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Old 20th August 2009, 10:52 PM   #1
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Default Back to Back transformers

I am looking for a cheap way to get 6V @2A for a heater and ~150V, maybe 175V, @ 100mA for a B+ supply. The cheapest, and most compact, I can find are a couple of Antek 50VA toroids back to back. They are $13 each. Can anyone comment on the wisdom of doing this? So long as I am not overstressing the transformers, is there an inherent reason that this is a bad idea?
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Old 20th August 2009, 11:07 PM   #2
tomchr is offline tomchr  United States
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So rather than using the $29 Antek AN-1T150 (2 x 150 V @ 2 x 0.4 A + 2 x 6.3 V @ 2 x 3 A) you'd rather use two transformers to the tune of 2 x $13 = $26?! Sorry. That makes no sense to me. Granted $3 is $3, but still. That's a cup of coffee...

But technically, there isn't anything that would prevent you from doing what you describe. In fact, I've done it once with scrap iron to get an old HAM radio working.

~Tom
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Old 20th August 2009, 11:30 PM   #3
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Twice the size and weight, but regulation is only half (voltage drop in both transformers). 100 VA worth of iron for maybe 30 VA output. Your B+ may come out lower than you think...
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Old 20th August 2009, 11:37 PM   #4
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by dsavitsk View Post
I am looking for a cheap way to get 6V @2A for a heater and ~150V, maybe 175V, @ 100mA for a B+ supply. The cheapest, and most compact, I can find are a couple of Antek 50VA toroids back to back. They are $13 each. Can anyone comment on the wisdom of doing this? So long as I am not overstressing the transformers, is there an inherent reason that this is a bad idea?
I just did something like this a couple weeks ago. I used a pair of transformers salvaged from two 12VAC "wall wort" plug in power supplies. I wired the the two 12VAC "secondary" together. This supplied about 160V DC B+ and a 12V heater (for 12AX type tubes) I used 1000 ma size transformers.

The only problem was the size. The completed assembly is twice or more the bulk of a small PT.

Look at Edcor. I think they make the PT you need for $20.01 list price.
http://www.edcorusa.com/Products/Sho...ct.aspx?ID=598

I used the back to back design only because I already had the two transformers, so for me it was zero cost. I would not do that if I had to buy both,

Last edited by ChrisA; 20th August 2009 at 11:44 PM.
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Old 20th August 2009, 11:45 PM   #5
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I've only done back-to-back transformers when using a wall-warts. I've used 12V and 24V Wall-warts back-to-back with their radioshack trannies to get the B+ for preamps, usually with a voltage doubler. I did this for both a 5687 and 12B4 preamp, with the wall wart feeding a regulator, in turn feeding the heaters wired in series.

With the 1.83A/24V wall-warts, I had no issues with using a 24V 2A radioshack transformer at the other end to get a 330V B+, even with the heater draw.

There are some cheap sources of 12V to 240V and 24V to 240V transformers at some of the surplus sites to make it even easier.

There's a good article in AudioXpress from a few years ago that discusses all sorts of wall-wart tricks and tips to do tube amps/preamps, etc.
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Old 20th August 2009, 11:53 PM   #6
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Originally Posted by tomchr View Post
So rather than using the $29 Antek AN-1T150 (2 x 150 V @ 2 x 0.4 A + 2 x 6.3 V @ 2 x 3 A) you'd rather use two transformers to the tune of 2 x $13 = $26?!
It isn't the $3, it's that the big antek is too large for my case while the two little ones will fit.


Quote:
Originally Posted by Tom Bavis View Post
Twice the size and weight, but regulation is only half (voltage drop in both transformers). 100 VA worth of iron for maybe 30 VA output. Your B+ may come out lower than you think...
Worth considering -- but I can just use the 10 or 12V instead.

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Originally Posted by ChrisA View Post
Look at Edcor. I think they make the PT you need for $20.01 list price.
http://www.edcorusa.com/Products/Sho...ct.aspx?ID=598
Good find -- it's OK except for being only 60mA -- If I could get 100mA out of it then it would be perfect.
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