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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Apr 2019, 4:45 pm   #1
Pitchfork
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Default Combined Choke & Output Transformer

I have come across an old guitar single-ended amp where the output transformer primary has a tap fed from the cathode of the rectifier valve. One end of the secondary then goes to the HT line as normal, the other to the anode of the output valve. Has anyone ever seen this type of combination choke/transformer before?
Since the transformer has failed a replacement is needed but I have never seen anything like it before or for that matter for sale.
Any ideas?
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Old 13th Apr 2019, 5:01 pm   #2
ms660
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Default Re: Combined Choke & Output Transformer

If it's the same arrangement I'm thinking of, many radio receivers use that type of transformer, the attachment shown below explains that particular type.

Lawrence.
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Old 13th Apr 2019, 7:53 pm   #3
Ed_Dinning
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Default Re: Combined Choke & Output Transformer

Hi , a fairly common hum reduction technique used in quite a few AC/DC sets.
The "choke" part of the winding is only about 5% of the total turns.
These units can easily be rewound.

Ed
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Old 13th Apr 2019, 9:09 pm   #4
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Default Re: Combined Choke & Output Transformer

Very common in later radios. Almost every other 50's set seemed to use this method to reduce hum.
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Old 14th Apr 2019, 6:36 pm   #5
Pitchfork
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Default Re: Combined Choke & Output Transformer

Thank you all, it's exactly per MS660 described
Had no idea that it was that "common" a technique
The Amp in question dates from the mid-late 50's which explains why it was done this way & I suppose that suitable transformers were readily available at that time
I wonder if any still are?
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Old 14th Apr 2019, 7:02 pm   #6
Ed_Dinning
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Default Re: Combined Choke & Output Transformer

Hi, it is actually a bog standard transformer that has a few extra turns on it and an additional leadout. A full description of how it works is in Langford- Smith's Radio Designers Handbook.

Ed
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Old 15th Apr 2019, 12:24 pm   #7
G8HQP Dave
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Default Re: Combined Choke & Output Transformer

The extra winding does not work much like a choke. It is essentially a hum-canceller, a bit like the extra winding sometimes found on guitar pickups.
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Old 15th Apr 2019, 12:39 pm   #8
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Default Re: Combined Choke & Output Transformer

Quote:
Originally Posted by G8HQP Dave View Post
The extra winding does not work much like a choke. It is essentially a hum-canceller
And this bit of winding has a resistor in series with it, which is important. Too high a value, or too low a value, and the hum cancellation does not work because it's over-cancelled or under-cancelled.

Of course, if you make this resistance ZERO it won't work anyway, because the reservoir and smoothing capacitors are then effectively across the winding, so would shunt away all your audio!
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