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Bigun 9th October 2012 07:04 PM

dc filament heater - high impedance const. voltage
 
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I may need a dc heater supply. There are several different designs out there, some of them proprietary. I only need one, for a mono project, so TentLabs is not a very economical option (sell only in pairs). But I like their approach of using a current source for high impedance and controlling it by measuring the voltage across the filament.

I thought about how I might go about doing that and came up with the attached concept.

Thoughts ?

DF96 9th October 2012 07:08 PM

If you adjust the current by measuring the voltage then you no longer have a high impedance supply; you have turned it into a low impedance supply. Fortunately, valve heaters prefer a low impedance supply as it gives better temperature stability.

tomchr 9th October 2012 07:11 PM

For indirectly heated tubes, all you need is a quiet 6.3 V supply.

For directly heated tubes, there is much debate about whether one should use a current source or a voltage source for the filament supply. I've tried both and could not tell a difference neither in measurements nor in listening tests. So I developed a voltage regulator. My regulator has an output impedance on the order of 1~2 mOhm so it's about as close to an ideal voltage source as you can get. The schematic and boards are on my website if you're interested.

~Tom

Bigun 9th October 2012 07:27 PM

Quote:

Originally Posted by DF96 (Post 3195306)
If you adjust the current by measuring the voltage then you no longer have a high impedance supply; you have turned it into a low impedance supply.

The key here is not the d.c. impedance of the heater supply seen by the filament - it is best if it were low since a large current has to flow. Rather what is important is that there is a high impedance to a.c. across the two terminals of the heater supply. Does not my concept provide that - since the lower end of the filament 'looks' into a 1k resistor in parallel to the collector of a BJT ?

Quote:

Originally Posted by tomchr (Post 3195309)
For directly heated tubes, there is much debate about whether one should use a current source or a voltage source for the filament supply. I've tried both and could not tell a difference neither in measurements nor in listening tests. [...]The schematic and boards are on my website if you're interested.

Thanks for the offer Tom. Did your current source have a high a.c. impedance across the heater ?

p.s. I'm talking DHT

Wavebourn 9th October 2012 07:43 PM

Make floating voltage stabilizer with current limited right above it's nominal value, to limit inrush current. It's the best you can do. Then use one of leads as a cathode, anyway dynamic resistance between them will be low.

One more option is to shunt filament by 2 3.15V zeners (or shunt regulators) in series, and feed this thingy from floating current source.

Bigun 9th October 2012 07:47 PM

Yes, this is my plan, a floating supply. In my concept, the inrush is limited, in a subtle way. The op-amp drives the base of the power BJT through a resistor. By choosing a higher value resistor and/or specifying an op-amp with an appropriate output current limit, you have placed a cap on the current that can flow through the bipolar (limited by it's finite Hfe).

Zener's - this is a non-starter for too many reasons.

Wavebourn 9th October 2012 07:59 PM

You already have Zener as reference.

You may use TL431 and one transistor (well, couple of both) in series if don't like Zeners. Then turn your stabilizer into a current source (- in on emitter with resistor to ground, 2.1V on +in as reference )

tomchr 9th October 2012 08:20 PM

Quote:

Originally Posted by Bigun (Post 3195335)
Thanks for the offer Tom. Did your current source have a high a.c. impedance across the heater ?

In order to provide a heater voltage that is constant across the audio band, I designed my regulator to provide low impedance across the audio band. I don't see a point of allowing the heater voltage to be modulated by the signal current. I'm talking about the output impedance of the regulator - i.e. the impedance the filament "sees" when "looking back into" the regulator. Not the impedance from input to output of the regulator which is quite high at RF.

Are you trying to design a regulator that acts as a voltage source at DC but as a current source from 1 mHz and up? Why? If you want a current source, design one. If you'd rather have a voltage source, build one of those... It's not clear from your post, but maybe you're looking for a current source with a voltage compliance no higher than, say 10 % above the nominal filament voltage. Is this correct?

~Tom

Bigun 9th October 2012 08:38 PM

Quote:

Originally Posted by Wavebourn (Post 3195383)
You already have Zener as reference.You may use TL431 and one transistor (well, couple of both) in series if don't like Zeners.

The problem with any kind of shunt regulator is that it is essentially a Class A device and will burn lots of juice - the filament is already hungry enough without adding to that. The other problem is that it offers an exceedingly low a.c. impedance across the filament - the opposite of what I want to achieve. I agree, the TL431 might be a good replacement as a voltage reference.

Quote:

Originally Posted by tomchr (Post 3195415)
Are you trying to design a regulator that acts as a voltage source at DC but as a current source from 1 mHz and up? Why? If you want a current source, design one. If you'd rather have a voltage source, build one of those... It's not clear from your post, but maybe you're looking for a current source with a voltage compliance no higher than, say 10 % above the nominal filament voltage. Is this correct?

Yes, more or less you are right. Rod Coleman's filament supply provides a good guideline to the design goals - low d.c. impedance, high. a.c. impedance.

A pure current source is a possibility, but you have to include a sense resistor in series with the filament current. And people using Rod's current source have found that the sound is sensitive to the quality of this resistor. This resistor also dissipates wasted heat and increases the voltage headroom required for the filament supply. I'll give this some thought though.

Wavebourn 9th October 2012 08:46 PM

Quote:

Originally Posted by Bigun (Post 3195446)
The other problem is that it offers an exceedingly low a.c. impedance across the filament - the opposite of what I want to achieve.

Sorry, it was hard to imagine that you wanted it.


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