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
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
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. 2500øF) 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
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. 2500øF) 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
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.
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
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
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.
JojoD818 said:
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
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.
Cheers 😉
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.
Cheers 😉
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
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
Hi,
Cathode poisoning will occur.
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.
Ir. van Mossevelde ?
Generally speaking that would be the wisest.
Cheers,😉
Cathode poisoning will occur.
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.
Ir. van Mossevelde ?
Generally speaking that would be the wisest.
Cheers,😉
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?
My opinion is that unless the mains variations are greater than normal: say >+-8%, it is unnecessary.
Of course there is no reason why you shouldn't, just as an engineering exercise 😉
I'm sure some will disagree with me 🙂
Of course there is no reason why you shouldn't, just as an engineering exercise 😉
I'm sure some will disagree with me 🙂
Hi,
Regulators have an added advantage; they limit inrush current.
For anything at preamp level I always use DC regulated supplies.
In some situations even the input stages of an amplifier may benefit form regulation as well.
Cheers,😉
Regulators have an added advantage; they limit inrush current.
For anything at preamp level I always use DC regulated supplies.
In some situations even the input stages of an amplifier may benefit form regulation as well.
Cheers,😉
...IF you design it with current limiting. A 7806 has no internal current limiting that I am aware of and is easily popped under say, a short circuit. I should know.. 🙄
Of course, the low cold resistance doesn't affect anything anyway.
Tim
Of course, the low cold resistance doesn't affect anything anyway.
Tim
Here's my little circuit for dc filament supplies. It's 'low dropout', current limiting, rugged and easily scalable for a wide range of output currents by changing the current sampling resistor value & some of the transistor current ratings, plus puts out 5.8-6.1 VDC into load using a common 1N4735B zener as a reference.
Attachments
Hi,
I suppose it all depends on which one you pick but I found the Fairchild range very reliable:
The MC/LM series is good for 1A:
From their datasheet:
"The MC78XX/LM78XX/MC78XXA series of three terminal positive regulators are available in the TO-220/D-PAK package and with several fixed output voltages, making them useful in a wide range of applications.
Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible.
If adequate heat sinking is provided, they can deliver over 1A output current.
Although designed primarily as fixed voltage regulators,these devices can be used with external components to obtain adjustable voltages and currents."
Cheers,😉
I suppose it all depends on which one you pick but I found the Fairchild range very reliable:
The MC/LM series is good for 1A:
From their datasheet:
"The MC78XX/LM78XX/MC78XXA series of three terminal positive regulators are available in the TO-220/D-PAK package and with several fixed output voltages, making them useful in a wide range of applications.
Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible.
If adequate heat sinking is provided, they can deliver over 1A output current.
Although designed primarily as fixed voltage regulators,these devices can be used with external components to obtain adjustable voltages and currents."
Cheers,😉
Hi,
The 78xx have a wide spread on their current limiting. Found values from 1.5A to 2.7A for the TO220 parts. Thermal protection mechanism is very unreliable so good heat sinking is mandatory. If you want to limit the current more defined you can use the attached circuit provided you have sufficient input voltage. Current limit is app. 0.6/R1
Alternatively use the L200 from ST. You can set current limit and output voltage at wish. I am using this part and it has proven very reliable. I set the current limit to twice the nominal current needed to not slow down the start-up time too much.
Cheers 😉
The 78xx have a wide spread on their current limiting. Found values from 1.5A to 2.7A for the TO220 parts. Thermal protection mechanism is very unreliable so good heat sinking is mandatory. If you want to limit the current more defined you can use the attached circuit provided you have sufficient input voltage. Current limit is app. 0.6/R1
Alternatively use the L200 from ST. You can set current limit and output voltage at wish. I am using this part and it has proven very reliable. I set the current limit to twice the nominal current needed to not slow down the start-up time too much.
Cheers 😉
Attachments
Hi,
Low filament voltage poisoning cathod quikly, 5.7V low limit.
May be better damage tube by high voltage than spoil sound.
DC compromising sound. Filament wire has different potential
along its length. Cathod sense it and we here. 🙂 Esp. for
double triods as 12ax7. I think there are no problems with
AC suppl. Just any filament coil in PT must have middle ground
point. Each tube must have dedicated fill.coil. In my amps all
input tubes got AC and hum could'nt be hear.
Regs, Igor.
Low filament voltage poisoning cathod quikly, 5.7V low limit.
May be better damage tube by high voltage than spoil sound.
DC compromising sound. Filament wire has different potential
along its length. Cathod sense it and we here. 🙂 Esp. for
double triods as 12ax7. I think there are no problems with
AC suppl. Just any filament coil in PT must have middle ground
point. Each tube must have dedicated fill.coil. In my amps all
input tubes got AC and hum could'nt be hear.
Regs, Igor.
Duh ??? Heaters in indirect heated cathodes are double spiralled just to minimise hum when AC powered. So there is virtually no potential across the cathode from the filament voltage. Maybe if you put the cathode at high DC potential (like in SRPP) there can be some leakage if you ground the filament voltage.
Cheers 😉
fdegrove,
How does operating filament below 6V cause cathode poisoning ?
Cathode Poisoning: Keeping cathode at full temperature without drawing any current. Am I right ?
MB
How does operating filament below 6V cause cathode poisoning ?
Cathode Poisoning: Keeping cathode at full temperature without drawing any current. Am I right ?
MB
>>Heaters in indirect heated cathodes are double spiralled just to minimise hum when AC powered.<<
If only it were so. Most indirectly heated tubes have an Alumina coated FOLDED heater. Some types, like genuine NOS 7025s and Sovtek 12AX7LPSes, have the spiral wound hum bucking construction.
An experiment to test the effectiveness of hum bucking heaters is to build a RCA phono stage with Sovtek 12AX7LPSes and energize the heaters with AC. Will the residual hum level be acceptable?
If only it were so. Most indirectly heated tubes have an Alumina coated FOLDED heater. Some types, like genuine NOS 7025s and Sovtek 12AX7LPSes, have the spiral wound hum bucking construction.
An experiment to test the effectiveness of hum bucking heaters is to build a RCA phono stage with Sovtek 12AX7LPSes and energize the heaters with AC. Will the residual hum level be acceptable?