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 Post subject: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 2:59 am 
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I have a Liberty Electric Six radio that uses six Kellogg 401 tubes which each draw one amp at three volts. There are already at least two threads about this radio of mine and one is at viewtopic.php?f=6&t=361619&p=3059427 . This time my question is not really about the radio per se (I think I have it figured out) but rather about what kind of power supply to use for the six three volt tubes that draw a total of six amps.

I have a power supply that will product the necessary voltage and current but it is a switching power supply and it appears that it simply generates too much noise in the radio or any other radio that is reasonably nearby. The "A" supply on my ARBE-III will only give me about three amps thus I need something else. After Googling I came up with a page that states "A standard D-size carbon-zinc battery has an Ah (amp-hour) capacity of approximately 4.5 to 8 Ah (4500-8000 mAh). This means that a D battery could supply 6.25 amps of current for about one hour, more or less. This can also be calculated as the D battery supplying a current of 1 amp for about 6 hours, or any other combination with this same formula." If I was reading it correctly and it was correct it seemed to me that if I put six sets of two D cells in series in a parallel configuration and made myself a power supply that would work. I really didn't think that this would work but again if I read it correctly and it was correct it appeared that it should.

With the above information I went to work building my power supply. I have attached two pictures, one before the sides are on and the other is when the unit is complete except for the top latch. When I check the voltage I get 3+ volts as expected but the current is a totally different story. When I connect this unit to just one tube my meter(s) show that it is only drawing .16 amps and of course when I connect it to all six tubes it still shows .16 amps. What is really interesting is if I try just two D cells I still get approximately the same current.

Okay, is that what should be expected with this arrangement of batteries or did I build the unit totally wrong? Also, any suggestions for a power supply until to power these tubes would be greatly appreciated.

Cheers and stay well,

Don


Attachments:
supply1.jpg
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supply2.jpg
supply2.jpg [ 307.77 KiB | Viewed 2496 times ]
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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 3:15 am 
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Your power supply is probably OK. Trickiest thing about that particular arrangement is making sure you get good contact on all batteries. Looks like you've got a compressible layer in there, so that should do the trick. I'm not sure your contact material is low enough in resistance (is it steel, or something better like plated copper or brass?).

Using a bunch of batteries in series parallel like this is a good way to go. Although you maybe could draw 6 amps out of a single D cell, its internal resistance would cause a serious voltage drop. By using many in parallel like this, that problem should not occur.

If you're only drawing 0.16 amp when you wire this up to your radio, I would expect two possible problems:

1. The device you are using to measure the current has a significant resistance. Best to use a DMM on its 10 amp scale, and one which has a separate input just for 10 amp current measurement.

2. The wiring between the battery pack and tubes has too much resistance. Too thin of wires, a switch with contacts that need to be cleaned, or solder joints that aren't robust enough. To run 6 amps at 3 volts without much voltage drop, you need less than 0.05 ohm total in your wiring, connectors, and switches. You're having problems already with just one tube. Hopefully the problem is #1 above, and not #2, since the problem will be 6X worse with six tubes.

Before you fix anything, measure the voltage on the tube filament right at the tube socket. If it is drawing only 0.16 amp, you'll find that the actual voltage making it to the tube is less than 1 volt.

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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 3:38 am 
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If it isn't working right, a layer of copper foil or copper plate down the center of each steel strip will make the sparks fly, IMO.

I "built" this 4D battery pack from a Home Depot nail box and some metal strips, tight fit, works well, ugly. These are steel strips hammered from curtain rod supports, plated with something like brass, copper is too soft by itself.


Attachments:
A, B & C batts.jpg
A, B & C batts.jpg [ 250.78 KiB | Viewed 2466 times ]

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Last edited by westcoastjohn on May Sun 17, 2020 3:56 am, edited 1 time in total.
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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 3:46 am 
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westcoastjohn wrote:
If it isn't working right, a layer of copper foil or copper plate down the center of each steel strip will make the sparks fly, IMO.


You may well be correct. See my post above and if the resistance in the aluminum is more than I expected I will find a copper alternative.

Don


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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 4:01 am 
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Hi Don, aluminum is a good conductor, but tarnishes right away in open air. Maybe polish it up a bit and try again.
That will be a nice battery pack when you're done.

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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 4:09 am 
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Kellogg 401's are an AC filament tube with a cathode.

Chas

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Last edited by Chas on May Sun 17, 2020 4:24 am, edited 1 time in total.

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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 4:16 am 
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Agree with the above that aluminum is a poor choice as a contact material. It oxidizes immediately in air and has no surface conductivity. If you're not sure about that, try measuring the resistance between two probes pushed lightly against clean aluminum. Infinite resistance. Only if you push the probe into the metal will you get temporary conduction.

If you can put copper or brass strips in place of (or on the surface of) the aluminum, you will do better. Even copper or brass will tend to oxidize, so something like nickel or silver plating can help a lot. Solder "plating" at the contact points will also work -- a crude but effective solution.

I'd be tempted to solder copper wire directly to the batteries. Not so convenient when it comes time to replace the batteries, but it works.

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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 4:48 am 
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Wiring primary cells in parallel would seem to be a good way of obtaining a desired amount of current. But it has an undesirable side-effect in the real world: unless every cell is exactly matched to all others in the array (which they never are), current will flow between cells even if the battery isn't being used to drive an external load. Best case, this will lead to premature battery pack depletion; worst case, it can cause a fire or mechanical damage to the holder.

Unless switching is provided to interrupt the parallel cell connections, removing the cells from their holder whenever the system is not in use is recommended.


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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 5:20 am 
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Although the above can be a "worst case" scenario, as long as the batteries are installed as a group and of the same type, nothing terrible happens.

Some battery radios (in particular, tube portable A batteries, and 1950s portable transistor radios with big 9V batteries) use batteries in parallel without switching to no obvious disadvantage. I've had a pair of D cells in parallel on my Radiola IIIA for perhaps 10 years, and they still light the filament OK (obviously I don't use it very often!).

If the cells are a mix of brand new and well-depleted cells, the new ones may end up trying the charge the old ones, which probably isn't very efficient. If you mix alkaline and carbon-zinc, you may end up with different cell voltages and a similar effect.

But for cells of the same type installed together when new, the odds are they will last very well.

Your project here is a bit of an extreme case, so it would be interesting to see what your experience is. Now that you know roughly what the energy capacity of D cells is, you know about how long they should be able to run. See if you get the expected life, while leaving them in parallel over a period of a month or so. If the lifetime is significantly shortened, I'll learn something new.

As pointed out above, opening your holder so the top contact no longer contacts the cells will solve the problem if indeed drainage is noticed when not in use.

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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 6:39 am 
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I'm having a hard time wrapping my head around the suggestion that a D size battery can supply one amp for up to an hour. Be that as it may, why not use a rechargeable lead acid battery? Here's one supplying 6 volts at 7AH. You could use a resistor in series to drop the voltage.
https://www.allelectronics.com/item/gc- ... ery/1.html

Here are their other batteries;
https://www.allelectronics.com/category ... ble/1.html

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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 1:30 pm 
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My go-to with this, these days, is a Shorai Lithium Iron battery. Designed for motorbikes, available
in 6 V versions. Will run my AK 19 for quite some time. No spillable electrolyte, can be operated
in any position. Lightweight.


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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 4:58 pm 
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Location: Santa Rosa, CA
Datasheets are actually pretty useful.

A-H rating for D-cells is based on discharge to 0.8 V. For most of us, that's impractically low.

The internal resistance of a D-cell can be up to 400 milliohms. Try to draw 1 amp and you lose 0.4Volts.

https://data.energizer.com/PDFs/LR20BA_CEU.pdf

I never parallel batteries. May work well if all the batteries are same brand and age. Hard to remember that later on... And we hope that all the batteries age in the same manner.

Rich


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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 5:16 pm 
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fifties wrote:
I'm having a hard time wrapping my head around the suggestion that a D size battery can supply one amp for up to an hour. Be that as it may, why not use a rechargeable lead acid battery? Here's one supplying 6 volts at 7AH. You could use a resistor in series to drop the voltage.
https://www.allelectronics.com/item/gc- ... ery/1.html

Here are their other batteries;
https://www.allelectronics.com/category ... ble/1.html


I also was having a problem believing that a D cell could do that so that is why I am trying it with all of this free time at home. I am familiar with the battery that you mentioned (6 volts at 7AH) but I am not sure what sized resistor would work when one would be drawing 6 amps. It would really be neat if I could just use a potentiometer or rheostat so I could make the output variable.

Don


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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 6:01 pm 
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You stated that no matter what, the amp draw is only .16 amps. I suspect something wrong with the meter, the leads, or you are not correctly measuring the amps.

If you hook up the batteries (3 volts) directly to the filament of 1 tube, does it light up? Or, test the battery pack with a flashlight bulb instead of the rare tube.

Brute force test: attach a jumper wire to one battery contact and brush the other end against the other contact. Should get a whopper spark.

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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 6:37 pm 
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Thanks for posting, Rich!

Rich, W3HWJ wrote:

The internal resistance of a D-cell can be up to 400 milliohms. Try to draw 1 amp and you lose 0.4Volts.

https://data.energizer.com/PDFs/LR20BA_CEU.pdf


Unfortunately, if the internal resistance is as high as 0.4 ohm, then this approach really isn't going to produce a satisfactory result. Although other places show the internal resistance of D cells at 0.1 ohm, which might be acceptable, with the manufacturer showing 0.2 - 0.4 ohm for brand new cells, you'll have 2.2 - 2.6 V at the battery pack, and a little less at the tube filament (taking into consideration wiring and contacts), which is probably too low for things to work properly.

So, if you're going to use D cells, you'd want 3 in series (and 6 of these stacks in parallel) to give you 3.3 - 3.9 V at the batteries, which will drop to something closer to what you want with the extra drop from wiring and contacts.

If you use a large rechargeable battery, they often have very low internal resistance. If the battery manages to stay close to 6 V at 6 amps, then you need a resistor of 0.5 ohms in series. A more stable approach would be to get some 10 amp diodes. Each one added in series will drop 0.6 - 0.7 V. Almost as good as a potentiometer. Just choose the number in series that give you about 3 V at the tube socket.

As has also been pointed out above, you don't need DC for these tube filaments. Could go with AC from a transformer.

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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 7:03 pm 
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[quote=As has also been pointed out above, you don't need DC for these tube filaments. Could go with AC from a transformer.[/quote]

Tom, I have a variac that will adjust very accurately down to the three volt range but unfortunately it only has a 4.75 amps output. (see attachment)

Don


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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 7:39 pm 
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If you get a 6 V 6 amp transformer, and put a 10 amp diode in series between the secondary and the tube filaments, you’ll be close. The rectification drops the effective RMS voltage to 4.2 V, and the diode drop knocks an additional 0.7 V off, leaving 3.5 V. Throw in some resistance in your wiring, and you may be very close. If it’s too high, put one more diode in series.

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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 8:51 pm 
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Well, I have not got out to my shop yet today but it seems like the group has pretty well decided that my D cell arrangement will not work as I had hoped so now for an alternative. I don't have a 6 volt 6 amp transformer, nor can I find one on eBay so that leaves me with using a battery as shown in the attachment if you think that it will work. I realize that I will need a resistor, potentiometer, or rheostat (would really like to have it variable) to get me down to the voltage that I need but it seems like it would have to be rather heavy duty.

With the above in mind can someone point me to a resistor, potentiometer, pr a rheostat (or all three) that will work for me with the battery. A URL would really be appreciated...either to a merchant or a unit on eBay.

Don


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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Sun 17, 2020 10:24 pm 
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I just got back from my shop and it appears that Rich and Tom were correct, what I have is just not going to work because of the internal resistant. :(

I did some more testing just to verify the above and got the following results which bear out their opinion...thanks guys.

- No load voltage at battery case = 3.22V
- Voltage at battery case with 1 amp load (1 tube) = 2.95V
- Only showing .8 amp when connected to a 1 amp tube.
- Only showing 2.7 amp when connected to six 1 amp tubes.
- Voltage at battery case when connect to six 1 amp tubes = 2.32V
- Aluminum contacts vs. copper contact makes no difference.

All in all, I need to find an alternative as described in my previous post and any and all help will be appreciated.

Cheers and stay well,

Don


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 Post subject: Re: Back to the basic "basics" so bear with me
PostPosted: May Mon 18, 2020 1:00 am 
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Whoa, Don! What happened to .16 amps, no matter what? Unresolved questions cause confusion.

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