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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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13th Apr 2019, 4:45 pm | #1 |
Tetrode
Join Date: Jan 2015
Location: Tadley, Hampshire, UK.
Posts: 77
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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? |
13th Apr 2019, 5:01 pm | #2 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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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. |
13th Apr 2019, 7:53 pm | #3 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
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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 |
13th Apr 2019, 9:09 pm | #4 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,580
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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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14th Apr 2019, 6:36 pm | #5 |
Tetrode
Join Date: Jan 2015
Location: Tadley, Hampshire, UK.
Posts: 77
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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? |
14th Apr 2019, 7:02 pm | #6 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
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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 |
15th Apr 2019, 12:24 pm | #7 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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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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15th Apr 2019, 12:39 pm | #8 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,088
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Re: Combined Choke & Output Transformer
Quote:
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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