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 Post subject: 8 olm vs 2000 olm
PostPosted: May Sun 29, 2022 5:28 pm 
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unable to find a working 2000 olm speaker for my Atwater 20c, is there any way to get 4 or 8 olm speakers to work??


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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: May Sun 29, 2022 6:55 pm 
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atwater20c wrote:
I am unable to find a working 2000 ohm speaker for my Atwater-Kent 20c, is there any way to get 4 or 8 ohm speakers to work?
Yes, a salvage speaker with the output transformer from a common AA5/6 tube radio. The blue lead would go to the plate of the output tube and the red lead to the B+. An automobile speaker is far less sensitive than even an AA5 speaker.

Use a baffle with the speaker for better reproduction. Expect the volume to be somewhat low as the replacement speaker has a less efficiency.

Chas

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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: May Mon 30, 2022 12:31 am 
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2000 ohm speakers are usually the horn type.

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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: May Mon 30, 2022 2:47 am 
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SO you want an audio output transformer, commonly salvaged from old table tube radios, if you can find it. What you seek is a transformer with a primary impedance in the absolute minimum 1000 ohms to about 5000 ohms range. Not critical within that range for most purposes. Then alot of the old radio transformers are made for 3.2 ohm speakers on the secondary but you can get away with a 4 ohm speaker there. Occasionally they have 8 ohm outputs on those transformers. But you want to get the matching speaker and transformer from the old radio to make your life easy. There is probably a way he can take one of those 70.7V PA speaker matching transformers and use that too. Those are not very expensive. You have to select the windings you need on that, as there are multiple taps and leads.

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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: May Mon 30, 2022 3:28 pm 
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something like this Standard 12HB001 (EEB3) Audio Output Transformer 2500 Primary Impedance, self-lead 3.2?


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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: May Mon 30, 2022 3:51 pm 
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atwater20c wrote:
something like this Standard 12HB001 (EEB3) Audio Output Transformer 2500 Primary Impedance, self-lead 3.2?


Yes. You can probably find something in the $10 and under range, particularly if you get a used one.

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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: May Mon 30, 2022 4:28 pm 
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Thanks for the info I will set it up and let you know what I experience


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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: Jun Sun 19, 2022 3:09 am 
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I got a transformer 2500 olm to 3.2 olms connected to a 4 olm speaker but something interesting is going on, I get voltage from the speaker output terminals 9v with the volume all the way down and up to 52v full volume but when I connect the transformer the voltage goes away when I touch the speaker wires directly to the speaker ( no transformer)there is no clicking sound like it does when I touch it with a 9v battery,
Any ideas why the speaker output voltage goes away immediately when it's connected


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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: Jun Mon 20, 2022 12:40 am 
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I'm not sure what you are touching to what. But first of all, the ohms spec of old speakers may be the DC resistance (like it is for headphones) The rating of an output transformer is the AC impedance; the DC resistance would be a few hundred ohms. So if you are measuring DC voltage, that would cause a large difference. Connecting a modern speaker without an output transformer would short out both the AC and DC voltages.

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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: Jun Mon 20, 2022 3:49 pm 
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I am using an output transformer and connecting the red to B+ and blue to plate as indicated by the installation instructions. With the volt meter connected to the radio output terminals B+ and plate as soon as I connect to the transformer input, the voltage reading at the radio goes to zero? I am assuming the output from radio is DC and the input for the transformer is also DC since the transformer has a positive B+ and negative plate.


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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: Jun Mon 20, 2022 5:03 pm 
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atwater20c wrote:
I am using an output transformer and connecting the red to B+ and blue to plate as indicated by the installation instructions. With the volt meter connected to the radio output terminals B+ and plate as soon as I connect to the transformer input, the voltage reading at the radio goes to zero? I am assuming the output from radio is DC and the input for the transformer is also DC since the transformer has a positive B+ and negative plate.


Umm, (<< means "lookout" lengthy diatribe ahead :roll: )

Either something is broke/wrong or interpretation of what is being observed is incorrect.

May be a mixture of both...

The measuring device (meter) if it is a digital hand-held it likely has a 1 meg-ohm input resistance. It can measure most DC/AC voltage and draw a very tiny amount of current from the circuit under test. In some instances this can be miss-leading.

There are two versions of the 20c, below are the modern drawings of both variations:
Attachment:
A-K_20c_7570.png
A-K_20c_7570.png [ 60.92 KiB | Viewed 264 times ]


Attachment:
A-K_20c_7960.png
A-K_20c_7960.png [ 71.54 KiB | Viewed 264 times ]


Here is the web site that provided that information:

http://www.atwaterkent.info/

Note that how the output to the speaker connections are arranged is different for the two models.

The strange voltage readings seem to agree that the radio is one of those models such as connected erroneously to an output transformer with yield an open circuit reading :D .

Compare the wiring of your radio and/or the model type number which should be on a nameplate attached to the chassis. The wiring should agree to one of those model types.

For the 7570 & the 7960 the output transformer primary should be connected across the "Phone" connections, the plate side getting the blue wire.

BTW the red/blue can be reversed it will still work...

Note the connections for the B+ in reference to how the zero voltage could have been measured.

When measuring voltages in a battery radio where there is no "negative chassis" the B- is the return or reference point. When describing measurements always include both points of measurement.

Be aware, that a DVM can measure tiny leakage voltages yet the measurement will be incorrect.

For example if the grid side of a audio transformer is bad, the grid voltage measured from C+ (not indicated in these drawings and is usually A- & C+ connected together) to the grid of the driven tube can indicate full C- voltage, that is because the grid does not draw current in this instance.

Even measuring the resistance WITH A DVM of a audio transformer can be problematic as the DVM tries to negotiate a combined resistance and a high inductance.

I always recommend using an analog ohmmeter for work on a radio as the meter does draw current and is not confused by high inductance when measuring the resistance of that device.

I also recommend that all observed soldered joints be heated and re-soldered. Solder alloys will suffer from inter-granular corrosion, though will pass DC for a measurement will be problematic at RF frequencies. CAVEAT! some radio parts are made of a thermoforming "MUD" compound, soldering to lugs fastened to this material can cause the material to melt and deform, always loosen that lug and/or heat sink the re-soldering process. Loosen and re-tighten all other mechanically made electrical joints too. dissimilar metal interfaces can also be a problem.

Clean the grey colored oxide off of the tips of the tube pins when bayonet sockets are in use. Use a gritty "Steno" eraser on the blades of the tube socket connections.

Never try to read the grid voltage on the detector, it is meaningless as there is no current available to activate any meter even a DVM.

The use of transistor radio batteries for B+ is not a good idea, the internal resistance of a 9 volt battery is too high and may cause the radio to oscillate.
Never use lower than recommended voltages, the "A" supply for this radio is 6 to 6.6 volts NOT 5 VOLTS!.

Running a 20's era radio on reduced filament voltage depletes the tubes of the thorium coating, although this can be repaired the process does shorten the overall life of the tube. The same result if the tubes are operated with too much plate current for example without a recommended "C" voltage. This fault in the tube can be repaired but again shortens the overall life.

I would highly recommend getting a high impedance speaker, they will always be more sensitive than a modern make-do unless in poor condition. Cones will always sound better than a horn. Does not have to be an Atwater-Kent speaker.

Do not use a power supply that grounds to earth via the electric socket the "A" or the B- in this radio, use the ground as recommended at the radios connections ANT/GND and always as short as possible and never to the electrical outlet. Despite that such a connection may "work", don't. Be sure that no bypass capacitor makes the same connection in any power supply, such a condenser can bring noise into the radio via the "U" ground connection at the electrical outlet.

YMMV

Chas

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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: Jun Tue 21, 2022 1:55 am 
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Posts: 7
Thanks for the info
I do have a service manual reprint and an original instruction book vol.3
I will go through the radio clean terminals, solder, and check connections next before moving on
I am powering it with an Arbe 111 and have the A output adjusted to 6v
the antenna I am using is a DX engineering AFHD-4
no label but I am assuming it's a 7960 because the C- 4.5v wire is green
dig out my analog meter nice to know about how the DVM works
search for a high impedance speaker
then take another run at getting it to work
will let you know how that plays out


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 Post subject: Re: 8 olm vs 2000 olm
PostPosted: Jun Tue 21, 2022 4:23 am 
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The DX-Eng antenna has a low impedance output for a modern receiver.

Attachment:
DX-Eng.png
DX-Eng.png [ 144.79 KiB | Viewed 237 times ]


May do better erecting an antenna as suggested by Atwater-Kent. One type even suggests using an under the carpet wire as a counterpoise that connects to the ground on the radio. The Atwater-Kent has a high impedance input and reference a true earth ground.

http://www.atwaterkent.info/akAntennas.html

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