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Rewinding old iron-core transformer
Rewinding old iron-core transformer
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Old 6th February 2018, 04:47 AM   #1
LDA is offline LDA
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Default Rewinding old iron-core transformer

Hi there,
Haven't made many posts here but I've lurked pretty hard previously. Have made a low powered SET amp with some mods from what was initially a kit but I'm still pretty new to electronics.
Anyway I'm a frequent dumpster diver and have found a number of tubes over the years, but recently found these an old transformer and am now wondering if either are any good to build an amp from.
It's an ironcore brand, quite old, has inputs for mains 230/240V(Australian) and neutral, a 6.3V @ 3A output, a 5V @ 2A output, a 385V-0-385V output @ 80mA (I think, see pictures), a 2.5V @ 5A output, and a 4V pin(with no paired neutral or amperage rating?) and an E pin(earth), (Have a look at the pictures to make sure I've assigned the voltages and pins correctly but hoping this is correct.)

I haven't fired it up yet to see if it doesnt catch fire but am wondering if its at all useful. I'm just not sure if it'll handle the currents needed for the filaments (or the plates) of a few tubes between both channels. I'm looking to build a low powered(10-15W), two channel (P-P circuits) amplifier, which means a minimum of 6 tubes from that 3A of heater current.

My other option is rewinding it, it's quite old so I'm unsure how feasible it will be pulling the slightly rusted laminations apart and putting them back together. So was just wondering if anyone has any advice/experience rewinding older transformers. I also worry that if I rewind it, whether I'll have space to fit all the windings around the same core... Hopefully if I remove the other secondaries..?
I've never wound a transformer before but I figure the power one is a good one to start, and it if works can try winding some OPTs with another pair of ironcores I found.

Thanks!
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Old 6th February 2018, 06:51 AM   #2
Eli Duttman is offline Eli Duttman  United States
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Restacking a core is a job for an expert. I suggest you steer clear. Brush the loose rust off with a SOFT toothbrush, apply a rust converter to the lamination stack, and paint.

The 80 mA. number is subject to more than 1 interpretation. Is that the RMS rating or is that 80 mA. what can be drawn from a cap. I/P filter. Assume the worst and the rating is RMS. You can access all of that 80 mA. as B+, if you choke I/P filter. Unfortunately, 4X EL84s or 6V6s for the desired 15 W. power O/P require more than 80 mA.

A stereoblock was not a consideration, when the "iron" was wound. 5 WPC is feasible with SE EL84s or 6V6s. The 6.3 VAC winding is adequate for 2X O/P tubes and a 12AX7/ECC83 as the voltage amplifiers.

Do you wish to continue?
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Old 6th February 2018, 11:26 AM   #3
lexx21 is offline lexx21  United States
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Rewinding old iron-core transformer
Find Tony over in the power supply area. He winds his own so he would be able to help.
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Old 6th February 2018, 11:37 AM   #4
AndrewT is offline AndrewT  Scotland
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Before you do anything with it, you should check it is safe to use connected to the mains.

An Electrician with a Megger can check the insulation between Primary and each of the Secondaries and from the High Voltage secondary to all the low voltage Secondaries.

This is a quick test and the labour charge should be small.

Google shorted turn test. There is a simple and cheap method of detecting a shorted turn.
After this is done you can start it up via Mains Bulb Tester. Use a very low wattage tungsten filament bulb. This will allow you to measure the secondary voltages and/or detect if there is a serious internal fault. The bulb should stay dim.
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Last edited by AndrewT; 6th February 2018 at 11:41 AM.
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Old 8th February 2018, 03:54 AM   #5
LDA is offline LDA
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I was about to plug it in and test it with the bulb tester and check the secondary voltages when I first checked the resistances of the windings. Turns out one of the 0-385V turnings is an open circuit
Looks like I'll be partially unwinding it at least.... Hopefully it's an easy enough break to find... If not it may turn out to be a good little experiment in transformer winding. Or a huge headache...
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Old 8th February 2018, 07:41 AM   #6
Eli Duttman is offline Eli Duttman  United States
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That open leg may not be all that bad. You may be able to get away with hybrid bridge rectifying the good leg.
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Old 8th February 2018, 05:52 PM   #7
pcan is offline pcan  Italy
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Rewinding old iron-core transformer
Check the magnet wire that goes from the lug to the winding itself. On that style of transformers, it is easy to broke it when the trasformer is removed from the chassis. You may be able to resolder it with a bit of patience. Remove the top bell and check for mould/degradation of the exterior paper insulation and for dead bugs. Use kapton tape when needed. You may want to replace the screws. Do not even try to rewind it unless you have the proper equipment and experience. As Eli Duttman said, a broken HT winding is not a showstopper, but excessive degradation of the paper insulator is definitely a issue.
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Old 9th February 2018, 11:37 AM   #8
indianajo is offline indianajo  United States
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I tried rewinding a golf cart charger transformer once. I shorted the winding to the core. If trying this, buy or make a mylar core wrapper. Those sharp corners can cut right through even polyurethane insulation. Maybe use a heat gun to bend sharp corners in the plastic.
I had extra room, the secondary was aluminum and I replaced it with copper. But it didn't work. Would have made a good mig welder with a few less turns on the secondary.
You could gain some extra room for less than tight winding by deleting the 5 v winding for the rectifier tube heater. You could use silicon diodes for the rectifier and achieve the soft start feature by putting a NTCresistor in series with the primary , say a GE CL-80 or 70.
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Last edited by indianajo; 9th February 2018 at 11:43 AM.
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