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 More AK 37 Advice Needed

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atwaterkent
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PostSubject: More AK 37 Advice Needed   August 25th 2018, 2:35 pm

I need to replace the three chokes in an AK37 power supply (2 power, 1 speaker). As I am a 20's battery radio guy, I am not sure what the buy to replace them.  Any advice would be appreciated.
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atwaterkent
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PostSubject: Re: More AK 37 Advice Needed   August 25th 2018, 7:54 pm

This might help. There's two 675 ohm filter chokes and a 550 speaker choke that's partially cutoff at the top.


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Rod Clay
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PostSubject: Re: More AK 37 Advice Needed   August 25th 2018, 8:36 pm

Hi Atwaterkent,

I just saw your new post so I may have to revise this a bit but you'll get the idea. My Riders Manual search of Volume (1) and (3) lists 800 Ohm filter chokes and a 500 Ohm speaker choke. Also, the 1.5uF and 1.0uF condensers are reversed on the Riders schematic.

I have looked over the schematic of the power supply and the set's power requirements. Here is what I have found and what I have come up with as suggestions for replacement parts as well as a little power supply history: The power supply is choke input which should be maintained to get the proper output supply voltage and also reduces the size and number of filter condensers needed. 

The total normal B+ current requirement is 40 mA. 

50 mA rated filter chokes in the power supply section should be fine as replacements-higher current rated chokes are ok as long as they will fit in. It was common in those days to use filter chokes rated at 20 Henries in power supplies. The schematic does not give a value in Henries but the dc resistance given of 800 Ohms each indicates that they would be about 15-20 Henries each. 

So for your list of parts that would be (2) 20H chokes rated at 50 mA of current. 

Also, it was thought then that using 20H filter chokes was a better way to go than using larger values of filter capacitance as good paper filter condensers were more expensive than chokes were at that time. 

Getting 20 H chokes now may be a bit difficult but you can start looking around for them. Also, 20 H chokes have a higher internal resistance than say 10 H 50mA chokes which is a factor in getting the right voltage out of the power supply as some DC voltage drop in the chokes is to be expected (and wanted in some cases). 

The 10 H 50mA chokes are more common to find (and physically smaller) but their use may require larger filter condensers say 4UF-8uF. Actually, the values of filter capacitance given on the schematic are not that critical and 2uF could be used for both the 1.5UF and 1.0uF condensers for convenience in getting replacements. 

If the chokes you end up using have considerably less DC resistance than the originals then you can make up for that by adding a series wire wound resistor to the power supply circuit that will bring up the total series resistance to the rated 1600 Ohms given for the (2) chokes. Any value that gets you close to 1600 Ohms will be ok.

Now for the Speaker or Audio Output choke. Here the rated 71A plate current given is 18mA so a choke rated at 25mA or more will be fine. No value in Henries is given but again the 500 Ohm DC resistance value given suggests a 10-15 Henry choke. A 10H-15H choke rated at 50 mA will be commonly available, small in size and should give good output response.

Chokes are often rated in Test Voltage and Working Voltage for HV power supply use and since this is for a receiver just about any choke you run across will be fine as far as voltage ratings go.

Hope this helps.

73, Rod  N4QNX
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Rod Clay
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PostSubject: Re: More AK 37 Advice Needed   August 25th 2018, 8:48 pm

I'll add one other recommendation: The B+ supply voltage in this set is typically rather low at around 180VDC so filter condensers (caps) rated at 300VDC should be fine. 450V or 600V  rated caps would of course be ok if available but might be more expensive (and larger if film caps). 

Rod
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atwaterkent
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PostSubject: Re: More AK 37 Advice Needed   August 26th 2018, 12:15 pm

Thanks for the desperately needed information Rod. I already have the 630v film caps I'm going to use, now I'll start hunting for the chokes. If you know what the Henrys and amperage rating for a choke, is there any way to calculate or know the resistance of it? For instance, a 3H choke rated at 250ma.
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Rod Clay
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PostSubject: Re: More AK 37 Advice Needed   August 26th 2018, 2:12 pm

3H @ 250 mA sounds more like a choke meant for later model audio amplifiers and TV sets (post WWII equipment). It would also be physically large and heavy. It would also have a low dc resistance.

That information (dc resistance) is usually provided by the choke manufacturer and can be found in catalogs and such. It can also be found on the original box for the choke and/or data sheet that comes with it. If you are shopping around for used chokes you could ask the seller to measure it for you. 

One can also make an educated guess about the dc resistance of a choke based on its physical size if given the inductance and current rating to start with. 

Another way to tell is from its intended purpose-chokes made for supplying class B amplifiers and other large current requirements in radio transmitters (Class C amplifiers) usually have a low dc resistance for less loss and better voltage regulation. Chokes meant for use in TV sets usually have a low internal resistance often at lower values of inductance as high inductance chokes usually were not needed by that time.

Measuring a choke's dc resistance is easy to do with a dmm or other meter. 

One thing important to remember is that the manufacturers rate the inductance in Henries at a given amount of working DC current. Measuring a choke's inductance with a bridge with no direct current flowing (0 mA DC) thru it will give a higher H value and is not representative of what you'll get in an actual circuit.
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atwaterkent
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PostSubject: Re: More AK 37 Advice Needed   August 26th 2018, 4:28 pm

I prefer at this point to buy new chokes and unfortunately when I look online they often do not give resistance. I was told that I could add a resistor to a modern choke to make up the difference. Hard to know what the Henries  value was on the original ones. How important is it?
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Rod Clay
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PostSubject: Re: More AK 37 Advice Needed   August 26th 2018, 7:05 pm

I'd stick to a minimum of 10 Henries on the replacement chokes rated from 50-100 mA each. These are available, small and lightweight. I read you have the caps already but if you use chokes of less than 20 Henries each then I recommend going to a minimum of 2uF film caps or you might get noticeable 120 Hz hum in the radio's audio. Of course you could experiment a bit with values of caps and so forth. Will you be putting the tar back in the can?

Hammond Transformers of Canada, has a full line of radio chokes to choose from. These are available from the various dealers like AES and Radiodaze. 

If you like ordering on eBay then there are plenty to choose from there. One seller (vivatubescom) I just checked on has two Triad types that would be good. Check out his Triad C-3X offering which is rated 10H at 50 mA and 500 Ohms. The C-3X is priced at $24.95 new boxed. Also check out the Triad C-5X rated at 12H 75 mA and 390 Ohms. The C-5X will be a bit larger than the C-3X but still reasonable to use. The C-5X is also priced at $24.95. Either one will do the job for your project. 

After your selection then you can choose an appropriate series dropping resistor to use. Incidentally, the dc resistance can vary widely from the published value so after you get your chokes go ahead and measure their actual resistance for your dropping resistor calculations.
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atwaterkent
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PostSubject: Re: More AK 37 Advice Needed   August 27th 2018, 12:29 pm

I won't be putting tar back in Rod, I use GE silicone (acid free) for keeping everything in place.  Do I use the same chokes to replace all the chokes, including the speaker choke?
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Rod Clay
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PostSubject: Re: More AK 37 Advice Needed   August 27th 2018, 1:07 pm

Yes, you could order (3) of the same chokes for the job. 10-20 Henry at 50-100 mA will work for the speaker choke too. The plate current for a 71A tube is about 18-20 mA so a smaller 10-15 Henry 25 mA choke could be used there. The dc resistance of the speaker choke is not really as important a consideration as the filter chokes.
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atwaterkent
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PostSubject: Re: More AK 37 Advice Needed   August 27th 2018, 3:01 pm

Thank you again Rod.
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atwaterkent
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PostSubject: Re: More AK 37 Advice Needed   September 13th 2018, 6:53 pm

I am going to wire 1 and 1.5 micro farad caps in parallel to get to 2.5 for each of the filter caps. I plan to use .47 film caps to replace the .5 caps which I think should be close enough.
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