12 volt supply

[andrew]

I have an unused 5v ac supply (rectifier) and I would like to convert it to 12v dc to power one tube that requires 300ma. Would this be suitable? http://www.ebay.com/bhp/12v-dc-500ma-power-supply Could something be rigged up using the 5v ac fairly easily? Thanks for any help.

[Firestorm]

If you rectify, voltage double, then voltage regulate the 5VAC, you can get your 12VDC. Why did you include the link to the wall wart?

[martin manning]

Am I understanding correctly that you have a 5VAC winding and you want to convert it to 12VDC to power a filament that requires 300mA? That is do-able with a voltage doubler circuit and a 1A regulator like LM7812. I'm not sure why you have a link to 12V wall adaptors, unless that is another option you are considering? They are usually not well regulated and noisy.

[andrew]

thanks for the replies. The wall wart looked like an easy alternative but if it is noisy I'll go the other route. Thanks

[gingertube]

If you voltage double the 5V using schottky diodes and say 2 x 10,000uF capacitors you will get 12V DC. You will not have enough voltage for a regulator.

For guidance ONLY, I used a schottky bridge on a 5V rectifier heater winding into a 10,000uF capactor, with a 12AX7 heater load (300mA) I got 5.95V which was close enough. to the ideal 6.3 volts. The above suggestion is based on this experience.

Cheers,
Ian

[martin manning]

Thanks Ian. Going unregulated is simpler, and probably close enough as you say. The only drawback is the size of the 10,000 uF caps.

If this 5V winding is right on 5VAC, it would produce (5x1.414 - 0.7)x2 = 12.8V peak. Since it's lightly loaded the AC voltage might be a bit higher than 5, 5.5 say, yielding a bit over 14V, but that would still be cutting it close even for a low-dropout regulator.

Assuming 5VAC, a voltage tripler would produce 18.2V, which is easily enough for an ordinary regulator to function. This means a few more parts, but they are small. I'm not sure how well the regulator will like the high initial current a filament will draw, though.

[printer2]

I bought a switching power supply off EBay and it has no problem with two 12AQ5's ans a pair of 12AX7's.

[img935]http://i406.photobucket.com/albums/pp14 ... 32d06a.jpg[/img]

[img784]http://i406.photobucket.com/albums/pp14 ... 78c87a.jpg[/img]

I find you have to double the rated current if you want to use them for 120V, normally the cheap stuff needs 240V to put out the rated current.

[dorrisant]

http://www.ebay.com/itm/110831075196?ss ... 1423.l2649

$6.98 shipped. 2A@12v with adjustment trimmer.
Approximately 2 5/16" wide, 3 5/16" long and 1 5/16" tall.

Gonna be trying these out in some Altec 1566A rebuilds. Got one on my bench now... Looks cute, we'll see.

Tony

[ampfab]

why not go to radio shack and pick up a small 12v transformer for about $5?

[Ned]

Could I as what tube you intend to supply?

[andrew]

12sn7,12sj7, and a 12ax7. Four el84 output tubes using the 6 volt supply from the PT.
http://www.radioshack.com/product/index ... RadioShack That does look good. Thanks

[Structo]

I wanted to buy one of those 12v transformers from Radio Shack before.
The website said it was in stock so I drove up there to get it.

Nope, and I drove to another store, nope.

So I wasted about $5 in gas trying to get something I would have to order anyway.....

I think it is a discontinued item.

[ampfab]

just at radio shack today. multiple 12v transformer choices.
I would choose the 450ma one.
small and about $7.50

[Milkmansound]

http://www.ebay.com/itm/110831075196?ss ... 1423.l2649

$6.98 shipped. 2A@12v with adjustment trimmer.
Approximately 2 5/16" wide, 3 5/16" long and 1 5/16" tall.

Gonna be trying these out in some Altec 1566A rebuilds. Got one on my bench now... Looks cute, we'll see.

Tony

I bought a few supplies from digikey a few years back based on a post that Dr Z made. I have used them to power 4 preamp tubes and they are great. I will have to try some of these too. Silence is truly golden - especially when it easily mounts to the side of the chassis out of the way

http://www.digikey.com/product-detail/e ... ND/1304581