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Old 17th April 2006, 09:43 PM   #1
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Default A Transformer question

I am using the cheap-o Hammond 125SE transformers. and I look at the schematic an I see that I need a center-tap to run to the grid. looking at the transformer digram, there is no center tap!!! what do I do?
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Old 17th April 2006, 09:53 PM   #2
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What grid, G2? What are you trying to do make a ultra linear with a SE output xfmer that has no primary tap?

In the case of a xfmer used to drive and also act as phase splitter for push pull grids it is possible to create a phantom CT by using a center tapped resistance between the two ends of the transformer secondary. It is not ideal and you lose voltage swing. Good "desert island" trick though.
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Old 17th April 2006, 09:57 PM   #3
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What grid, G2? What are you trying to do make a ultra linear with a SE output xfmer that has no primary tap?
yeah, pretty much. Is there anything i can do?
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Old 18th April 2006, 12:41 AM   #4
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Get a sewing needle and insulate it competely except for the very tip. Connect G2 to it with a piece of wire. Poke the needle into the primary winding and hope you get lucky.

I'm mostly kidding. Don't do this.
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Old 18th April 2006, 02:29 AM   #5
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haha, sounds like a good idea!!!!


What I am going to do is connect the tubes in a Triode configuration untill i order some cheap transformers with center taps, which leads me to another question.

I need a primary resistance of 2.5k and a secondary of 8 (both are ohms)

The Hammond 125SE shows the following diagram

Click the image to open in full size.

This has a universial primary winding (from 2.5k, to 10k) so assume I wanted 2.5K primary, and 8 secondary, then I would use the white connection, however If i had a 5k primary the same tap would be the 16 ohm connection.

look at the edcor equvilent with a middle tap, and no universal primary

Click the image to open in full size.

This model has a fixed primary at 5k ohms, and a fixed secondary at 16 ohms, now, after all this explination my question!

could I use this fixed 5k, 16 ohm transformer with a primary of 2.5k, and a secondary of 8?
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Old 18th April 2006, 02:54 AM   #6
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Yes, you could use the Edcor that way but if a 2.5K to 8 ohm Edcor is available, buy that instead.

FYI, the screen tap on an UL xfmer is not set at 50%. It is most often 40% from the B+ end. On some xfmers it is less than 40%.
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Old 18th April 2006, 02:57 AM   #7
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Yes you can. I used the Edcor XSE-15-8-5K (8 ohm model) with a 4 ohm load to get a 2500 ohm primary. It worked quite well. I am now using these transformers with UL in a 6L6 amp. These transformers sound far better than $18 transformers should.

Cheap SE output transformers

The experiment was in post #43 and 44
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Old 18th April 2006, 03:33 AM   #8
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Exactly the purpose I was going to use them for 6l6 UL, thanks for the advice
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Old 18th April 2006, 02:00 PM   #9
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I have the amp working, but am still tweaking on it. As soon as I have the details worked out I will post the schematic on my web site. I will also put it in the previously mentioned Cheap SE output transformers thread. Given my current work load it might be a week or two.

I am currently running a 5K load for all of the experiments that I am doing. The amp sounds fantastic. I may try 2.5K to see if there is much degradation in sound quality.
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