How to build a variable 3volt 6amp power supply ?

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Old 05-06-20, 09:34 PM
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How to build a variable 3volt 6amp power supply ?

For an antique radio I need a 3 volt power supply that provides at least 6 amps (6 tubes at 1 amp each). I have a cheap 110 volt power supply but it produces a lot of static in all radios thus I need a battery powered system. I can get a 6 volt battery like the one shown in my attachment. To me it appears that it would have enough power to power the radio for approximately two hours (correct me if I am wrong).

My problem is how do I reduce the six volts down to a variable voltage of approximately zero up to the max of six volts. I would normally use three volts but would really like to have it variable. Can I do something as simple as putting a potentiometer or rheostat in the circuit and still maintain my required 6 amps that I need or do I need something more elaborate.

Your advice in detail would be greatly appreciated.

Cheers and stay well,

Don

 
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Old 05-06-20, 09:56 PM
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Welcome to the forums Don.
(I clarified your title)

What exactly are you trying to power..... the tube filaments ?
A tubed radio requires 6-12 volts to run the filaments.
Six tube filaments should only need about 2-3a of power.

That same radio will also require +/- 150v AC to run the high voltage on the tubes.
 
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Old 05-07-20, 01:34 PM
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Pete, you are correct that MOST tubed radios require only 2 to 3 amps of power but this case is totally different. I restore many old tubed radios and many of them are battery powered from the 1920's. In about 1925 they came out with the first AC tube, it would also work with DC, and it was called the 401_McCullough (actually made by Kellogg) and it required 1 amp at 3 volts. I have attached a picture of the tube and related specs. I am currently working on a Liberty Electric Six radio that someone at sometime had removed the AC power supply and converted it to battery powered...at least they attempted to do so. I can used my ARBE-II, or a supply that I have built, for the B and C power but neither one will put out over 3.25 amps thus I need a separate power supply for the filaments. As I mentioned I do have a power supply that will give me the required voltage and amps but it is cheap (China made) and generates noise/static in the radio that I am working on and all other radios in the area. In short, it will not work to power the filaments thus I would like to use a battery powered supply. It does not have to work for a long time...an hour or two would be fine.

If you are not familiar with the ARBE-II you can see it at https://www.arbeiii.com/

Hopefully I have explained my problem correctly and now you can see what I need. If you have a suggestion on how to build my needed power supply please reply with information.

Cheers and stay well,

Don
 
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Old 05-07-20, 06:26 PM
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My first radio used a 1T4 tube. Yes, one volt filament. Lots of ways to attack the power issue. One way is a simple bridge rectifier and filter cap, driven from a variac. Dial your voltage. .
 
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Old 05-07-20, 06:42 PM
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Your cheap power supply is a switching supply. Very effective but noisy.
Technically you just need an AC transformer of the right voltage and current.
transformer

I was thinking to reduce the battery voltage...... you could use a PWM (pulse width modulated) controller. Very cost effective and reliable but I'm not sure if that will be a noise problem too. Wouldn't know until you try one.
Search for PWM controllers

You could also build a basic mosfet regulator for reducing the voltage with variable control.
The first diagram using the IRF540 would very effective.
Voltage reducer circuits
 
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Old 05-08-20, 01:23 PM
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Telecom Guy, unfortunately I believe that my variac only goes up to approximately 5 amps which is not enough for the six tubes. This is why I wanted to use the battery that I mentioned in my first post but I still don't know if I could simply use a rheostat or potentiometer with it or not and if so I would have enough current once it is turned down to three volts.

Don
 
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Old 05-08-20, 02:42 PM
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No. Resistive dropping at multi amps is not great. Bad regulation, high losses, burn hazard, large $ resistors. Iím surprised ur variac is small. 10a are very common.
soooo, I would drive a 6.3v filament transformer with ur variac. A diode bridge and cap to follow. Low losses, good regulation.
 
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Old 05-08-20, 05:04 PM
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Telecom Guy, I just looked and my variac is 4.75 amps output. Really a nice little outfit but not enough output for what I currently need. Again, I will be drawing 6 amps at 3 volts (may need a bit more than 3 volts input because of the wire loss). Isn't there something like my electric train transformer that I had when I was eight to ten years old? Something variable between around 1.5 volts to 6 volts so I could possibly use it for other things. I think that a diode bridge and cap is beyond me building without a schematic.

Cheers and stay well,

Don
 
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Old 05-08-20, 05:38 PM
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The variac current will be only 1/20 what the 6v transformer will be. Check ebay for a 6.3v, 5 amp transformer. The primary will attach to ur variac. When the variac is at 50%, the filament transformer will output 3.2 volts. Still need a simple bridge and cap to make dc.
 
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Old 05-08-20, 07:25 PM
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I really don't need dc, those filaments will work with ac or dc. In fact they were originally made for ac. You are saying that once the transformer output is at 3.2 volts I will have enough current (6 amps) to power my tubes, correct?

Don
 
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Old 05-08-20, 08:04 PM
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Close. Size the 6,3v transformer secondary for your expected load.
 
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Old 05-08-20, 09:10 PM
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Check ebay for a 6.3v, 5 amp transformer.
I've been looking. Either very slim pickings or big bucks.
I left a link to a transformer in my previous post. That will be the most bang for the buck.
Many in stock and available "in country".
 
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Old 05-08-20, 09:59 PM
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PJmax, I looked at that one and in fact have it in my "watched" list on eBay. I would still like to figure out a way to do it with a rechargeable battery and a variable output so I could use it for other things. Maybe even a noise filter to use with my current power supply if that is possible.

Don
 
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Old 05-09-20, 05:46 AM
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For just dc, consider a simple adjustable linear voltage regulator. A LM317 or the like. For your current of 6 amps, you will need a high current rated transistor, perhaps a darlington. Iím sure the LM317 has an app note to schematic exactly what u want.
at these power levels, u will need a piece of aluminum to bolt the T03 package to.
 
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Old 05-09-20, 10:56 AM
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Just curious.... did you read post 5 ?
I left several links there for circuits. The bottom link is a very basic high current voltage reducer.
Linear regulators are easier to use but run very hot at high amperages.

Question..... why couldn't you put two tubes in series. That series pair would be 6v.
You could have three series pairs in parallel.

 
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Old 05-09-20, 11:37 AM
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This old radio was perhaps designed for an A battery for the filaments and a B battery for the plates? If so, I suspect all the heaters are in parallel.
but no, Iím not suggesting u try to find an A cell🤯
 
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Old 05-09-20, 11:42 AM
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Exactly..... a plate battery is required too.
 
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Old 05-09-20, 11:48 AM
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There is a cheap lambda 0 to 10v supply on the bay. 14a. But shipping is a killer. I have a couple lambdas. Diff outputs though. I really like the classic , made in NY brand.
 
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Old 05-09-20, 12:00 PM
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Is the output pretty quiet or is that a DC supply?
I think the OP's original power supply is a switching supply.

I was just thinking.... since AC is used on the filaments.... that must have been a nasty power supply to make that much audible noise.
 
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Old 05-09-20, 06:28 PM
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PJmax, yes I did read post #5 and went to the last link. I showed a 12 volt battery and a 12 volt motor which was speed controlled with a circuit. I am wanting to go from a 6 volt battery to six tubes drawing a total of 6 ohms at 3 volts thus I did not have the expertise to convert the circuit in the link to my needs. All the tubes were originally wired in parallel and still are. It would be quite a job to convert them plus I would like for the interior to look original.

Telecom guy, the radio was originally designed for the filaments to be powered through a transformer from 110 volt ac. It also needed a B and C battery which I have a power supply for. Actually I have two power supplies that have A, B and C outputs but the A output us only around 3 ohms. The is why I tried using my switching power supply. It will provide me with 6 ohms at 3 volts but generates to much interference. Can you give me the auction number of the cheap lambda 0 to 10v supply on the bay?

When time permits I will post the schematic of the original radio or a similar one that uses the same tubes.

Don


 
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Old 05-09-20, 08:00 PM
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333594095166. Is the item number. It would have to be a pickup. If you do a lot of this stuff, it could power 1v, 5v or 6.3v tubes.
 
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Old 05-09-20, 08:07 PM
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Duplicate message ..............
 
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Old 05-09-20, 10:00 PM
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Telecom guy, is that auction #333594095166 really worth over $100 or are they just trying to gouge someone on shipping?

Just curious, how many D cell batteries would it take to give me 6 amps at 3 volts? It would no doubt be several sets of two in series placed in parallel, but how many?

Don
 
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Old 05-10-20, 06:12 AM
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D cells will work. How many will depend on what length of time is involved. Plan on 5 to 10 amp hours per cell, depending on load.
 
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Old 05-10-20, 11:07 AM
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D cells...... ouch.

That circuit is not voltage dependent. It work as it is on 6v also.

If you want to get down and dirty..... put 6A+ diodes inline to drop the 6v battery voltage.
Each diode will drop approx .7v.
 
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Old 05-10-20, 03:12 PM
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PJmax, sorry but I don't understand your "ouch" concerning the D cell batteries. I would think that if I put 6 sets of 2 D cells in series I would end up with 3 volts and adequate current (6 amps) for my purpose. I would only need the voltage and current for enough time to get the balance of the radio working and aligned...maybe about an hour. After that the radio, without the power supply, simply goes on one of my shelves with many more finished radios. Do you really see a problem with using the D cells for this?

As far as working with 6 volts I am almost sure that it would burn the filaments out.

Don
 
 

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