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Old 22nd February 2013, 07:38 AM   #1
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Default ST-70 Power Transformer

I am building an ST70 clone and need to select a power transformer. I am ordering my output transformers from Edcor , CXPP60-MS-4.2K, and I would like the power transformer to also come from Edcor to save on shipping cost.

So far I found the XPWR030-120. Since there is no 5V winding I plan on buying another small transformer to supply this voltage to the rectifier tube. I also will need to drop the 130V bias to 55V, which I can do using a voltage divider. Will this work out?

Thanks!

Edit: From searching the forums the XPWR002-120 is also another option, but I am wondering if the maximum output current is too low at 220mA. Has anyone used this transformer?

Last edited by jonnykun; 22nd February 2013 at 07:48 AM.
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Old 22nd February 2013, 08:36 AM   #2
SY is offline SY  United States
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Easiest thing to do is use this one:

PA060 S Dynaco ST70 Upgrade Power Transformer Replacement MADE IN USA

It's not Edcor, but it's a 100% drop in. Alternately, Edcor will custom make you a drop in replacement if you ask.
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Old 22nd February 2013, 08:48 PM   #3
SandyG is offline SandyG  United States
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If your circuit can tolerate a bit more B+ voltage take a look at the EDCOR XPWR233-100. It has 360-55-0-360 with both 6.3Vac and 5Vac filament windings.

As mentioned above you could specify exactly what you want, and sometimes is not much more money. Remember not much come off the shelf from EDCOR so any way you do it you have to wait a couple of weeks so custom might just be fine.

I like their products, very nice finish and good value and quality.

Sandy
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Old 23rd February 2013, 04:17 AM   #4
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Sandy, I noticed that the XPWR233-100 is designed for 100VAC. What would happen if I used it in a typical 120VAC outlet? Other than that, the specs on that transformer is exactly what I am looking for, nice find!
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Old 23rd February 2013, 10:21 PM   #5
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Hi,
Well, the old ST-70 is up and running again, but there's still some transformer humm.
It's MUCH quieter, but the hum is still audible from the listening chair, which I don't find acceptable. It's only a mechanical humm, it's DEAD quiet through the speakers, you can only hear a slight tube rush if you have your ear right up to the tweets. I'll see if it gets quieter later at night when the AC is better.
It's still good to have it working again. It's sooo much sweeter than the Adcom GFA535II that I picked up as a temporary replacement, it's silly.



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Old 24th February 2013, 01:34 AM   #6
SandyG is offline SandyG  United States
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Shoot, didn't catch the 100vac primary on that transformer. It would not work unless you plan on running a power cord to japan I would just shoot EDCOR that you want the same transformer with 120vac primary windings, or spec the exact one you want. It may not be more then a couple of bucks more for the custom. Also I think I saw some specs for the ST70 xformer with a 360-0-360 winding so it might be fine even at that voltage.

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Old 24th February 2013, 07:56 AM   #7
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I sent EDCOR a request to change the primary to 120V, hopefully I hear back from them on Monday. It's only my second amp build, and I'm really excited to order and put together all the parts. Maybe I'll even post some pictures along the way, since I probably will have some newbie questions for the forums.
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Old 24th February 2013, 02:13 PM   #8
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This is probably blasphemy here, but you can dump the tube rectifier. Tubes are particularly ill-suited to that job. You get a much better power supply with solid state diodes there. That would solve your 5V issue.

I'm using the transformer that Sy referenced above. It's a good one. I don't use the 5V winding, so I have a bit more headroom in that thing as well.

Sheldon
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