DC Filament Supply questions

[ccaudle]

No, in this case I'm stepping up to 12.6V

Assuming that is the usual 12.6V RMS rating, it will be higher than that at loads lower than the rated current of the transformer.
12.6V AC RMS will be almost 18V peak, so around 16.5V give or take at the output of the rectifiers. Your current simulation shows 10.7V at the output of the rectifiers, so I doubt that will accurately represent the real power supply.
And don't forget that the output scales with input voltage, so given typical AC line voltage variations you should design for at least +5%/-10% variation in input voltage.

This is a case where you should really grab a volt meter and validate that your values you are using for simulation are accurate before you spend very much time tweaking the fine details of the simulation. You could spend hours playing in the simulator only to get to the bench and find that you misunderstood the transformer rating and so were using values in the simulator which were off by 40% compared to the real world values.

 

[jonasnoble]

Assuming that is the usual 12.6V RMS rating, it will be higher than that at loads lower than the rated current of the transformer.
12.6V AC RMS will be almost 18V peak, so around 16.5V give or take at the output of the rectifiers. Your current simulation shows 10.7V at the output of the rectifiers, so I doubt that will accurately represent the real power supply.

That's also a good point that I didn't catch. I do have about 16V after the bridge, which is why I wanted to divide it down before the regulator. But if a 7806 can handle up to 25V, my only concern is the heat dissipated. I only have small to-220 heatsinks.

 

[ccaudle]

my only concern is the heat dissipated

10V across the regulator at 0.6A is 6W dissipated in the linear regulator.

I only have small to-220 heatsinks.

Do you mean something like one of these small clip on heatsinks?


I found that at Mouser, the datasheet says with natural convection (i.e. no fan) it has around 30deg/W thermal resistance. That means that at 6W the heatsink will be around 180 deg C higher than ambient temperature, or over 200deg C in room temperature. Regulators are often rated to 100 deg or higher, but I would not recommend running them that hot if you can avoid it.

You could use something like one of these switching regulators in a LM78xx compatible pin out either to generate your heater voltage, or to pre-regulate the input to 9V, and then use a passive noise filter and 7806 regulator to generate the 6V if you are nervous about using a switching regulator to generate the heater voltage directly.
Recom switching LM78xx replacements at Digikey

 

[scott2000]

Recom switching LM78xx replacements at Digikey

Neat

 

[C H]

I might have missed something, but why not run the secondaries in parallel as suggested? You're heating the circuitry as much as the heater running them in series, hugely wasteful.

 

[amplexus]

I've used these and they work excellently well. They're a bit closer to the footprint of a TO220 than some of the others.

https://www.digikey.ca/en/products/detail/gaptec-electronic/LMS78-05-2-0R/13691812

10V across the regulator at 0.6A is 6W dissipated in the linear regulator.



Do you mean something like one of these small clip on heatsinks?
View attachment 114916

I found that at Mouser, the datasheet says with natural convection (i.e. no fan) it has around 30deg/W thermal resistance. That means that at 6W the heatsink will be around 180 deg C higher than ambient temperature, or over 200deg C in room temperature. Regulators are often rated to 100 deg or higher, but I would not recommend running them that hot if you can avoid it.

You could use something like one of these switching regulators in a LM78xx compatible pin out either to generate your heater voltage, or to pre-regulate the input to 9V, and then use a passive noise filter and 7806 regulator to generate the 6V if you are nervous about using a switching regulator to generate the heater voltage directly.
Recom switching LM78xx replacements at Digikey

I've used these

 

[jonasnoble]

I might have missed something, but why not run the secondaries in parallel as suggested? You're heating the circuitry as much as the heater running them in series, hugely wasteful.

When I run them parallel, I only get 6.3V into the bridge, and for some reason I cannot get more than 5.7VDC out after filtration. I have tried this once before with similar results. Not sure if I'm doing something wrong.

So I need to start with higher voltage so I can regulate it down.

 

[jonasnoble]

I removed the voltage divider and got 6VDC unloaded. As soon as I connected the tube, the voltage dropped to 1.6V. Maybe this transformer doesn't have enough juice. But both windings are rated at 2A.

 

[ccaudle]

I only get 6.3V into the bridge, and for some reason I cannot get more than 5.7VDC out after filtration. I have tried this once before with similar results. Not sure if I'm doing something wrong.

You are definitely doing something wrong. For 6.3V AC all the numbers I calculated above should be half, so 9V peak, a little under 8V after the rectifier (although for single supply you could use a two-diode full-wave configuration and only drop 0.6-0.7V instead of 1.2-1.4V through the diodes).
That is a little bit low to use a 7806, the regulator drops out of regulation right about 8V input, but the transformer should be able to put out twice the current, no way it should drop to 1.6V when you put a 0.6A load on it if the series configuration still gives 16V. Unless you meant 16V with no load and you have not tried it with the load yet.

 

[Khron]

When I run them parallel, I only get 6.3V into the bridge, and for some reason I cannot get more than 5.7VDC out after filtration. I have tried this once before with similar results. Not sure if I'm doing something wrong.

So I need to start with higher voltage so I can regulate it down.

What exact transformer ARE you using? What are its two secondaries rated at?

 

[RoadrunnerOZ]

When I run them parallel, I only get 6.3V into the bridge, and for some reason I cannot get more than 5.7VDC out after filtration. I have tried this once before with similar results. Not sure if I'm doing something wrong.

So I need to start with higher voltage so I can regulate it down.

The min output of a 7806 is 5.75V, max 6.25V. The minimum input voltage is 8V. If the input voltage is too low it’ll stop working. Sounds like your transformer is outputting 6.3VAC loaded - designed for AC filament supply? Why not put a dummy load across the bridge - like a wirewound 10Ω 10W and see what voltage you get in single and parallel modes. Maybe the transformer can’t deliver the current you need - if it sags too much then your regulator will go below minimum threshold. Also I’d check your bridge rectifier. Maybe put a scope on the output of it with a load in place.
Weird you get a lower voltage when both windings are in parallel - have you measured the AC output of each separately - maybe one is a different output voltage to the other for another purpose?

 

[RoadrunnerOZ]

You could use something like one of these switching regulators in a LM78xx compatible pin out either to generate your heater voltage, or to pre-regulate the input to 9V, and then use a passive noise filter and 7806 regulator to generate the 6V if you are nervous about using a switching regulator to generate the heater voltage directly.
Recom switching LM78xx replacements at Digikey

Just have to watch the selected wattage - those particular ones are only 3W - 500mA @ 6.5V - can these work in parallel?

 

[jonasnoble]

Guys, I appreciate all the help, but I think there's some confusion. My PT has two filament windings, each rated for 2A. I have them run series for 12.6V @2A, because when I run them parallel I don't get enough voltage to supply the regulator.

So it should be real simple, 12.6V into the bridge, two filter caps, then the regulator, final filter, into the load. Unloaded, the voltages look right. All I can think is maybe it's the shitty old caps I had laying around that I used to prototype this build. I planned on putting good ones in it when I was done designing, but maybe now is the time.

It could also be the Chinese regulator just not doing what it's designed to. Or the tube is pulling more than 600ma.

What kind of filter caps do you guys like to use? In high powered tube amps I've always used Sprague, until recently I've liked the F&T's. But for high uF, low V caps, I don't really know what to buy.

 

[jonasnoble]

 

[Khron]

If you connect blue-blue and green-green, what AC voltage are you getting, unloaded?

According to the "usual"(?) way to provide specs, you should get more or less 6.3v on the nose, with each of the two heater windings loaded with 2A. Normally you should see at least 7v AC unloaded, on each.

But for high uF, low V caps, I don't really know what to buy.

Rubycon, Nichicon, United Chemi-Con, Panasonic, Elna. Don't bother with anything else.

 

[RoadrunnerOZ]

Does this need the two primary windings in parallel to achieve the rated output current? Some gear I’ve worked on has the primaries either in series or parallel - others have a multi tap input - they have single (115 CT) or dual (230) for primaries. Seems like this transformer can handle the current without using parallel secondary. I’d be looking at a second transformer for your DC filament supply that outputs a slightly higher voltage or using the DC-DC converter to drop your voltage into a regulator as suggested previously by ccaudle and running the two secondaries in series to get the higher voltage.

 

[jonasnoble]

If you connect blue-blue and green-green, what AC voltage are you getting, unloaded?

I think I get the right voltages, 6.3 when parallel, 12.6 in series. Haven't looked with a scope, but a Fluke 45 should give accurate RMS.

According to the "usual"(?) way to provide specs, you should get more or less 6.3v on the nose, with each of the two heater windings loaded with 2A. Normally you should see at least 7v AC unloaded, on each.

Again, you said something that allowed me to think in a new way. I will check unloaded versus loaded tonight.

Rubycon, Nichicon, United Chemi-Con, Panasonic, Elna. Don't bother with anything else.

Thanks Khron.

 

[jonasnoble]

Does this need the two primary windings in parallel to achieve the rated output current? Some gear I’ve worked on has the primaries either in series or parallel - others have a multi tap input - they have single (115 CT) or dual (230) for primaries. Seems like this transformer can handle the current without using parallel secondary. I’d be looking at a second transformer for your DC filament supply that outputs a slightly higher voltage or using the DC-DC converter to drop your voltage into a regulator as suggested previously by ccaudle and running the two secondaries in series to get the higher voltage.

I do have the primaries run parallel.

You think I need more than 12.6V at 2A to run a DC heater circuit?

edit: I just reread your comment, I will look into the DC-DC converter you guys have mentioned. I just haven't gotten there yet.

 

[My3gger]

I used 6,3V secondaries with LDO regulator like LM1085 and 1N582x low drop diodes with god success. Loss in diodes and regulator is ~2V, leaving enough voltage for 6V heaters which is enough for small 6,3V tubes like 6N1p, etc. Your power trafo has more than enough current capability to not sag under load.
Having unregulated heater voltage would be even simpler, cheaper and more reliable. Regulation isn't needed for a DI box or even mic preamps. Look at the figures Ian (Ruffrecords) got with AC voltage, Telefunken didn't use regulation or stabilization for some of their well known stuff...

 

[Gus]

Good day everyone, could you help me with a simple DC Filament Supply? I've searched the forums and don't see exactly what I need.

This is my first attempt at running DC heaters, and I thought it would be really simple. I have 12.6V into a bridge rectifier, filtered, the voltage divider feed to a 7806 or a Zener, and I cannot get either one to work. The Zener method will give me the 6.3VDC, but as soon as I load it with anything (even a light load) the supply drops to ~2V.

Here are some sims I'm working with. If anybody could direct me to a schematic or thread, I'd be very grateful. (I'm sure this has been covered.) Or, if you can tell me how to provide enough juice that my supply doesn't get dragged down so hard...

Search the web

https://sound-au.com/power-supplies.htm
search for the National Semiconductor Voltage Regulator Handbook or other voltage reg or power supply books and app notes