is rated at 1W.)

These make sense to me, but I don't know the tube guys say. So back where we are now.

Thanks again,

T

P.S. The writing on the tube saids it's 6DJ8. The box told me the same, too. Nonetheless, I will check the pin 9 over the weekend. If it is 12AU7 or something, you get 6V there right?
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Old 1st June 2005, 07:09 AM   #10
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The resistance of the heater will increase as it heats up, however the heater voltage requirement is based on the steady state (i.e. hot) condition of the heater. It should be drawing 365mA at 6.3V, which is around 17 ohms hot resistance. This resistance will be less when it cold.

When fed by a constant current supply, the voltage will rise slowly, increasing as the heater wire gets hotter and finally settling at 6.3V. Provided your measurements are correct, you are getting around 43 ohms hot resistance. Either your measurements are wrong, you have a different valve, or the 6DJ8 is messed up.

Quote:
Furthermore, "I" think the amount of the heat emission is what determines whether the tube die or not. So voltage or current alone would not matter. (100V and 10mA should be ok if the heater is rated at 1W.)
You misunderstand. For that to happen, the resistance of the heater would have to change from a few tens of ohms when cold to 10K when hot. This does not happen, no heater wire has a temperature coefficient that large.
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