EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: SK_Caterpilar_SK on November 10, 2018, 12:07:26 am
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So recently I repaired a old vacuum tube voltmeter. And it has a weird bulb. Apparently its called a "variator" or some sort of thing. When I unscrew it its marked as "NARVA 3-9V 1,4A". According to the schematics (which I will include) its used as a very simple constant current regulator for heating the cathodes of the tubes. Seems like its basically a hot piece of wire just changing its rezistance based on teperature. And the reason im worried is that the wire inside looks almost crackled it has lighter noticeable spots and it looks original from the 1950s.
I tryed to find a replacement "bulb" and I did not. So im wondering if its possible to replace the "hot piece of wire" with a semiconductor current source that does not require much of aditional power to work. AC current source in short. Simply cant find anything at all. Any help is very much appreciated.
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That would be an https://en.wikipedia.org/wiki/Iron%E2%80%93hydrogen_resistor
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That would be an https://en.wikipedia.org/wiki/Iron%E2%80%93hydrogen_resistor
Thanks. Seems youre right. But the problem is that I cannot buy any of thhoose and I heard that you could replace it with some sort of circuitry with semiconductors. It would be nice If I could just replace it with a some sort os a circuit. If it would have been DC powered I would just replace it no question but its AC...
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That would be an https://en.wikipedia.org/wiki/Iron%E2%80%93hydrogen_resistor
Thanks. Seems youre right. But the problem is that I cannot buy any of thhoose and I heard that you could replace it with some sort of circuitry with semiconductors. It would be nice If I could just replace it with a some sort os a circuit. If it would have been DC powered I would just replace it no question but its AC...
Is it heater supply? You can use DC power
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Is it heater supply? You can use DC power
+1 I was just going to comment the same.
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Youre right. I could but the tranformer has a heater winding, A 50V winding, And a 300V winding. How can I regulate it? That leaves 0 options other than a rezistor which is really not ideal for this and unacceptable
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It's basically a PTC.
Interesting are "Glüwo" and "NARVA", both of which were light bulb manufacturers in Eastern Germany.
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i think there are thermal tubes you can use that act as a CC source but I don't think their AC, they regulate by the tempco of their emission coating, more advanced then the hydrogen barretter, in contrast to most vacuum tubes that use emission coatings that are considerably more temperature stable.
You should try to stick to old school components because your thermal design will be easier, the meter probobly gets unacceptably toasty inside for using modern semiconductors. Measure chassis temperature before making the mod so you know how to overrate.
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+ SK_Caterpilar_SK
Could you give the type number of the valve that requires the heater supply. Am I correct in thinking that the heater is 6.3V at 1.4A AC. Is it also correct that the heater winding on the transformer is 9V AC.
Based on the above assumptions, I have done a circuit to provide a constant current for the valve heater, but to save unnecessary work can I describe it to you to see if you think it would be acceptable?
The circuit has four small rectifier diodes, similar to 1N400x types, one SOT223 MOSFET, 1 BJT transistor (BC546 etc) and two resistors. The rectifier diodes could be surface mount as could the BJT.
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As your schematic shows, these regulators commonly known as ‘ballast tubes’, can run on A.C. because they are just specialized resistors. Amperite was a company in the U.S. that made most of them and here is an article describing their use:
https://www.radiomuseum.org/forum/ballast_tubes_in_filament_regulation_of_vfo_tubes.html (https://www.radiomuseum.org/forum/ballast_tubes_in_filament_regulation_of_vfo_tubes.html)
The article also discusses solid state replacements and you could make your own. Where their purpose is to produce a steady voltage to a tube heater by regulating the current that feeds the tube (and also produce a ‘soft start’), you could just rectify the output from the 50 volt transformer winding and use a 3-terminal regulator that gives the proper voltage to the tube heater. You can test it by using an adjustable D.C. supply connected to the left side of the regulator marked E6 on your schematic (with the regulator unscrewed) and with a voltmeter across the heaters, adjust the supply until the heater voltage is correct. This will tell you what you need for the built-in regulator voltage.
There still are some of the Amperite ballast tubes on Ebay if you could figure out what their ratings are. There are also very small inexpensive power supplies of various voltages on Ebay if you don't want to build one. Here is an example.
https://www.ebay.com/itm/Mean-Well-RS-15-5-Enclosed-Switching-5-Volt-3-Amp-AC-DC-Power-Supply-5V-3A-15W/112158655353?epid=631063195&hash=item1a1d2d4379:g:TwgAAOSwJ7RYUaOm:rk:10:pf:0 (https://www.ebay.com/itm/Mean-Well-RS-15-5-Enclosed-Switching-5-Volt-3-Amp-AC-DC-Power-Supply-5V-3A-15W/112158655353?epid=631063195&hash=item1a1d2d4379:g:TwgAAOSwJ7RYUaOm:rk:10:pf:0)
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Any MCU easily does the job.
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Build a bog standard constant current circuit (couple of diodes, bipolar transistor, emitter resistor, you know the drill), and wire it between the + and - terminals of a bridge rectifier, connect the AC terminals to the rest of the circuit.
Regards, Dan.
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Should a tube heater be driven with a constant current? I'd have thought the tungsten filament would have a positive TC, making constant drive undesirable.
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mikerj - "Should a tube heater be driven with a constant current? I'd have thought the tungsten filament would have a positive TC, making constant drive undesirable. "
If you check the reference in my post above, you will see how these constant current ballast tubes do a pretty good job to control a tube heater voltage. These ballast tubes were designed around the 1930s long before solid state voltage regulators were available to do a much better job.
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Precision vacuum tube circuits regulate the heater voltage or current to prevent offset voltage changes.
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mikerj - "Should a tube heater be driven with a constant current? I'd have thought the tungsten filament would have a positive TC, making constant drive undesirable. "
If you check the reference in my post above, you will see how these constant current ballast tubes do a pretty good job to control a tube heater voltage. These ballast tubes were designed around the 1930s long before solid state voltage regulators were available to do a much better job.
I don't doubt that. I was responding to the post above mine, suggesting the OP builds a solid state constant current drive for the heaters. Constant current drive into a heater with a positive TC doesn't provide great regulation of it's output compared to voltage drive.
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Maybe a couple of depletion mode mosfets acting as a bidirectional current source?
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out of curiosity, what is the chassis temperature in the region you have available?
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Maybe a couple of depletion mode mosfets acting as a bidirectional current source?
Or place a DC current regulator inside of a bridge rectifier. Or an AC shunt voltage regulator which is just two back-to-back zener diodes or a zener diode inside of a bridge rectifier could be used. There are lots of options including changing the heat voltage to DC making regulation easier.
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Uff...I was away for a loong time.. The thing is not getting so unnaceptably hot inside. The chassi is huge and has a lot of ventilation. The tubes are EF800 (two) and one ECC83 (12AX7 as you may). The tubes are being run on low voltage purposely, the barreter was there because of the voltage fluctuations back in the days of 220V..you know your grandmother threw the oven on and all of the sudden the light were dimmer and your voltage in the wall was all of the sudden 200V. Voltages are relatively low on the tubes. The ECC83 has its filaments in parallel. The whole point of it was to reduce innacuracies in the masurement if and when the line voltages was fluctuating. It also has a dual diode for the AC range. That is a EAA91.
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There are 4 tubes with filaments. EF800 x2, ECC83(paralel filaments), 6B32 diode for the AC range.
The schamtic of the entire VTVM is in one of the posts above (I believe)
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not more than 50C. Warm but not toasty. Just like your average sauna.
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These iron filament, hydrogen filled AC constant current "barreters" are very old, they were first used in 1899 by Walther Nernst to regulate the current through his Nernst Glower Lamps and were initially made in 250 and 500mA models. Due to power dissipation, they could only sustain a voltage drop of 5-10v. Below this range they ceased to current limit and tended to behave like resistors, above this voltage range the iron filament started to glow red hot and eventually melted, open-circuiting the device permanently.
To see an original 1901 model, follow this URL...
https://www.lighting-gallery.net/gallery/displayimage.php?album=2434&pos=149&pid=69895 (https://www.lighting-gallery.net/gallery/displayimage.php?album=2434&pos=149&pid=69895)
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These iron filament, hydrogen filled AC constant current "barreters" are very old, they were first used in 1899 by Walther Nernst to regulate the current through his Nernst Glower Lamps and were initially made in 250 and 500mA models. Due to power dissipation, they could only sustain a voltage drop of 5-10v. Below this range they ceased to current limit and tended to behave like resistors, above this voltage range the iron filament started to glow red hot and eventually melted, open-circuiting the device permanently.
To see an original 1901 model, follow this URL...
https://www.lighting-gallery.net/gallery/displayimage.php?album=2434&pos=149&pid=69895 (https://www.lighting-gallery.net/gallery/displayimage.php?album=2434&pos=149&pid=69895)
great website
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Yeah I found out about that. The thing is that the filament looks like it had its days gone and has not much left. So Im either looking for a direct replacement or designing a semiconductor circuit as the replacement for the bulb
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I’d say they were trying to regulate the valve heater voltage and the easiest way to do that back in the day was to regulate the current, and as a consequence the voltage was more or less regulated. Modern solution is simply regulate the voltage.
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You can still buy NOS barretters.
Over here they cost 5-20PLN or (with current exchange of 3.75PLN/USD) 1.3-5.3USD each.
Depends on model.
Since Poland is next to Slovakia you may find similar stuff in your country or talk with sellers to send it to Slovakia.
The problem is finding one which directly replaces your device.
Mostly 1A ones are avaiable.
Perhaps you could put together a working thing using such 1A barretteralong with parallel resistor to bump the current.
Or parallel a 1A barretter with 400mA one. It's possible to find a pair of such devices.
Running a modern day regulator shouldn't be a big problem.
Others have already mentioned the regulated DC way.
Since barretter already "eats" couple volts anyway there's already sufficient headroom for a regulator.
You could do either constant voltage or constant current.
Constant current would be easier since we already know the value to be at precisely 1.4A.
Default resistor network will do the rest.
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That would be the best solution. Were technically neighbours so where can I buy it? Cause on ebay its unobtanium and very...VEEEERY expensive.. Narva 6-9 V 1,4A.
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Search on "Allegro" under the name "bareter".
I could even find those on a flea market.
Bit more expensive but still cheap.
The best solution would still be the DC semiconductor-based regulator.
Barretters have a limited lifespan.