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Old 7th April 2004, 05:34 AM   #1
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Default Heater Voltage Range, Hi or Low

Hi,

I have some questions regarding tube heaters.

1. What happens if the heater voltage is too low?
2. What happens if the heater voltage is too high?
3. AC or DC, which is generally preffered?
4. Advantage/disadvantage of low heater voltage?
5. Advantage/disadvantage of high heater voltage?
6. If my tubes are rated at 6.3V (AC or DC) +/- 0.6V, then, the operating range is from 5.7V up to 6.9V correct?

Thanks in advance!

Regards,
JojoD
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Old 7th April 2004, 05:51 AM   #2
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1. Reduced emission. Not a problem with small tubes but watch out with power tubes or those near the end of their life ("heater-sensitive cathode").
2. Reduced life. Extra heat = more boiled-off stuff.
Which reminds me, has anyone seen a 6W6 without a shiny spot right above the heater? All mine glow yellow-white (approx. 2500F) at normal voltage too!
3. AC for convienience, DC for hum-sensitive tubes or circuits. Phono preamp for instance. In 95% of the cases, a balanced AC supply, maybe with a bit of positive bias, is more than sufficient.
4. Longer life, lower transconductance and perveance (er, current capacity essentially). [1]
5. Shorter life, should have a good bit of current available though. [1]
6. Yes

[1] The way a cathode works, emission increases exponentially towards a limit, such that at absolute zero, no electrons are emitted; at room temperature, a few escape. At glowing temperatures, there are enough electrons being "boiled off" to make use of. Due to the exponential curve, you gain very little emission by increasing temperature past where it really gets going. Like a silicon diode: it can be assumed to have a .7V forward drop almost no matter what the current. Replace the words "diode" with "tube cathode", "forward drop" with "emission" and "current" with "temperature".

Tim
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Old 7th April 2004, 06:27 AM   #3
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Excellent response from Sch3matic1c. I would just add that, if the power supply to your house is prone to large fluctuations in voltage, then having a slightly lower average heater voltage (say -7%) for your tubes may help protect them from damage in cases where the mains voltage becomes abnormally high.
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Old 7th April 2004, 10:01 AM   #4
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OK, so lower heater voltage is better. If my tubes are 6.3V +/-0.6V heater rating, then it would be best to be just at about 5.7V.

But does lower heater voltage affect the sound? More distortion, noise? Or would it be just fine.

To sum it all up, all I want is for my tubes to be happy, sound well, have a longer life, then I would really be happy.

I know that there are a lot of factors to consider, but I will start with the heaters.

Cheers,
JojoD
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Old 7th April 2004, 10:06 AM   #5
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The tolerance built into heater specification is to cater for mains variations and other factors. I wouldn't worry about it too much. Just arrange that the nominal voltage appears on the valve pins. A surprising amount of voltage gets dropped through heater wiring and valve sockets.
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Old 7th April 2004, 11:04 AM   #6
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Quote:
Originally posted by JojoD818
OK, so lower heater voltage is better. If my tubes are 6.3V +/-0.6V heater rating, then it would be best to be just at about 5.7V.

But does lower heater voltage affect the sound? More distortion, noise? Or would it be just fine.

To sum it all up, all I want is for my tubes to be happy, sound well, have a longer life, then I would really be happy.

I know that there are a lot of factors to consider, but I will start with the heaters.

Cheers,
JojoD
Hi

NEVER go below 6 V, it will shorten tube life.

A little bit below 6,3 is OK, eg 6.2V.

I learned these lessons from a tube design engineer from Mullard.

cheers
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Old 7th April 2004, 12:49 PM   #7
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Hi,

Usually filament voltage is specified as 10% for regular tubes. But I was also told long time ago that too low filament voltage will shorten tube life either. Did that Mullard engineer also told you why Guido? Curious about it.

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Old 7th April 2004, 04:58 PM   #8
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Guido, Pjotr,

Wait a second, now I'm learning new things from you guys. All this time I thought (and practice) lower tube heater voltage just to extend the tube's life, but now you say that lower heater voltage will also shorten it's life.

Maybe the post of dhaen is right on target. Maybe I should aim at almost exactly the specified tube heater voltage on the datasheet?

JojoD
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Old 7th April 2004, 11:23 PM   #9
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Hi,

Quote:
NEVER go below 6 V, it will shorten tube life.
Cathode poisoning will occur.

Quote:
A little bit below 6,3 is OK, eg 6.2V.
If using regulated DC you can use 6VDC on a 6.3V heater with no problem at all and with doubled life expectancy of the cathode.

However, other than the convenience of commonly available 6VDC regulators the life expectance is only as good as the getter is still functional.
The getter is usually the first part to be exhausted. Without one no vacuum tube works correctly.

Quote:
I learned these lessons from a tube design engineer from Mullard.
Ir. van Mossevelde ?

Quote:
Maybe I should aim at almost exactly the specified tube heater voltage on the datasheet?
Generally speaking that would be the wisest.

Cheers,
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Old 8th April 2004, 05:47 PM   #10
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Can it be that the best solution for stable tube heater voltages is a regulator? I mean, a regulated dc supply for the heaters will mean it's safe from mains fluctuation and a more stable supply right?
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