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Old 9th August 2012, 06:45 AM   #1
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Lightbulb Dont have a power transformer?

Sometimes I need test some tube, or just do a simple amplifier, but dont have any power transformer around.. Well, here where I live, any PT is expensive (120-300R$), so, this is a great ideia that I found, and is very easy to do:

Click the image to open in full size.

I grabed this two transformers from scrap eletronic (am-pm radio and microwave), and I connect together the two secondaries. The two transformers need be different if you want more that 220V, like on this case. In this case, the microwave transformer give to me 24V, and the another have a 12V secondary, so on the output have 440-220V, but with loss and load, I get after rectified, around 340V. The first transformer get a little warm, but nothing really hot, so dont worry.
For the filament, I used another power source, but this work pretty good with 12v winding from the microwave transformer...
In total, I spended 8R$, 2 on the radio, 6 on the microwave mainboard, and considering the result, Im really very glad. Note: The vacuum tube is a EL84.

As I said, it's simple, but work pretty good.
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Old 9th August 2012, 07:51 AM   #2
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It's a good idea if you have them laying around. Pretty common actually. Probably b/c it's cheaper since 12V transformers are so abundant.
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Old 9th August 2012, 07:58 AM   #3
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Old 9th August 2012, 08:55 AM   #4
d a o is offline d a o  Denmark
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HI felipemuniz Super idea :-)

And if you make a generator that can control the last transfomator for example. with 400 Hz so you will see that all electrolytes multiplies their Capacity.

Like 12v-dc-220v-ac-converter/
Or 12v-dc-to-220v-100w-inverter-4047-irf540
Power inverter-12v-to-110v-220v-500w

However, the generator must preferably be a sine wave generator, So it's not high pitched noise it produce.
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Last edited by d a o; 9th August 2012 at 08:59 AM.
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Old 10th August 2012, 01:27 AM   #5
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I also did this thing seems i have plenty of main xformers around, connecting them back to back to get high voltage source for the plates. For higher current...i use two step down autoformer (0-110-220v) of about the same capacity if not exact...i disconnect the center tap to make two windings separated and connect in series to form two (0-220vac) isolated...
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Old 10th August 2012, 11:30 PM   #6
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i use this trick on cd players when modding them for tube output, usually the 12v power transformer in a good cd player will deal with another 500mA load, i have a huge pile of minature pcb mount 12 transformers that i connect up, sec to sec for the high voltage,and the first secondary 12v is also used for the heater supply... this combination works very well indeed
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Old 11th August 2012, 08:57 PM   #7
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Hey Felipe - ever worried about the insulation of the wire on those transformers? Do you think it is ok to handle 440V?

I hate to ask it, but I just don't want to see you or a friend of yours get a lethal shock.

Some here wind their own transformers to save cost (always use NEW wire).

Here is one tip I can give you for low power filaments -> look for old laptop power supplies. Try IT departments at large companies, hospitals, governement offices, etc. Ask if they have any old laptop power supplies that they need to recycle. You might be surprised at how many nice 12V supplies you can get this way.

I find some of the older laptop power supplies from Dell to be especially good quality.
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Old 14th August 2012, 08:00 PM   #8
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Hi, d a o.
I'll look for this first (electroschematics.com), maybe soon I'll start some tests with this inversors on the tubes.. Thanks

Hi, soulmerchant.
Dont worry, I always respect the components. Of course, I take some precautions while I'm working, like using thick shoes, one hand on the pocket, pliers, screwdrivers, etc with good isolation, discharging capacitors after any test.. I work with eletronic's since I was 12 years old (no high voltage, tubes), and this gave-me kinda experience with this precautions... Recently (since end of last year) I'm trying work with tubes amplifiers, and I'm loving working with this. On the last months, I made 6 amplifiers (two Hi-fi Single-Ended Stereo El84, and four guitar amps, Tiny Terror, AX84, JCM800 tube pre + solid state power and a 30W 4x EL84 that I developed by my self) and I sold they all, but that was hard to do it, because I spend very time looking for components like transformers, tubes and chassis, and I cant move much money yet... I'm 20 years old, and this is the only job that I have, but I think that is gonna get better sometime ^^
About made own transformers, I'm reading about, but I dont have chance to try yet
... And thank you about the tip about the power supplies, I always go to "second-hand stores" (we call here by "brechó"), and I buy power supplies, old radios, cables, to recycle, sometimes I even fix they, hehe... thank you.

Well, thanks for all replies, really ^^
I put the amp into a plastic box this week, and looks that is working pretty fine I think that the amp is getting about 4 or 5 watts, with my cellphone playing with phone output.

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Old 31st December 2012, 12:56 PM   #9
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the second transformer is operating at twice the rated voltages. is that ok. Is it possible to use a 110-6v 5A transformer as a 220-12v 2.5A one. i have a few 110v walwarts, could use the transformers out of those if it works with 220v.
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Old 31st December 2012, 04:03 PM   #10
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Quote:
Originally Posted by doors666 View Post
the second transformer is operating at twice the rated voltages. is that ok. Is it possible to use a 110-6v 5A transformer as a 220-12v 2.5A one. i have a few 110v walwarts, could use the transformers out of those if it works with 220v.
I think so.. You can also use a EL86 to replace the EL84. Is very similar to EL84, but this tube need only about 200v, with rate max 250v. If you get 220V with a good current, the tube can work without problems. The output transformer, I think that will have less impedance on secundary that EL84 OTs. Check the pdf from this tube.
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