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Filament DC power supply for 2 valves

02GF74

Member
2010-07-13 1:38 pm
My circuit will have two valves, 6.3v heater and 175mA. (One for each stereo channel)

As my power source is 12vac, i would like to have heaters in series so the va of my transformer is less.

Is a constant current source a good idea? I realise the heaters will take longer to warm up, no idea if this is seconds or minutes but the heater element will never get more than its rated current i.e. 175mA.

Alternative is connect both in series to 7812 regulator with diode between the middle pin and ground to give regulated 12.7v. Are valve heater resistances well matched for this to be a good solution.

Of the two methods, which is the better for ensuring valves operate at matching conditions?
Or does it not matter if one heater say has 10% more power than the other?
.
 
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kylej1050

Member
2013-11-22 7:55 pm
You're going to find that regulating a 12VAC input to 12.7 to be difficult. There isn't really enough overhead for anything like a regulator.

The heaters will somewhat match and since they are in series will always have the same current through both but the voltage dropped may vary a bit.

I have some 16VAC transformers I use for DC preamp heater circuits where I use a CCS. It takes a bit longer to warm up but is extremely easy on the heaters.

Any reason you can't run AC heaters?
 
Or does it not matter if one heater say has 10% more power than the other?

It doesn't matter.

Are valve heater resistances well matched

In my experience, yes, very well matched.

Using any kind of regulation on an indirectly heated tube heater supply would be required only if it's a particularly demanding situation, like a microphone amplifier or a RIAA corrector or such, or maybe because the circuit will be in such a small space that normal precautions cannot be observed.

Or, if it's a headphone amp. Though even then, not always required.

So unless you specifically want to take this on as a technical challenge, or you are making a special kind of circuit, you don't have to think about regulation.


One more special case I just thought about; if the tubes in question are very rare and/or expensive. I regulate the heater supply of my CRTs.
 
02GF74 said:
As my power source is 12vac, i would like to have heaters in series so the va of my transformer is less.
If you are worried about transformer VA you should not use DC heaters, as these use about 3 times as much VA as AC heaters.

Reson for suggesting regulated is that it guarantess correct power, otherwise i would need to mess around with resistor values to get the correct voltage.
Regulated heaters are a common newbie fetish. Unnecessary in almost all circuits. Often creates problems, which catch newbies out. Much better to learn to do proper AC heater wiring.

Running from 35V rails suggests you are making an effects box.
 

jean-paul

diyAudio Moderator Emeritus
2002-09-20 7:20 am
Germany
You're going to find that regulating a 12VAC input to 12.6 to be difficult. There isn't really enough overhead for anything like a regulator.

Newer LDO regulator ICs have no problem in regulating to 12V DC when fed with 16V unregulated DC voltage. Just make sure you have enough capacitance to cater for the ripple voltage. It might be new to some but this really is an old problem that has long been solved. If you pay attention to the DC filament supply you make it slow starting so filaments don't get "shocked". It will make them live longer, just like using 12V instead of 12.6V. It is more or less the same as with light bulbs. They mostly break down when switched on because of the inrush current in cold state.

Using DC for filaments has some virtues when done right: less hum, easier wiring, slow starting. With modern parts losses can be quite low when compared to the classic circuits. This means using an LDO and Schottky diodes for instance. In the tube world newer semiconductors are seen as a kind of heresy so 78XX/LM317 still are used with their minimum drop of 3V. Higher voltage must be used for that reason and you will end up with a regulator circuit using as much power and creating as much heat as the tubes themselves. Look around for low drop higher current regs and your trouble is solved. Saves you from buying a large heatsink too.

If you are worried about transformer VA you should not use DC heaters, as these use about 3 times as much VA as AC heaters.

This is interesting, I always thought a 3W heater is 3W whether fed with AC or DC. Transformer will be loaded higher when a rectifier and filter caps are used that is true but I doubt the heater itself will draw more power. In fact I don't doubt it, I am sure it does not draw more power as it is plain rubbish. After using my "understanding english from native speakers" skills I think the grammar of the sentence could be way off and that something else is meant.
 
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Im worried that if i put 12vac (thats rms, peak would be 17v) it will be too much for the heaters and theyll be damaged.
You need to understand what RMS is about. 12 volts AC does have peaks at around 17 volts, but it also has dips where there is nothing. The heater gets the same amount of energy from a 12 volts AC supply (with peaks and dips) as from a 12 volts DC supply (which is constant), provided that the AC is a true sine wave and can therefore be describes as 12 volts AC RMS.

https://en.wikipedia.org/wiki/Root_mean_square
 
02GF74 said:
Im worried that if i put 12vac (thats rms, peak would be 17v) it will be too much for the heaters and theyll be damaged.
12V AC RMS has exactly the same heating effect as 12V DC - that is what RMS means!

jean-paul said:
This is interesting, I always thought a 3W heater is 3W whether fed with AC or DC. Transformer will be loaded higher when a rectifier and filter caps are used that is true but I doubt the heater itself will draw more power. In fact I don't doubt it, I am sure it does not draw more power as it is plain rubbish. After using my "understanding english from native speakers" skills I think the grammar of the sentence could be way off and that something else is meant.
A 3W heater consumes 3W whether fed 6.3V AC RMS or 6.3V DC. However, the DC fed heater will use up more of the VA rating of the transformer secondary, because of the peaky nature of the charging pulses. I didn't say that the heater will use more W, but more VA. I see no problem with the grammar, apart from a minor semantic point that when I said "DC heaters" I meant "DC heaters and the rectifier/reservoir arrangement feeding them" - this should have been clear from the context so any confusion is a matter of technical issues not language.
 

jean-paul

diyAudio Moderator Emeritus
2002-09-20 7:20 am
Germany
Sentence: "If you are worried about transformer VA you should not use DC heaters, as these use about 3 times as much VA as AC heaters." ----> Extracted meaning: DC heaters will use about 3 times a much VA as AC heaters.

Intended meaning: If you are worried about the transformer VA rating you should not use DC power for the filaments, as using DC power for the filaments will result in about 3 times as much load (as AC power for the filaments would do) to the transformer.

As filaments are ohmic/purely resistive in nature their consumed power will be the same whether it be in W or VA.
 
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jean-paul

diyAudio Moderator Emeritus
2002-09-20 7:20 am
Germany
Mr. 02GF74, you could use an LT3080 with added capacitor of 10 nF from the Set pin to GND and have a very low drop low noise soft starting filament PSU....

With 11DQ10 or similar diodes and a 2200 µF filter cap (I would probably use either 2 x 2200 or one 4700 µF just in case) this will work out OK for 12V AC input and 12V DC [email protected]

A cheap complete solution (less elegant as it has 1.3 V dropout voltage and no true soft start) but please use Schottky diodes and a 4700 µF filter cap with this one:

http://www.ebay.com/itm/LV30-Variable-Voltage-Regulator-1-5-30V-2A-DIY-Kit-/221266731333

You could also copy my SBT PSU design as found elsewhere on the forum. Shameless commercial message: I have several Rcore 6V transformers for sale that are very compact, have 10x less stray field compared to toroids and they are absolutely silent. i have used these for powering filaments with 6V DC myself with good results.
 
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Funny that the thread is labelled : power supply for two valves while it should be: filament supply for two valves
Would love to see the full schematic , or at least the tube section.
Just curious, what are these 175mA filament tubes?

Also curious about the power transformer windings: is there a separate 12VAC winding just for the filaments or do they come from a tap in a higher voltage winding which also feeds something else?
Not the same thing and again an image is worth 1000 words.
I mean a schematic, not just a transformer picture.
 
jean-paul said:
Sentence: "If you are worried about transformer VA you should not use DC heaters, as these use about 3 times as much VA as AC heaters." ----> Extracted meaning: DC heaters will use about 3 times a much VA as AC heaters.

Intended meaning: If you are worried about the transformer VA rating you should not use DC power for the filaments, as using DC power for the filaments will result in about 3 times as much load (as AC power for the filaments would do) to the transformer.
If you find your restatement of what I said easier to understand than my original then so be it. My version and your version both say exactly the same thing, but my version is shorter and slightly less clumsy as it does not include a sub-ordinate clause with a difficult ending.

Yeah it is way easier to understand the english non native speakers use
On my Master's course (all taught in English) the Chinese students found it easier to understand a Russian lecturer than an English lecturer. The English students found the Russian difficult to understand.

Give it a few years more and severe smart phone and iPad use will force us to communicate with hands and feet when facing a real person.
Perhaps. I can't really comment as I don't have a smartphone or an iPad, and almost always send texts using English spelling and punctuation.

Of course as a moderator you could introduce a forum rule that all posts by native English speakers are checked by a non-native speaker to ensure that they cannot be misunderstood by other non-native speakers.
 

jean-paul

diyAudio Moderator Emeritus
2002-09-20 7:20 am
Germany
I am not a moderator (anymore). The latin word Emeritus says "retired"...I did not choose that term myself AFAIK.

https://en.wikipedia.org/wiki/Emeritus

What you refer to as "clumsy" might be the right way of making thing clear on an international forum with many foreigners that are not used to cross linking data with data given in previous posts. "Shorter" is not a positive thing in some cases. I am confident that "DC heaters use about 3 times a much VA as AC heaters." will be misunderstood by many as it is a wrong statement because it is not the "heaters" themselves that use more VA/W. It is the transformer that "sees" a higher load because of rectification/filtering.
 
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In English 'emeritus' is often used to mean someone who is formally retired but carries on his work at a slower pace as an unpaid hobby; such a person is sometimes seen as a respected authority figure by younger folk. Hence I was unclear about your current role.

People often say I am too succinct in my writing. I don't intend learning how to add padding.