simple linear DC heater supply?

Milkmansound · Post by **Milkmansound** » Sat Mar 15, 2014 5:48 pm

does anyone have a simple, and good circuit idea for this?

I would like to run 3 12AX7s off of DC - and I have a 1A 6.3VAC winding to work with.

Currently I have el-cheapo SMPS in there and its actually, quite perfect. But I am wondering if there is a way to use the heater taps instead.

So far my experiments have not yielded good results, so I am putting it out there.

Post by **xtian** » Sat Mar 15, 2014 6:19 pm

I can't answer your question, but I would like to hear about your "SMPS" solution.

Dr-Joned · Post by **Dr-Joned** » Sat Mar 15, 2014 6:33 pm

I have not tried it, but possibly using a voltage tripler may get you close to 12.6 after loading. Or you could use this boost converter to 12vdc.

http://www.mpja.com/LM2577-Step-Up-Adju ... 0300%20PS/

Milkmansound · Post by **Milkmansound** » Sat Mar 15, 2014 6:40 pm

ooh - that might work. Its smaller and cheaper than the SMPS I have in there

Xtian - I am using a small Condor 12v supply right now. There is also the Meanwell 12V supply, which is even smaller and cheaper - but less friendly to mount in a chassis. Make sure you ground negative to the chassis, otherwise it'll be a noisy mess.

jaysg · Post by **jaysg** » Sat Mar 15, 2014 7:10 pm

I am not sure you can do much because you barely have enough for AC. You lose current capacity with a doubler.

If you had a 2A supply, you could double it and use an LM317 or 7806 for a well regulated supply. Mr. Fuchs recently pointed out that there are many crap 12a_7 tubes out there that hum on anything simpler.

Milkmansound · Post by **Milkmansound** » Sat Mar 15, 2014 7:28 pm

yeah - even a 1A AC supply barely has enough to cover the 3 tubes.

Maybe I should just stick with the 2A condor supply and call it a day

Leo_Gnardo · Post by **Leo_Gnardo** » Sat Mar 15, 2014 7:29 pm

jaysg wrote:If you had a 2A supply, you could double it and use an LM317 or 7806 for a well regulated supply.

FWIW you can always use a 7805 and "stand up" the ground reference on a pair of forward biased silicon diodes/rectifiers to get close to 6.3V. 7806 not so common. '05's at any RatShack.

pops · Post by **pops** » Sat Mar 15, 2014 9:59 pm

Built a 5f6a twin a while ago and after the power tubes i put a small bridge rectifier in the heater circuit, used caps between the preamp tubes and let the heaters be the resistance, was really quiet. Don't remember what values i used, small ecaps tho as voltage didn't need to be much.

Leo_Gnardo · Post by **Leo_Gnardo** » Sat Mar 15, 2014 10:29 pm

pops wrote:Don't remember what values i used, small ecaps tho as voltage didn't need to be much.

Easy & cheap to find: 6.3V or 10V from 4700 to 15,000 uF.

Gaz · Post by **Gaz** » Sun Mar 16, 2014 8:42 am

If you are just trying to reduce noise, just the first 12ax7 should be fine which frees up some current capability.

markusw · Post by **markusw** » Mon Mar 17, 2014 7:55 am

Milkmansound wrote:does anyone have a simple, and good circuit idea for this?

I would like to run 3 12AX7s off of DC - and I have a 1A 6.3VAC winding to work with.

You could use 1N5820, -21, oder -22 Schottky diodes and a low dropout voltage regulator like the LT1086 for regulated 6,3VDC.
The Schottky diodes in a bridge rectifier shouldn't even drop 1V in total.
So with 6.3VAC*1,414= 8,9VDC minus 1V for the Schottkies = 7,9VDC.
The LT1086 has a drop out voltage of about 1V. With 7,9VDC after the rectifier you'd still have 1,6V input-to-output differential for the LT1086.
1A is however as you already stated is pretty low for three 12AX7. With three 12AX7 you might not get 8,9V after the rectifier.
Probably still worth a try.

rp · Post by rp » Mon Mar 17, 2014 12:13 pm

Seeing how you're making amps for a living it might end up cheaper in terms of time to just use something like this, granted you still have to take the time to stuff it:

http://www.ebay.com/itm/DIY-PCB-2x-Regu ... 255bd64898

http://www.pmillett.com/DC_filament_supply.htm

or this, though it's likely too expensive for non hobbie work though it's drop in and go.

http://www.tentlabs.com/Products/Tubeam ... index.html

The Millet, if I got it right need 6.5V in min and the TentLabs 4V. Both are pretty small, height I guess depends on what caps you use. I was going to try one or the other for my 5C8 but got it pretty quiet by turning it into an octal 5D8. When I have some extra dough I'd like to get a pair of the Millet boards and throw then into the parts box just to have on hand.

Milkmansound · Post by **Milkmansound** » Mon Mar 17, 2014 3:29 pm

those are great links thanks!

the Meanwell and Condor supplies are both under $20 (assembled)

So for the mean time, I am going to stick with them and leave the 6.3V taps capped. There's already 2 other class D supplies in this particular model, so whats one more?

Hopefully they are all FCC approved

The Amp Garage

simple linear DC heater supply?

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