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Old 2nd October 2009, 06:39 PM   #1
Atilla is offline Atilla  Norway
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Default Any benefits of regulated current heater supply?

I'm not new at playing with electronic circuits, but I've something on my mind that confuses me a little bit and I wanted to see if the way I think about it is wrong.

Most tubes with seperately powered heaters have a voltage requirement. Let's say 12-12.6V. I will immediately think of providing regulated 12Vdc supply for them. If they're rated for this voltage, they must come with fillament resistances within an acceptable tolrance, so that the current trough the tube is at the necessary level. That's fine, designing a regulated circuit, for 150 to 600mA is not a big deal.

But they *also* specify the current. Commonly 150mA per fillament. The thing is - the resistance of the heater is not fixed, it has a PTC. It's a fair amount lower than the specification at turn on and it takes time to settle. Heater warm-up times for tubes like 12AU7 are specified at times like 12sec.

Now, with a resistance that varies, a CCS would provide a more stable working environment than a CVS, especially at turn-on time, where it would prevent excessively large currents flowing. One extra advantage, which is situational, is that if the transformer providing the power is with a very low rating, it will prevent the supply sagging under the initial load. (Fair enough, it's a moot point, if the supply sags, there'll be less current flowing in anyway, it's regulation, but it's the transformer that takes the toll).

My question is: is there an advantage of the CCS over a CVS? Both can be realized with the same parts, for the same price. It wouldn't even hurt to have both installed - overkill maybe, but you get a stable DC for not too much effort, so where is the problem?

It seems logical to me that any piece of sensitive wiring would like to avoid the inrush current as well, where a CCS helps. Even if extended lifetime is debatable, or the price of the tube insignificant, an LM317 is still a cheap piece of hardware.

Any thoughts on this? For my practical scenario, I've a supply that's perfectly happy to supply 150mA at 12V, but will sag under the initial, high current draw, for 2-5 seconds. It'll then go too much above 12V, if not regulated. Perfect stability is obvious - regulate both and the tube will be happy. But then, if a tube needs 150mA, a CCS is enough...

I know I'm overthinking this, but I'm just starting to use tubes (yeah .. 2009, I know, right) and some of their requirements puzzle me.

Cheers,
A fellow tube soon-to-be-addict.
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Old 2nd October 2009, 07:09 PM   #2
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If you're going to do that, then it's CCS for series connected heaters, CVS for heaters in parallel. As for the whole DC heater deal, it's beneficial for very low level stages, such as a preamp for a MM or MC cartridge, a guitar pick-up, or the earliest stages in a receiver (first mixer, and/or front end RF amp if one is used). If you already have an amp that's set up for the usual Vi= 1Vrms, then it's highly doubtful if it's worth it. One project I did has the whole heater winding floating (to avoid busting Vhk specs) and there's very little noise at the output, just a few mV, and you can't hear it from a foot away. AC heater noise usually isn't a big deal, and if it is, the spec sheet will say so. The capacitance between the heater and cathode is so tiny that it doesn't become a problem below the VHF band, and certainly not at power line frequencies.

DH tubes are a different story, and will probably need DC for heating. As for whether CCS or CVS heater power is better sonically, that's an on-going debate that's never been settled. So far as heater current in-rush, it's less than for light bulbs, and heater failures are quite rare, and almost always occur with those power xfmr-less designs that series connect all the heaters in series across the mains. You shouldn't be doing that anyway.
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Old 2nd October 2009, 07:20 PM   #3
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Old 3rd October 2009, 09:42 AM   #4
Atilla is offline Atilla  Norway
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I guess I'll try both approaches when I've got the time. For the first try - voltage regulation it is.
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Old 3rd October 2009, 10:17 AM   #5
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There may not be any clear technical advantages but i prefer the sound with a CCS. I also prefer regulated dc to ac heaters but capacitors/regulators are clearly audible. The CCS approach eliminates the ouput cap and may be the reason i prefer it.
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Old 3rd October 2009, 10:24 AM   #6
SY is offline SY  United States
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There's extensive data and design information on series vs. parallel heaters and the use of constant current vs. constant voltage supplies in "Valve Amplifiers" 3rd edition. If you're starting to get into tubes, it's a book you need.
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Old 3rd October 2009, 10:45 AM   #7
Atilla is offline Atilla  Norway
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Quote:
Originally Posted by SY View Post
There's extensive data and design information on series vs. parallel heaters and the use of constant current vs. constant voltage supplies in "Valve Amplifiers" 3rd edition. If you're starting to get into tubes, it's a book you need.
Great, a nice book is good to keep around.
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Old 3rd October 2009, 07:37 PM   #8
Arnulf is offline Arnulf  Europe
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Quote:
Originally Posted by analog_sa View Post
There may not be any clear technical advantages but i prefer the sound with a CCS. I also prefer regulated dc to ac heaters but capacitors/regulators are clearly audible. The CCS approach eliminates the ouput cap and may be the reason i prefer it.
Which "output cap" would that be ?

You can hear the difference between tubes with their heaters powered by a constant current source versus a constant voltage source ?
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Old 3rd October 2009, 07:53 PM   #9
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I might try CCS string with MOSFETs strapped as voltage regulator diodes in shunt.
That way heatup can be a bit faster (set the CCS a little higher than needed) , but
no huge rush like cold filaments on fixed voltage with nearly unlimited current.

Last edited by kenpeter; 3rd October 2009 at 07:56 PM.
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Old 3rd October 2009, 09:06 PM   #10
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I use a zener regulated supply in one of my amps and it works fine.
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