Author Topic: Strange AC current source..("variator"????)  (Read 703 times)

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Offline SK_Caterpilar_SK

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Strange AC current source..("variator"????)
« on: November 10, 2018, 11:07:26 am »
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.
 

Offline amyk

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Offline SK_Caterpilar_SK

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Re: Strange AC current source..("variator"????)
« Reply #2 on: November 10, 2018, 07:53:10 pm »
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...
 

Online 001

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Re: Strange AC current source..("variator"????)
« Reply #3 on: November 10, 2018, 09:15:36 pm »
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
 

Offline PA0PBZ

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Re: Strange AC current source..("variator"????)
« Reply #4 on: November 10, 2018, 09:37:11 pm »
Is it heater supply? You can use DC power


+1  I was just going to comment the same.
Keyboard error: Press F1 to continue.
 

Offline SK_Caterpilar_SK

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Re: Strange AC current source..("variator"????)
« Reply #5 on: November 11, 2018, 06:45:21 am »
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
 

Online Benta

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Re: Strange AC current source..("variator"????)
« Reply #6 on: November 11, 2018, 06:57:18 am »
It's basically a PTC.
Interesting are "Glüwo" and "NARVA", both of which were light bulb manufacturers in Eastern Germany.
 

Online coppercone2

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Re: Strange AC current source..("variator"????)
« Reply #7 on: November 11, 2018, 07:19:56 am »
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.
« Last Edit: November 11, 2018, 07:24:46 am by coppercone2 »
 

Offline spec

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Re: Strange AC current source..("variator"????)
« Reply #8 on: November 11, 2018, 08:36:31 am »
+ 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.
« Last Edit: November 11, 2018, 08:44:12 am by spec »
 

Offline ArthurDent

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Re: Strange AC current source..("variator"????)
« Reply #9 on: November 11, 2018, 09:09:22 am »
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

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
« Last Edit: November 11, 2018, 09:14:43 am by ArthurDent »
 

Offline SuzyC

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Re: Strange AC current source..("variator"????)
« Reply #10 on: November 11, 2018, 11:19:35 am »
Any MCU easily does the job.
 

Offline dmills

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Re: Strange AC current source..("variator"????)
« Reply #11 on: November 11, 2018, 12:09:14 pm »
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.
 

Offline mikerj

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Re: Strange AC current source..("variator"????)
« Reply #12 on: November 11, 2018, 11:10:08 pm »
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.
 

Offline ArthurDent

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Re: Strange AC current source..("variator"????)
« Reply #13 on: November 12, 2018, 04:06:44 am »
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.   
 

Online David Hess

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Re: Strange AC current source..("variator"????)
« Reply #14 on: November 13, 2018, 12:24:55 am »
Precision vacuum tube circuits regulate the heater voltage or current to prevent offset voltage changes.
 

Offline mikerj

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Re: Strange AC current source..("variator"????)
« Reply #15 on: November 13, 2018, 03:11:55 am »
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.
 

Offline orolo

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Re: Strange AC current source..("variator"????)
« Reply #16 on: November 13, 2018, 06:04:22 am »
Maybe a couple of depletion mode mosfets acting as a bidirectional current source?

 

Online coppercone2

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Re: Strange AC current source..("variator"????)
« Reply #17 on: November 13, 2018, 07:47:27 am »
out of curiosity, what is the chassis temperature in the region you have available?
 

Online David Hess

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Re: Strange AC current source..("variator"????)
« Reply #18 on: November 13, 2018, 11:48:55 am »
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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