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    Tube filaments on 230v? What do I need to know?

    Hi I have a UCL82 and four UL84 tubes. The UCL82 filament is 100mA at 50V and the UL84 are 100mA at 45V.

    Now, can I connect all the filaments in series and connect them directly to the mains without any transformer or resistor?? (230v)
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    Re: Tube filaments on 230v? What do I need to know?

    connect them directly to the mains without any transformer or resistor?? (230v)
    Surely you can. If you find too much hum, you may have to switch to DC. Running the filaments below the rated voltage may add life to the tubes (with only a small loss in performance).

    If you are worried about the high surge current at turn on, perhaps a NTC thermistor can also be included in the series connection.


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    Re: Tube filaments on 230v? What do I need to know?

    Quote Originally Posted by c_mitra View Post
    Surely you can. If you find too much hum, you may have to switch to DC. Running the filaments below the rated voltage may add life to the tubes (with only a small loss in performance).

    If you are worried about the high surge current at turn on, perhaps a NTC thermistor can also be included in the series connection.
    I remember old television sets tubes filaments glowed brightly when the tv was switched on. Then they shut off and after a few seconds went red glow (normal operation). This is the case in almost all televisions I have seen. I do not think they worried too much for the inrush current when the filaments are cold. And I have never seen a tube filament broken from that era. So I guess since they do it in commercial sets that were switched on/off everyday, it is kind of ok to just leave it simple without any thermistor in place. Instead, I think I will include a fuse (100-200mA@250v) in series with the filaments, just in case.
    MAybe the fuse current should be higher (how much?) to deal with the inrush current.

    The TX will be CW only. Do you think that hum will cause significant problems in the "tone" of CW?

    Once I remenber I had hum in a transistor transmitter, but when I switched my receiver to CW, the hum was not noticeable.
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    Re: Tube filaments on 230v? What do I need to know?

    I wouldn't connect them directly across the mains, the surge current would probably kill them very rapidly. The idea of fixed current/various voltage heaters was to eliminate the need for a separate heater supply by joining them in series and using the voltage drop as appropriate to get the required heater power. However, as far as I know, in all circumstances there was still a power resistor in line to limit the surge current and an NTC to help it.

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    Re: Tube filaments on 230v? What do I need to know?

    Quote Originally Posted by betwixt View Post
    I wouldn't connect them directly across the mains, the surge current would probably kill them very rapidly. The idea of fixed current/various voltage heaters was to eliminate the need for a separate heater supply by joining them in series and using the voltage drop as appropriate to get the required heater power. However, as far as I know, in all circumstances there was still a power resistor in line to limit the surge current and an NTC to help it.

    Brian.
    I could reduce the number of tubes, which will require 185v for the filaments, and use a power resistor for the rest of the voltage drop. What current and value resistor should I use for the purpose?
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    Re: Tube filaments on 230v? What do I need to know?

    I do not think they worried too much for the inrush current when the filaments are cold. And I have never seen a tube filament broken from that era...
    I am still using old style filament lamps for household lighting and they go bad frequently. I am not really sure about the failure mode but the heat shock due to the frequent turn on/off is somewhat cumulative. But I do not understand why the filament will glow brightly for a few seconds, then turn off and after a few seconds went into red glow (normal operation) mode. Most tubes use a filament voltage of 6V (6.3V to be exact and I do not know the reason for this odd choice) and most use a thoriated indirectly heated cathode that do not need high temp for a decent emission. Also, at lower voltage the filament will be thicker and stronger.

    As you already have the tubes, it is very easy to measure the room temp resistance of the filaments with a multimeter and estimate the inrush current. It is nowhere as serious as a large capacitor.



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    Re: Tube filaments on 230v? What do I need to know?

    A cold filament is exposed to extreme surge current by direct hookup to mains.

    On the other hand a transformer self-limits current output, because voltage starts to drop when you draw more than its spec current. It's less stressful on a cold filament.

    I've seen schematics which have several tube heaters in series, although those were 6v or 12v types.


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    Re: Tube filaments on 230v? What do I need to know?

    These are a fairly rare kind of tube used in some European countries and maybe not familiar to the rest of the World.

    The tube types beginning with 'U' were somewhat unusual, the concept was that instead of using different heater voltages and currents to get the required heating power, they all ran at 100mA and their individual heater voltages were different. The idea was that they could all be wired in series to minimize the final drop needed to run them across the incoming line supply and as such they had a better insulating layer between heater filament and the internal cathode wall. Peak voltage between heater and cathode could exceed 500V which would be risky for normal 6.3V or 12V heated types.

    The dropper resistor calculation is simple Ohms law. Take the AC line voltage, subtract the sum of all the heater voltages and divide by the current (0.1A). For longest life, look to make up a few Ohms of the final resistance with the 'hot' resistance of an NTC thermistor so it works as a surge limiter, having a higher resistance when cold.
    The power drop is also a simple Ohms Law but expect several Watts, in almost all cases there was a fat wire-wound power resistor in the heater circuit rated at as much as 20W. The cold resistance of the heaters will be much lower than at normal operating temperature, hence the surge in current and bright flash when first powered on so use the manufacturers rated current and voltage rather than measured resistance and expect at least twice that current at cold when considering surge limitation.

    Brian.
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    Re: Tube filaments on 230v? What do I need to know?

    Quote Originally Posted by betwixt View Post
    These are a fairly rare kind of tube used in some European countries and maybe not familiar to the rest of the World.

    The tube types beginning with 'U' were somewhat unusual, the concept was that instead of using different heater voltages and currents to get the required heating power, they all ran at 100mA and their individual heater voltages were different. The idea was that they could all be wired in series to minimize the final drop needed to run them across the incoming line supply and as such they had a better insulating layer between heater filament and the internal cathode wall. Peak voltage between heater and cathode could exceed 500V which would be risky for normal 6.3V or 12V heated types.

    The dropper resistor calculation is simple Ohms law. Take the AC line voltage, subtract the sum of all the heater voltages and divide by the current (0.1A). For longest life, look to make up a few Ohms of the final resistance with the 'hot' resistance of an NTC thermistor so it works as a surge limiter, having a higher resistance when cold.
    The power drop is also a simple Ohms Law but expect several Watts, in almost all cases there was a fat wire-wound power resistor in the heater circuit rated at as much as 20W. The cold resistance of the heaters will be much lower than at normal operating temperature, hence the surge in current and bright flash when first powered on so use the manufacturers rated current and voltage rather than measured resistance and expect at least twice that current at cold when considering surge limitation.

    Brian.

    Ok so for 3x ul84 and 1x ucl82 this will be
    3x45 + 50 = 185v
    230 - 185 = 45v
    45v/0.1A = 450

    So the power resistor in series with the filaments should be 450 ohms?
    Professional engineering is the top, but amateur engineering is more fun.
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    Re: Tube filaments on 230v? What do I need to know?

    Correct - and expect continuous dissipation of about 4.5W (V * I) so use a 10W resistor for safety and to ensure reliable surge currents.

    Brian.
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    Re: Tube filaments on 230v? What do I need to know?

    Quote Originally Posted by betwixt View Post
    Correct - and expect continuous dissipation of about 4.5W (V * I) so use a 10W resistor for safety and to ensure reliable surge currents.

    Brian.
    How about if I use a thermistor and all 5 tubes? (4 x UL84 + 1 x UCL82). Their filament voltages add for 230v but I guess it is ok to run a bit lower. So can I use a thermistor in place and of what characteristics?
    I have quite a few of them from old SMPSUs.
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    Re: Tube filaments on 230v? What do I need to know?

    Quote Originally Posted by neazoi View Post
    How about if I use a thermistor and all 5 tubes? (4 x UL84 + 1 x UCL82). Their filament voltages add for 230v
    Don't do that. Filament wiring isn't design for save isolation at 230V against the chassis.



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    Re: Tube filaments on 230v? What do I need to know?

    Quote Originally Posted by volker@muehlhaus View Post
    Don't do that. Filament wiring isn't design for save isolation at 230V against the chassis.
    Sorry I did not understand well what you are saying. I am not going to connect anything to the chassis. A thermistor in series with the 5 tubes. You worry for isolation because the thermistor resistance is low compared to a power resistor replacing a tube?
    Professional engineering is the top, but amateur engineering is more fun.
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    Re: Tube filaments on 230v? What do I need to know?

    The wiring of the filament isn't sufficiently isolated for 230V mains voltage.



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    Re: Tube filaments on 230v? What do I need to know?

    Quote Originally Posted by volker@muehlhaus View Post
    The wiring of the filament isn't sufficiently isolated for 230V mains voltage.
    These are U type tubes.
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    Re: Tube filaments on 230v? What do I need to know?

    These are U type tubes.
    Yes, you want to read the data sheets. They have 200 V cathode to heater voltage limit.


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    Re: Tube filaments on 230v? What do I need to know?

    Quote Originally Posted by FvM View Post
    Yes, you want to read the data sheets. They have 200 V cathode to heater voltage limit.
    Since they are in series, it wouldn't matter would it? Because the voltage at the ends of each filament is 45-50v anyway. This is what I am talking about.
    Maybe it's a good idea to do what Brian suggested, to use a 450R 10W resistor and remove one of the tubes. This will bring the voltage applied to the tubes to less than 200v and limit the surge current as well.
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    Re: Tube filaments on 230v? What do I need to know?

    Quote Originally Posted by neazoi View Post
    Since they are in series, it wouldn't matter would it?
    It matters. What is your cathode voltage to GND? For one of your tubes onnected in series, the filament potential to GND will be 230V * sqrt(2) peak. What is the voltage between filament and cathode then?


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    Re: Tube filaments on 230v? What do I need to know?

    You need to observe the voltage difference between each cathode and both heater terminals. The heater circuit sketched in post #1 would most likely exceed the voltage limits.

    A classical AC-DC radio has one end of the heater chain connected to neutral and also cathode potential near to neutral. The number of series connected Uxx filaments is respectively limited. You didn't yet tell about the intended circuit.



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    Re: Tube filaments on 230v? What do I need to know?

    Quote Originally Posted by FvM View Post
    You need to observe the voltage difference between each cathode and both heater terminals. The heater circuit sketched in post #1 would most likely exceed the voltage limits.

    A classical AC-DC radio has one end of the heater chain connected to neutral and also cathode potential near to neutral. The number of series connected Uxx filaments is respectively limited. You didn't yet tell about the intended circuit.
    It is intended as a lightweight and cheap CW transmitter. Since I already have these tubes... But I was thinking of completely isolate the chassis from the circuit anyway (chassis floating) because I do not feel safe to mess around with mains. It is done on old TVs with Pxx type tubes and power resistors. It is done in old cheap radios with Uxx tubes and power resistors. I believe they used Uxx tubes in radios because these radios had fewer tubes than a TV (usually 4-5 max). I have seen none of these TVs or radios fail due to tube filaments, the flyback transformer usually failed.
    Professional engineering is the top, but amateur engineering is more fun.
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