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Current capability full wave bridge rectifier
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Old 12th April 2019, 06:22 PM   #11
Mooly is offline Mooly  United Kingdom
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Current capability full wave bridge rectifier
Yes it is 8.5 volts but remember that in LTspice that is the peak voltage we set. So the RMS value is just 6 volts.

The cap gives a non sinusoidal ripple and so the big question is what is the real equivalent DC heating power in that resistor. Thats the sort of question that could be endlessly debated
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Old 12th April 2019, 08:57 PM   #12
ferret is offline ferret  South Africa
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Default Over specified rectifiers

I was lucky enough to pick up a bunchem of NOS electronics from an industrial electronics place some two years ago. Amongst this was roughly 200 off P600J diodes (6A @ 600V) which I used on a filament supply - overkill, but ...
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Old 12th April 2019, 09:03 PM   #13
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Mooly View Post
...The cap gives a non sinusoidal ripple...
And a lot of ripple at that, about half a volt peak-to-peak from the graph. Even with a huge 10mF (10,000 uF) filter cap.

This is one of several reasons why a small switching power supply is a much better solution for this particular task. Ripple voltage tends to be much smaller. Because today's switchers operate at tens of kilohertz, it takes relatively little additional filtering to remove even that little bit of ripple. For heater power, I've never had to add any additional filtering.

With a switching power supply for power, the thermal inertia of a valve heater completely removes any ripple in the actual heater wire temperature, because of the high ripple frequency.


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Old 13th April 2019, 01:17 AM   #14
PRR is offline PRR  United States
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Quote:
Originally Posted by Mooly View Post
...The cap gives a non sinusoidal ripple and so the big question is what is the real equivalent DC heating power in that resistor.....
In Probe: RMS(Vheater). Let run several cycles.

On fingers:

The sawtooth is symmetric between 6.07V and 6.54V. Average is 6.305V.

RMS is something near the root of half of 6.07V^2+6.54V^2 which has numbers 36.845 and 42.77, giving near 6.309V.

The sub-0.1% discrepancy is clearly beneath practical notice.

The other question is whether the half-volt of ripple-spike may be more annoying than 6V of pure hum.
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Old 13th April 2019, 06:52 AM   #15
Mooly is offline Mooly  United Kingdom
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Current capability full wave bridge rectifier
So close enough for jazz then

(I can see the attraction of an SMPS for this kind of application)
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Old 15th April 2019, 05:47 AM   #16
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Mooly View Post
(I can see the attraction of an SMPS for this kind of application)
Here is some very interesting data on the subject. Post #32 by ruffrecords on this page ( REW Update ):
Quote:
Originally Posted by ruffrecords
...I found the biggest culprit was the 12V dc heater supply...Big toroid (transformer) followed by big bridge and reservoir cap followed by LDO (Low Drop Out voltage regulator) on big heatsink...The toroid is asked for some serious current pulses by the rectifier...
<snip>
The electronics was in a screened box and I even invested in toroids with mu-metal screens which helped but hum was still affecting the noise floor
<snip>
Almost in desperation I replaced the linear heater supply with a Meanwell 12V SMPSU (Switch Mode Power Supply Unit). Bingo, no more hum. REW (Room EQ Wizard) measured the best noise floor I have ever achieved with a tube mic pre.
There it is, a documented case where a cheap, light, small, efficient switching power supply did a better job of reducing hum than a hefty (and expensive) toroidal main transformer, a huge filter cap, and a linear voltage regulator!

This was the missing piece in the golden era of vacuum tubes. Back then, DC heaters meant battery power. Even after silicon rectifier diodes arrived, there was no practical way to produce clean DC power for heaters - even with the biggest and most expensive electrolytic filter caps available at the time. But now, decades later, switch mode supplies are ubiquitous, and they are the perfect solution for clean, hum-free, heater power.


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Old 15th April 2019, 07:26 AM   #17
Mooly is offline Mooly  United Kingdom
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Thanks It does make sense.
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Old 15th April 2019, 02:44 PM   #18
Max999 is offline Max999
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Thanks everybody. The DC heaters I wanted to implement are on this site The Valve Wizard

Unfortunately they start to look like a bad plan now, too much ripple spike. I elevated the heaters with 55V and this helped but there is still some hum.

Smps's sounds like the way to go .. does anybody knows a smps that is easy to screw in my amplifier chassis? Can I take just any led power supply or the Meanwell ones?
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Old 15th April 2019, 06:19 PM   #19
Gnobuddy is offline Gnobuddy  Canada
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...there is still some hum...SMPS sounds like the way to go...
I suggest first doing a quick test with whatever suitable SMPS you have lying around - use a series resistor to drop the voltage to 6.3V if necessary. The test is to make sure the hum really does go away when you use an SMPS.

For instance, if you want to power a 12AX7 heater, you can wire it for 12V operation and connect to a 12V DC SMPS. If you only have a 7.5V SMPS lying around, wire the 12AX7 for 6.3V operation, then use Ohm's law to estimate the series resistor to use:

R = voltage drop / current = (7.5 - 6.3)/ 0.3A = 4 ohms

Power dissipated in resistor = voltage dropped x current = (7.5 - 6.3) x 0.3 A = 1.2V x 0.3 A = 0.36 watt.

So you could put a 3.9 ohm, 1 watt or 2 watt resistor in series with the 6.3 V heater, and power it with the 7.5 V SMPS.

Remember to ground (or elevate to known DC voltage) one end of the heater voltage - you don't want the DC output of the SMPS to float to some high voltage and damage the heater-cathode insulation in the 12AX7, so do something to prevent that.

The second question you asked - the cheap and ugly solution is to wire the AC prongs of the wall-wart to your incoming switched, fused, AC mains voltage, inside your amp enclosure. Use heat-shrink tubing to make sure there is no exposed metal at mains voltage!

The slightly more expensive solution is to buy the guts of an SMPS, but not packaged in a wall-wart with a built-in AC plug. For example, Digikey has this very affordable 7.5V, 2A SMPS on a bare PCB you can mount inside your chassis: EPS-15-7.5 MEAN WELL USA Inc. | Power Supplies - External/Internal (Off-Board) | DigiKey

You will need connectors to fit that SMPS, and it should be mounted inside a plastic housing - there are deadly voltages on parts of the SMPS board.

I have also seen PCB-mount SMPS modules at Digikey and Mouser. Connections are made with pins sized to fit through typical PCB holes - you can use a generic proto-board PCB. And then there are the chassis-mount SMPS modules, which are already housed in some sort of metal enclosure, like this one: https://www.digikey.ca/product-detai...708-ND/7703186

While the more expensive solutions are elegant, I usually just hunt through the junk-box or visit the nearest thift store to find an SMPS with suitable ratings, and just use that.

For me, good engineering is all about performance, not appearance. (It should look good when the case is closed, but I don't care if it's "pretty" inside the box.) But I understand that not everyone feels the same way, particularly in DIY!


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Old 15th April 2019, 06:39 PM   #20
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by Max999 View Post
Thanks everybody. The DC heaters I wanted to implement are on this site The Valve Wizard
The tiny, cheap, reliable, SMPS that are everywhere these days are a pretty new phenomenon. Twenty years ago (very recent, in historical terms!), they were pretty rare, except inside you big desktop PC, or outside your expensive laptop.

It's only in the last ten years that these SMPS were suddenly found in all sorts of new consumer electronics products, and only in the last five years that they have become ubiquitous. Nowadays, most people have a small collection of wall-warts just lying around the house, left over from dead electronics products.

I don't know exactly when Merlin Blencowe (aka the Valve Wizard) originally put that page together. It might have been a few years ago, when cheap ubiquitous SMPS still didn't exist.

I also note that I've found on this very forum that many grizzled DIY valve veterans become quite angry if you suggest that a cheap modern SMPS is in fact the best heater power solution. They react as though you had just insulted their religion. They are emotionally invested in ancient technology, and do not want to even think about the possibility that something new might actually be a perfect complement to the old technology they love. And they want to show off their beautifully twisted and laid out heater wiring.

The fact is that, from the engineering (rather than emotional) point of view, AC heater power has never made any sense. Running several amps of audio-frequency current through the guts of a sensitive audio amplifier? Absolute insanity! It's the equivalent of putting 50 kilos of rotting fish in the back seat of your car.

Of course there was a reason why engineers from the golden era of valves were forced to use AC heater power - there was no affordable, practical alternative at the time. If there had been, I am quite sure they would have rejoiced and immediately switched to it.

But lacking anything but noisy AC heater power, they came up with all these clever tricks to manage the stinky fish, sorry, noisy AC: twisted heater wiring, heater wires carefully routed around valves and pushed into corners of the chassis, et cetera. It's like putting three trash bags, one over the other, around the 50 kilos of rotting fish in the back seat. If you bag and seal everything perfectly, it will only stink a little.

Times have changed. Today, we can use clean cheap DC from an SMPS - and it's not only better, it's even cheaper than paying for the extra transformer windings needed for AC heater power. We have no reason to keep rotting fish in the back seat any longer at all!


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